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Impacts of Climatic and Atmospheric Changes on Carbon Dynamics in the Great Smoky Mountains National Park
Abstract
We used the Dynamic Land Ecosystem Model (DLEM) to estimate carbon (C) storage and to analyze the impacts of environmental changes on C dynamics from 1971 to 2001 in Great Smoky Mountain National Park (GRSM). Our simulation results indicate that forests in GRSM have a C density as high as 15.9kgm(-2), about twice the regional average. Total carbon storage in GRSM in 2001 was 62.2Tg (T=10(12)), 54% of which was in vegetation, the rest in the soil detritus pool. Higher precipitation and lower temperatures in the higher elevation forests result in larger total C pool sizes than in forests at lower elevations. During the study period, the CO(2) fertilization effect dominated ozone and climatic stresses (temperature and precipitation), and the combination of these multiple factors resulted in net accumulation of 0.9Tg C in this ecosystem.
... Understating the terrestrial ecosystem feedbacks to climate change needs conducting long term controlled studies to measure/predict temporal and spatial variation of carbon fluxes and sizes across different soil, vegetation, climate, and land uses. The measurements may be supported with integrated process ecosystem models such as Dynamic Land Ecosystem Model (DLEM) to address responses of terrestrial ecosystems to multiple stressor including land use change, climate change, and ecosystem disturbances (Zhang et al. 2007). The aim of this review is to discuss terrestrial ecosystem carbon dynamics in regard with land use and climate aspects. ...
... However, it is difficult to generalize the rate of change in soil carbon stocks across diverse climate and soil conditions (Vesterdal/Feifeld 2010). Zhang et al. (2007) reported that greater precipitation and lower temperature in higher elevation forests resulted in greater amount of carbon to store in soils compared to those in lower elevation forests in Great Smoky Mountains National Park. Figure 3.1 shows MODIS-predicted net primary products (NPPs) for different land uses in Turkey in 2006. ...
... Altitude has a dramatic control on the ecosystem C storage and its responses to global change. The protected areas in the high altitudes may provide a unique opportunity to study ecosystem response to climate change since these ecosystems are highly sensitive to climate change (Zhang et al. 2007). ...
This book pursues a unique approach, investigating both the ecological and socio-economic aspects of carbon management in Mediterranean ecosystems. All chapters are based on papers originally presented at the 1st Istanbul Carbon Summit, held at Istanbul Technical University, 2–4 April, 2014, and revised following a peer-review process.
The book addresses the summit’s three main themes – carbon management, carbon technologies, and carbon trends – while also offering chapters on the economic aspects of carbon management and the ecological aspects of the carbon cycle. The chapters on economic aspects analyze the carbon trade and its institutional, political, and legislative structures in different Mediterranean nations, while those on ecological aspects review the discourse on and analysis of the related ecological factors and their feedback due to governance processes.
... Understating the terrestrial ecosystem feedbacks to climate change needs conducting long term controlled studies to measure/predict temporal and spatial variation of carbon fluxes and sizes across different soil, vegetation, climate, and land uses. The measurements may be supported with integrated process ecosystem models such as Dynamic Land Ecosystem Model (DLEM) to address responses of terrestrial ecosystems to multiple stressor including land use change, climate change, and ecosystem disturbances (Zhang et al. 2007). The aim of this review is to discuss terrestrial ecosystem carbon dynamics in regard with land use and climate aspects. ...
... However, it is difficult to generalize the rate of change in soil carbon stocks across diverse climate and soil conditions (Vesterdal/Feifeld 2010). Zhang et al. (2007) reported that greater precipitation and lower temperature in higher elevation forests resulted in greater amount of carbon to store in soils compared to those in lower elevation forests in Great Smoky Mountains National Park. Figure 3.1 shows MODIS-predicted net primary products (NPPs) for different land uses in Turkey in 2006. ...
... Altitude has a dramatic control on the ecosystem C storage and its responses to global change. The protected areas in the high altitudes may provide a unique opportunity to study ecosystem response to climate change since these ecosystems are highly sensitive to climate change (Zhang et al. 2007). ...
Recent increases in atmospheric CO2 concentration and increased climate variations enforced us to improve our understanding of the terrestrial biosphere to improve human-ecosystem harmony in regard with processes and feedbacks that have functions in the earth system as a whole. Terrestrial ecosystems are principal components of the main carbon pools and land use has a decisive impact on these pools. Studies showed that converting forest and grasslands to farmlands and urban areas can result in considerable amount of carbon losses to atmosphere. However, emitted amounts may depend on the geographical region as well as type of vegetation cover of the converted areas. Recent studies showed that feedbacks between climate change and vegetation is more complicated than it was thought. Combined with these feedbacks, the land use changes may have an intricate impact on carbon exchange between atmosphere and biosphere. Studies showed that the consequences of changes in land use are beyond the expected in terms of ecosystem functioning and environmental quality. Complex interactions among climate, soil, plant productivity, and land management should be understood well to balance ecosystem functions and human welfare. In this literature review, we discussed interactions and feedbacks among terrestrial ecosystems and global carbon balance in regard with global climate change.
... Understating the terrestrial ecosystem feedbacks to climate change needs conducting long term controlled studies to measure/predict temporal and spatial variation of carbon fluxes and sizes across different soil, vegetation, climate, and land uses. The measurements may be supported with integrated process ecosystem models such as Dynamic Land Ecosystem Model (DLEM) to address responses of terrestrial ecosystems to multiple stressor including land use change, climate change, and ecosystem disturbances (Zhang et al. 2007). The aim of this review is to discuss terrestrial ecosystem carbon dynamics in regard with land use and climate aspects. ...
... However, it is difficult to generalize the rate of change in soil carbon stocks across diverse climate and soil conditions (Vesterdal/Feifeld 2010). Zhang et al. (2007) reported that greater precipitation and lower temperature in higher elevation forests resulted in greater amount of carbon to store in soils compared to those in lower elevation forests in Great Smoky Mountains National Park. Figure 3.1 shows MODIS-predicted net primary products (NPPs) for different land uses in Turkey in 2006. ...
... Altitude has a dramatic control on the ecosystem C storage and its responses to global change. The protected areas in the high altitudes may provide a unique opportunity to study ecosystem response to climate change since these ecosystems are highly sensitive to climate change (Zhang et al. 2007). ...
... As put by McKenzie et al. (2007): " Dynamic fuel mapping is necessary as we move into the future with rapid climatic change ... " To project the ecosystem's response to environmental changes, process-based ecosystem models have been developed to simulate the dynamics of living or dead organic matter pools in daily or monthly time step. The Dynamic Land Ecosystem Mode (DLEM, Tian et al., 2010), for example, has been used to estimate the impacts of long-term changes in climate and atmosphere on the vegetation biomass, coarse woody debris, litter pools and soil organic matter of a forest ecosystem in the Southern US (Zhang et al., 2007; Tian et al., 2008). The compartments/pools of ecosystem models can be linked to different fuel types (Allaby, 1998; Tian et al., 2005 Tian et al., , 2010). ...
... Except for the coastal regions, most of the southern states were projected to experience declined cipitation and prolonged drought in 2050 (Fig. 2C). Previous studies indicated that the productivity of southern ecosystem was dominated by precipitation (Zhang et al., 2007; Wear and Greis, 2002). Our climate model predicted 10% declination of annual precipitation of SUS during the first 50 years of the 1st century. ...
... The FCCS was compiled and calculated using the best available data which has a well-documented scientific foundation (Riccardi et al., 2007). The DLEM has been validated and applied to study the ecosystem carbon dynamics in response to long-term climate changes in the SUS (Zhang et al., 2007; Tian et al., 2010 ). For the purpose of regional validation, we compared the model simulated state-level mean carbon density of the southern ecosystem against the reported value based on forest inventory dataset (http://www.fia.fs.fed.us/). ...
The model projected ecosystem carbon dynamics were incorporated into the default (contemporary) fuel load map developed by FCCS (Fuel Characteristic Classification System) to estimate the dynamics of fuel load in the Southern United States in response to projected changes in climate and atmosphere (CO2 and nitrogen deposition) from 2002 to 2050. The study results indicated that in 2002 the total fuel load of the Southern United States was about 1.15 P g (1 P = 1015), which will decrease to 1.11 P g in 2050. The declination of fuel load is mainly due to the climate change, especially the reduced precipitation in 2050, while the effects of elevated CO2 and nitrogen deposition will increase fuel load. Interactions among all factors will result in 1% reduction in the fuel load in 2050. In response to the spatial heterogeneity in environmental changes, the dynamics of fuel load from 2002 to 2050 vary strongly among the study states. The declined precipitation in the northern inland of the study region may lead to 20% fuel load reduction in Tennessee and Kentucky by the year of 2050, while the elevated precipitation and decreased daily mean temperature in the coastal states, especially in South Carolina, North Carolina, and Virginia, may result in fuel load accumulation. The temporal–spatial variation of the fuel load may be overestimated since the adjustments of forest management regime in response to climate change were not considered in current study.
... The Dynamic Land Ecosystem Model (DLEM) is a highly integrated process-based terrestrial ecosystem model that simulates daily carbon, water and nitrogen cycles driven by the changes in atmospheric chemistry including ozone, nitrogen deposition, CO 2 concentration, climate, land-use and land-cover types and disturbances (i.e., fire, hurricane, and harvest) (Fig. 1). The DLEM is well documented and well evaluated and has been extensively used in studying the terrestrial carbon, water and nitrogen cycles over Monsoon Asia, the continental U.S., and South America (e.g., Tian et al., 2005Tian et al., , 2008, submitted for publication; Chen et al., 2006b;Ren et al., 2007a,b;Liu et al., 2007Liu et al., , 2008Zhang et al., 2007). ...
... Historical land-use data: Approaches similar to those of Chen et al. (2006a) and Zhang et al. (2007) were used to combine the contemporary land-use and land-cover map (which was derived from NLCD, 2001 with the historical census datasets of cropland and urban area), with population to reconstruct annual distribution maps of cropland and urban/developed lands at 8 km  8 km resolution from 1895 to 2007. ...
... The DLEM has been parameterized and applied in several regional studies both in China and the United States using various field observational data for all defined plant functional types (Fig. 2), and then validated with independent field observational data, inventory data and regional estimations from other models and remote sensing tools (Chen et al., 2006a,b;Ren et al., 2007a,b;Zhang et al., 2007;Tian et al., 2008, submitted for publication). Specifically, the model was recalibrated for all the plant functional types except build-up land in SUS. ...
The effects of global change on ecosystem productivity and water resources in the southern United States (SUS), a traditionally ‘water-rich’ region and the ‘timber basket’ of the country, are not well quantified. We carried out several simulation experiments to quantify ecosystem net primary productivity (NPP), evapotranspiration (ET) and water use efficiency (WUE) (i.e., NPP/ET) in the SUS by employing an integrated process-based ecosystem model (Dynamic Land Ecosystem Model, DLEM). The results indicated that the average ET in the SUS was 710 mm during 1895–2007. As a whole, the annual ET increased and decreased slightly during the first and second half of the study period, respectively. The mean regional total NPP was 1.18 Pg C/yr (525.2 g C/m2/yr) during 1895–2007. NPP increased consistently from 1895 to 2007 with a rate of 2.5 Tg C/yr or 1.10 g C/m2/yr, representing a 27% increase. The average WUE was about 0.71 g C/kg H2O and increased about 25% from 1895 to 2007. The rather stable ET might explain the resulting increase in WUE. The average WUE of different biomes followed an order of: forest (0.93 g C/kg H2O) > wetland (0.75 g C/kg H2O) > grassland (0.58 g C/kg H2O) > cropland (0.54 g C/kg H2O) > shrubland (0.45 g C/kg H2O). WUE of cropland increased the fastest (by 30%), followed by shrubland (17%) and grassland (9%), while WUE of forest and wetland changed little from the period of 1895–1950 to the period of 1951–2007. NPP, ET and WUE showed substantial inter-annual and spatial variability, which was induced by the non-uniform distribution patterns and change rates of environmental factors across the SUS. We concluded that an accurate projection of the regional impact of climate change on carbon and water resources must consider the spatial variability of ecosystem water use efficiency across biomes as well as the interactions among all stresses, especially land-use and land-cover change and climate.
... Recently, we developed a process-based biogeochemistry model, the Dynamic Land Ecosystem Model (DLEM), to simulate biogeochemical cycling of carbon, nitrogen and water in the land ecosystems. The DLEM considers multiple factors including climate, atmospheric compositions (CO 2 , O 3 ), precipitation chemistry (nitrogen composition), natural disturbances (fire, insect/disease, hurricane, etc), landuse/land-cover change, and land management (harvest, rotation , fertilization, irrigation, etc.) (Tian et al., 2005Tian et al., , 2008Tian et al., , 2010 Ren et al., 2007a Ren et al., , 2007b Ren et al., , 2009 Zhang et al., 2007 Zhang et al., , 2008 Lu, 2009; Liu et al., 2008; Xu, 2010). This model has been successfully applied to simulate the effects of multiple environmental factors on carbon and water cycles in China (Ren et al., 2007aRen et al., , 2007b Lu, 2009; Liu et al., 2008; Xu, 2010) and USA (Tian et al., 2008Tian et al., , 2010 Zhang et al., 2007 Zhang et al., , 2008). ...
... The DLEM considers multiple factors including climate, atmospheric compositions (CO 2 , O 3 ), precipitation chemistry (nitrogen composition), natural disturbances (fire, insect/disease, hurricane, etc), landuse/land-cover change, and land management (harvest, rotation , fertilization, irrigation, etc.) (Tian et al., 2005Tian et al., , 2008Tian et al., , 2010 Ren et al., 2007a Ren et al., , 2007b Ren et al., , 2009 Zhang et al., 2007 Zhang et al., , 2008 Lu, 2009; Liu et al., 2008; Xu, 2010). This model has been successfully applied to simulate the effects of multiple environmental factors on carbon and water cycles in China (Ren et al., 2007aRen et al., , 2007b Lu, 2009; Liu et al., 2008; Xu, 2010) and USA (Tian et al., 2008Tian et al., , 2010 Zhang et al., 2007 Zhang et al., , 2008). In this study, we enhanced the model's capability by addressing the biogeochemical processes of CH 4 and N 2 O and simulated CH 4 and N 2 O fluxes over terrestrial ecosystems in North America from 1979 to 2008. ...
... The DLEM operates at a daily time step and at varied spatial resolutions, from meters to kilometers, from regional to global. The additional information on the processes, interactions and feedbacks in the DLEM and associated input/output data (Fig. 1) can be found in our previous studies (Tian et al., 2005Tian et al., , 2008Tian et al., , 2010 Ren et al., 2007a Ren et al., , 2007b Ren et al., , 2009 Zhang et al., 2007 Zhang et al., , 2008 Lu, 2009; Liu et al., 2008;). In this paper, we provide a detailed description of the CH 4 and N 2 O modules with an emphasis on major processes that control fluxes of CH 4 and N 2 O in terrestrial ecosystems (Fig. 2). ...
Continental-scale estimations of terrestrial methane (CH4) and nitrous oxide (N2O) fluxes over a long time period are crucial to accurately assess the global balance of greenhouse gases and enhance our understanding and prediction of global climate change and terrestrial ecosystem feedbacks. Using a process-based global biogeochemical model, the Dynamic Land Ecosystem Model (DLEM), we quantified simultaneously CH4 and N2O fluxes in North America's terrestrial ecosystems from 1979 to 2008. During the past 30 years, approximately 14.69 ± 1.64 T g C a−1 (1 T g = 1012 g) of CH4, and 1.94 ± 0.1 T g N a−1 of N2O were released from terrestrial ecosystems in North America. At the country level, both the US and Canada acted as CH4 sources to the atmosphere, but Mexico mainly oxidized and consumed CH4 from the atmosphere. Wetlands in North America contributed predominantly to the regional CH4 source, while all other ecosystems acted as sinks for atmospheric CH4, of which forests accounted for 36.8%. Regarding N2O emission in North America, the US, Canada, and Mexico contributed 56.19%, 18.23%, and 25.58%, respectively, to the continental source over the past 30 years. Forests and croplands were the two ecosystems that contributed most to continental N2O emission. The inter-annual variations of CH4 and N2O fluxes in North America were mainly attributed to year-to-year climatic variability. While only annual precipitation was found to have a significant effect on annual CH4 flux, both mean annual temperature and annual precipitation were significantly correlated to annual N2O flux. The regional estimates and spatiotemporal patterns of terrestrial ecosystem CH4 and N2O fluxes in North America generated in this study provide useful information for global change research and policy making.
... Moreover, there is heterogeneity over a span of thirty years and therefore cannot be measured directly on a regional scale [13]. Ecosystembased modeling can analyze complex ecosystem processes and potential internal linkages and has proven to be a powerful tool for analyzing environmental factors [14][15][16]. However, common models such as Biome-BGC [17], CENTURY [18], OCHIDEE [19], etc., lack analysis of vegetation structure and ecological physiological processes in arid regions. ...
... Therefore, it cannot be used for analysis in this area [20,21]. Zhang & Ren [15,16] first verified the arid ecosystem model (AEM) of the arid zone ecological model using field observation data, and evaluated the spatial and temporal patterns of NPP responses to climate and carbon dioxide changes in Central Asia from 1980 to 2014, and evaluated different environmental factors (precipitation, The relative contribution rate of temperature, ...
... This model has been extensively calibrated against various field data covering forest, grassland and cropland from the Chinese Ecological Research Network, US LTER sites and AmeriFlux network (e.g. Ren et al., 2007a,b; Zhang et al., 2007a; Liu et al., 2008; Tian et al., 2010a,b). DLEM has been used to simulate the effects of climate variability and change, atmospheric CO 2 , tropospheric O 3 , nitrogen deposition and land-cover and landuse change on the pools and fluxes of carbon and water in China (Chen et al., 2006a,b; Ren et al., 2007a,b; Liu et al., 2008), the United States (Zhang et al., 2007a; Tian et al., 2008; 2010a) and the North America (Tian et al., 2010b; Xu et al., 2010). ...
... Ren et al., 2007a,b; Zhang et al., 2007a; Liu et al., 2008; Tian et al., 2010a,b). DLEM has been used to simulate the effects of climate variability and change, atmospheric CO 2 , tropospheric O 3 , nitrogen deposition and land-cover and landuse change on the pools and fluxes of carbon and water in China (Chen et al., 2006a,b; Ren et al., 2007a,b; Liu et al., 2008), the United States (Zhang et al., 2007a; Tian et al., 2008; 2010a) and the North America (Tian et al., 2010b; Xu et al., 2010). The DLEM agricultural module enhances the ability of DLEM model to simulate the interactive effects of agronomic practices/land management and other environmental factors on crop growth, phenology and biogeochemical cycles in croplands (Fig. 1). ...
... This model has been extensively calibrated against various field data covering forest, grassland and cropland from the Chinese Ecological Research Network, US LTER sites and AmeriFlux network (e.g. Ren et al., 2007a,b; Zhang et al., 2007a; Liu et al., 2008; Tian et al., 2010a,b). DLEM has been used to simulate the effects of climate variability and change, atmospheric CO 2 , tropospheric O 3 , nitrogen deposition and land-cover and landuse change on the pools and fluxes of carbon and water in China (Chen et al., 2006a,b; Ren et al., 2007a,b; Liu et al., 2008), the United States (Zhang et al., 2007a; Tian et al., 2008; 2010a) and the North America (Tian et al., 2010b; Xu et al., 2010). ...
... Ren et al., 2007a,b; Zhang et al., 2007a; Liu et al., 2008; Tian et al., 2010a,b). DLEM has been used to simulate the effects of climate variability and change, atmospheric CO 2 , tropospheric O 3 , nitrogen deposition and land-cover and landuse change on the pools and fluxes of carbon and water in China (Chen et al., 2006a,b; Ren et al., 2007a,b; Liu et al., 2008), the United States (Zhang et al., 2007a; Tian et al., 2008; 2010a) and the North America (Tian et al., 2010b; Xu et al., 2010). The DLEM agricultural module enhances the ability of DLEM model to simulate the interactive effects of agronomic practices/land management and other environmental factors on crop growth, phenology and biogeochemical cycles in croplands (Fig. 1). ...
The spatial and temporal patterns of CO[subscript 2] and CH[subscript 4] fluxes in China's croplands were investigated and attributed to multifactor environmental changes using the agricultural module of the Dynamic Land Ecosystem Model (DLEM), a highly integrated process-based ecosystem model. During 1980–2005 modelled results indicated that China's croplands acted as a carbon sink with an average carbon sequestration rate of 33.4 TgC yr[superscript -1] (1 Tg = 10[superscript 12] g). Both the highest net CO[subscript 2] uptake rate and the largest CH[subscript 4] emission rate were found in southeast region of China's croplands. Of primary influences were land-cover and land-use change, atmospheric CO[subscript 2] and nitrogen deposition, which accounted for 76%, 42% and 17% of the total carbon sequestration in China's croplands during the study period, respectively. The total carbon losses due to elevated ozone and climate variability/change were equivalent to 27% and 9% of the total carbon sequestration, respectively. Our further analysis indicated that nitrogen fertilizer application accounted for 60% of total national carbon uptake in cropland, whereas changes in paddy field areas mainly determined the variability of CH[subscript 4] emissions. Our results suggest that improving air quality by means such as reducing ozone concentration and optimizing agronomic practices can enhance carbon sequestration capacity of China's croplands.
... Furthermore, as agricultural food production has doubled in the last 35 years of the twentieth century, another doubling is anticipated so as to meet the increasing human demands for food and fiber (Tilman 1999). The arid and semiarid region of northwest China plays an important role in the food security of China, the world's most populous country, not only because of the plentiful land available for cultivation in this region (Dai et al. 2008), but also because of the lower economic cost of reclamation compared with that in southern China (Zhang et al. 2004). A large land resource including desert land and grassland has been reclaimed for cropland since the 1980s. ...
... A quantitative assessment of the climate response to cropland transition should be undertaken by employing ecosystem models and climate models. For example, the dynamic land ecosystem model (DLEM) (Zhang et al. 2007; Tian et al. 2010) could be adopted to measure ET, heat transformation between land and atmosphere, as well as the changes on carbon stock. Regional climate model (RCM) (Giorgi and Shields 1999; Ju et al. 2007; Sylla et al. 2010) and general circulation model (GCM) (Joshi et al. 2003; Willett et al. 2010) could also be coupled to assess the changes of temperature and rainfall. ...
Land-use change is becoming an important anthropogenic force in the global climate system through alteration of the Earth’s biogeophysical and biogeochemical processes. Cropland, which provides people with food, is the most variable vegetation land-use type and is affected by natural and anthropic forces and in turn affects the environment and climate system. This paper investigates the temporal-spatial pattern of cropland transition in the arid and semiarid region of northwest China, using remote sensing data for the late 1980s, 1995, 2000, 2005, 2008, and 2010. The aim was to clarify the change intensity and conversion pattern of cropland with a view to identifying the effects of a series of governmental policies and their influence on the climate system. Mathematical methodologies including the use of a transition matrix model, dynamic degree model, area-weighted centroid model, and area percentage were employed to analyze the temporal change in cropland. Meanwhile, a gridded zonal model with 10-km2 resolution was used to detect the spatial pattern of cropland transition. During the period from the late 1980s to 2010, cropland increased dramatically by 23,182.17 km2, an increase of 13.61 % relative to the area under cultivation in the late 1980s. Cropland transition accumulated in the western oasis–desert ecotone of the study area while it declined in the eastern farming–pastoral ecotone, leading to the westward movement of the cropland centroid. A net decrease in natural vegetation and unused land along with a net increase in built-up land due to cropland conversion was observed in the monitoring period. The three major driving forces of the cropland transition were population growth, economic development, and land-use management governed by the Grain for Green Program. The climate response to different conversion patterns was simply analyzed. However, quantitative assessment of the effect should be undertaken by employing ecosystem and climate models.
... Meanwhile, the concurrent impacts of changes in climate, atmospheric composition, and land use and management practices are integrated into DLEM to retrieve reliable historical evolution and spatial patterns of the biogenic GHG budgets. DLEM has been widely applied to a variety of terrestrial ecosystem types across the globe and the detailed assumptions and processes are well documented in our previous work [Chen et al., 2006;Liu et al., 2008;Lu, 2009;Ren et al., 2007Ren et al., , 2011aRen et al., , 2011bTian et al., 2008Tian et al., , 2010aTian et al., , 2010bTian et al., , 2011Xu et al., 2010;Zhang et al., 2007]. ...
... [9] The simulated exchanges of carbon, water and nitrogen between the interfaces at both site and regional levels have been well calibrated and extensively validated against field observation and inventory results (see Text S1, available as auxiliary material). 1 Especially for the three biogenic GHG budgets, simulation experiments conducted in North America and China found that DLEM performed well in capturing both the interannual/seasonal and spatial patterns of CO 2 , CH 4 and N 2 O fluxes [Ren et al., , 2011a[Ren et al., , 2011bTian et al., 2010aTian et al., , 2010bTian et al., , 2011Zhang et al., 2007]. More detailed information concerning CH 4 and N 2 O production and emission can be referred to our other publications [Tian et al., 2010a[Tian et al., , 2010bXu, 2010;Xu et al., 2010]. ...
China's terrestrial ecosystems have been recognized as an atmospheric CO2 sink; however, it is uncertain whether this sink can alleviate global warming given the fluxes of CH4 and N2O. In this study, we used a process-based ecosystem model driven by multiple environmental factors to examine the net warming potential resulting from net exchanges of CO2, CH4, and N2O between China's terrestrial ecosystems and the atmosphere during 1961-2005. In the past 45 years, China's terrestrial ecosystems were found to sequestrate CO2 at a rate of 179.3 Tg C yr-1 with a 95% confidence range of (62.0 Tg C yr-1, 264.9 Tg C yr-1) while emitting CH4 and N2O at rates of 8.3 Tg C yr-1 with a 95% confidence range of (3.3 Tg C yr-1, 12.4 Tg C yr-1) and 0.6 Tg N yr -1 with a 95% confidence range of (0.2 Tg N yr-1, 1.1 Tg N yr-1), respectively. When translated into global warming potential, it is highly possible that China's terrestrial ecosystems mitigated global climate warming at a rate of 96.9 Tg CO2eq yr-1 (1 Tg = 1012 g), substantially varying from a source of 766.8 Tg CO 2eq yr-1 in 1997 to a sink of 705.2 Tg CO2eq yr-1 in 2002. The southeast and northeast of China slightly contributed to global climate warming; while the northwest, north, and southwest of China imposed cooling effects on the climate system. Paddy land, followed by natural wetland and dry cropland, was the largest contributor to national warming potential; forest, followed by woodland and grassland, played the most significant role in alleviating climate warming. Our simulated results indicate that CH4 and N2O emissions offset approximately 84.8% of terrestrial CO2 sink in China during 1961-2005. This study suggests that the relieving effects of China's terrestrial ecosystems on climate warming through sequestering CO2 might be gradually offset by increasing N2O emission, in combination with CH4 emission.
... Meanwhile, the concurrent impacts of changes in climate, atmospheric composition, and land use and management practices are integrated into DLEM to retrieve reliable historical evolution and spatial patterns of the biogenic GHG budgets. DLEM has been widely applied to a variety of terrestrial ecosystem types across the globe and the detailed assumptions and processes are well documented in our previous work [Chen et al., 2006;Liu et al., 2008;Lu, 2009;Ren et al., 2007Ren et al., , 2011aRen et al., , 2011bTian et al., 2008Tian et al., , 2010aTian et al., , 2010bTian et al., , 2011Xu et al., 2010;Zhang et al., 2007]. ...
... [9] The simulated exchanges of carbon, water and nitrogen between the interfaces at both site and regional levels have been well calibrated and extensively validated against field observation and inventory results (see Text S1, available as auxiliary material). 1 Especially for the three biogenic GHG budgets, simulation experiments conducted in North America and China found that DLEM performed well in capturing both the interannual/seasonal and spatial patterns of CO 2 , CH 4 and N 2 O fluxes [Ren et al., , 2011a[Ren et al., , 2011bTian et al., 2010aTian et al., , 2010bTian et al., , 2011Zhang et al., 2007]. More detailed information concerning CH 4 and N 2 O production and emission can be referred to our other publications [Tian et al., 2010a[Tian et al., , 2010bXu, 2010;Xu et al., 2010]. ...
Natural wetland ecosystem plays an important role in global climate change due to its large amounts of stored carbon and nitrogen. The Sanjiang Plain, Northeast China, encompasses large area of natural freshwater marshy wetlands. However, the magnitude and temporal patterns of major greenhouse gases (GHGs: CO2, CH4 and N2O) in this region remain far from certain. Here we used a process-based ecosystem model to examine GHGs fluxes and their underlying mechanisms in the marshland across the Sanjiang Plain over the period 1949–2008. Simulation results indicated that during the past 60 years, the Sanjiang Plain's marshland acted as a net CO2 sink of 4.20 ± 0.44 Tg C yr−1, while approximately 0.46 ± 0.02 Tg C yr−1 for CH4 and 0.02 ± 0.00 Tg N yr−1 for N2O were released to the atmosphere. Land cover and land use change (LCLUC) was the primary driver for GHGs changes. Climate change and tropospheric ozone (O3) pollution decreased CO2 uptakes, yet elevated CO2 concentration and nitrogen deposition increased CO2 uptake. Tropospheric O3 pollution and nitrogen deposition decreased CH4 emission by 7.94 Gg C and 0.41 Gg C, respectively, while elevated CO2 concentration increased the CH4 emission by 133.81 Gg C. Accumulatively, tropospheric O3 pollution and climate change contributed approximately 5.37% and 4.89% to the increased N2O emission, respectively, while elevated CO2 concentration reduced 2.81% of the N2O emission. The global warming potential (GWP) ranged from −1.81 Tg CO2eq yr−1 in the 1970s to 29.15 Tg CO2eq yr−1 in the 2000s. The decadal GWP by CO2 fluxes shifted from negative values between the 1950s and the 1990s to positive values in the 2000s, while CH4 and N2O emissions enhanced GWP from the 1950s to the 2000s. The total GWP decreased from the 1950s to the 1970s, but increased from the 1980s to the 2000s. This suggested that the reduced GWP by decreased CH4 emissions was gradually offset by the increased GWP by increased N2O emissions and decreased CO2 sink from the 1980s to the 2000s, implying that a full accounting of the greenhouse gas balance is essential in assessing global change impacts.
... The dynamic land ecosystem model (DLEM) is applied in this study; it is a highly integrated process-based ecosystem model that aims at simulating the fluxes and storages of carbon, water and nitrogen among/within terrestrial ecosystem components while taking into consideration multiple natural and anthropogenic perturbations [Tian et al., 2010b]. The DLEM is composed of five major submodules focusing on biophysics, plant physiology, soil biogeochemistry, vegetation dynamics, land use and management as well as disturbances such as hurricane, fire, insect etc. DLEM has been widely applied to a variety of terrestrial ecosystem types across the globe and the detailed assumptions and processes are well documented in our previous publications [Tian et al., 2008, 2010a, 2010b, 2011a, 2011b; Chen et al., 2006; Liu et al., 2008; Lu et al., 2011; Ren et al., 2007, 2011; Xu et al., 2010, 2011; Xu, 2010; C. Zhang et al., 2007, 2010, 2012]. [12] The methane module in the DLEM model mainly simulates the production, consumption, and transport of CH 4 [Tian et al., 2010a; Xu et al., 2010]. ...
... Currently, there was only one field study conducted on nitrogen input impacts on CH 4 emission from marshland in China [L. H. Zhang et al., 2007]; there was no study reported for the effects of elevated atmospheric CO 2 and O 3 pollution on CH 4 flux from marshland in China. We collected the observed CH 4 flux in response to elevated atmospheric CO 2 , O 3 pollution, and nitrogen input conducted in other countries for the factorial validation in this study. ...
1] We used a process-based ecosystem model to examine methane (CH 4) fluxes in the marshland across China as a result of multifactor global changes during 1949–2008. Our simulated results show a significant declining rate of 18.7 Gg Ca À1 (1Gg = 10 9 g) (with a 95% confidence boundary of 17.6 $ 19.8 Gg C a À1) at national scale, but substantially varying from the maximum annual CH 4 emission of 2.4 Tg C a À1 (1Tg = 10 12 g) (with a 95% confident boundary of 1.8 $ 3.4 Tg C a À1) in 1952 to the minimum annual CH 4 emission of 1.3 Tg C a À1 (with a 95% confident boundary of 1.0 $ 1.9 Tg C a À1) in 2003. The marshland loss made the largest contribution to the CH 4 emission reduction with an cumulative effect of 37.9 Tg C (with a 95% confident boundary of 28.0 $ 54.1 Tg C) for the past 60 years. Ozone pollution reduced CH 4 emission, while elevated atmospheric CO 2 , nitrogen deposition, climate change, and multiple-factor interaction, cumulatively stimulated CH 4 emission. Climate variability predominately controlled the inter-annual variations in CH 4 emissions. A substantial spatial variation in CH 4 emission was observed across China's marshland. At regional scale, the Northeast, followed by Northwest and Southeast, made the greatest contribution, while North and Southwest made minor contributions to the national CH 4 emission. This study suggests that it is necessary to consider multiple global change factors when estimating regional CH 4 fluxes in natural wetlands.
... This model has been extensively calibrated against various field data covering forest, grassland and cropland from the Chinese Ecological Research Network, US LTER sites and AmeriFlux network (e.g. Ren et al., 2007a,b; Zhang et al., 2007a; Liu et al., 2008; Tian et al., 2010a,b). DLEM has been used to simulate the effects of climate variability and change, atmospheric CO 2 , tropospheric O 3 , nitrogen deposition and land-cover and landuse change on the pools and fluxes of carbon and water in China (Chen et al., 2006a,b; Ren et al., 2007a,b; Liu et al., 2008), the United States (Zhang et al., 2007a; Tian et al., 2008; 2010a) and the North America (Tian et al., 2010b; Xu et al., 2010). ...
... Ren et al., 2007a,b; Zhang et al., 2007a; Liu et al., 2008; Tian et al., 2010a,b). DLEM has been used to simulate the effects of climate variability and change, atmospheric CO 2 , tropospheric O 3 , nitrogen deposition and land-cover and landuse change on the pools and fluxes of carbon and water in China (Chen et al., 2006a,b; Ren et al., 2007a,b; Liu et al., 2008), the United States (Zhang et al., 2007a; Tian et al., 2008; 2010a) and the North America (Tian et al., 2010b; Xu et al., 2010). The DLEM agricultural module enhances the ability of DLEM model to simulate the interactive effects of agronomic practices/land management and other environmental factors on crop growth, phenology and biogeochemical cycles in croplands (Fig. 1). ...
The spatial and temporal patterns of CO2 and CH4 fluxes in China's croplands were investigated and attributed to multifactor environmental changes using the agricultural module of the Dynamic Land Ecosystem Model (DLEM), a highly integrated process-based ecosystem model. During 1980–2005 modelled results indicated that China's croplands acted as a carbon sink with an average carbon sequestration rate of 33.4 TgC yr−1 (1 Tg = 1012 g). Both the highest net CO2 uptake rate and the largest CH4 emission rate were found in southeast region of China's croplands. Of primary influences were land-cover and land-use change, atmospheric CO2 and nitrogen deposition, which accounted for 76%, 42% and 17% of the total carbon sequestration in China's croplands during the study period, respectively. The total carbon losses due to elevated ozone and climate variability/change were equivalent to 27% and 9% of the total carbon sequestration, respectively. Our further analysis indicated that nitrogen fertilizer application accounted for 60% of total national carbon uptake in cropland, whereas changes in paddy field areas mainly determined the variability of CH4 emissions. Our results suggest that improving air quality by means such as reducing ozone concentration and optimizing agronomic practices can enhance carbon sequestration capacity of China's croplands.
... The DLEM model used in this study is a highly integrated, process-based terrestrial ecosystem model that aims to simulate the structural and functional dynamics of land ecosystems affected by multiple environmental factors including climate, atmospheric compositions (CO 2 and O 3 ), precipitation (PPT) chemistry (nitrogen deposition), natural disturbances (fire, insect ⁄ disease, hurricane, etc), LUCC, and land management (harvest, rotation, fertilization, irrigation, etc.). DLEM couples major biogeochemical cycles, the hydrological cycle, and vegetation dynamics to make daily, spatially explicit estimates of water, carbon (e.g., CO 2 and CH 4 ), and nitrogen fluxes (e.g., N 2 O) in terrestrial ecosystems (Tian et al., 2005;Chen et al., 2006;Ren et al., 2007b;Zhang et al., 2007;Tian et al., 2008). ...
... Dynamic Land Ecosystem Model has already been carefully calibrated and widely applied to regional studies in both China (e.g., Chen et al., 2006;Ren et al., 2007a;Liu et al., 2007;Tian et al., 2008) and the United States (e.g., Zhang et al., 2007;Tian et al., 2008). Field data from the Chinese Ecological Research Network, U.S. Long-Term Ecological Research Network, and AmeriFlux network have been used to calibrate and validate the model. ...
China has experienced a rapid land-use/cover change (LUCC) during the 20th Century, and this process is expected to continue in the future. How LUCC has affected water resources across China, however, remains uncertain due to the complexity of LUCC-water interactions. In this study, we used an integrated Dynamic Land Ecosystem Model (DLEM) in conjunction with spatial data of LUCC to estimate the LUCC effects on the magnitude, spatial and temporal variations of evapotranspiration (ET), runoff, and water yield across China. Through comparisons of DLEM results with other model simulations, field observations, and river discharge data, we found that DLEM model can adequately catch the spatial and seasonal patterns of hydrological processes. Our simulation results demonstrate that LUCC led to substantial changes in ET, runoff, and water yield in most of the China’s river basins during the 20th Century. The temporal and spatial patterns varied significantly across China. The largest change occurred during the second half century when almost all of the river basins had a decreasing trend in ET and an increasing trend in water yield and runoff, in contrast to the inclinations of ET and declinations of water yield in major river basins, such as Pearl river basin, Yangtze river basin, and Yellow river basin during the first half century. The increased water yield and runoff indicated alleviated water deficiency in China in the late 20th Century, but the increased peak flow might make the runoff difficult to be held by reservoirs. The continuously increasing ET and decreasing water yield in Continental river basin, Southwest river basin, and Songhua and Liaohe river basin implied regional water deficiency. Our study in China indicates that deforestation averagely increased ET by 138 mm/year but decreased water yield by the same amount and that reforestation averagely decreased ET by 422 mm/year since most of deforested land was converted to paddy land or irrigated cropland. In China, cropland-related land transformation is the dominant anthropogenic force affecting water resources during the 20th Century. On national average, cropland expansion was estimated to increase ET by 182 mm/year while cropland abandonment decreased ET by 379 mm/year. Our simulation results indicate that urban sprawl generally decreased ET and increased water yield. Cropland managements (fertilization and irrigation) significantly increased ET by 98 mm/year. To better understand LUCC effects on China’s water resources, it is needed to take into account the interactions of LUCC with other environmental changes such as climate and atmospheric composition.
... In recent years, relevant scholars have performed many studies on the temporal and spatial characteristics, influencing factors, simulation and prediction, carbon emission policies and other aspects of national park carbon sinks. Zhang [11]. Cambule et al. evaluated the total amount, spatial distribution and reasons for changes in soil organic carbon in the Limpopo National Park in southwest Mozambique [12]. ...
National parks, as an important type of nature protected areas, are the cornerstone that can effectively maintain biodiversity and mitigate global climate change. At present, China is making every effort to build a nature-protection system, with national parks as the main body, and this approach considers China′s urgent goals of obtaining carbon neutrality and mitigating climate change. It is of great significance to the national carbon-neutralization strategy to accurately predict the carbon sink capacity of national park ecosystems under the background of global change. To evaluate and predict the dynamics of the carbon sink capacity of national parks under climate change and different management measures, we combined remote-sensing observations, model simulations and scenario analyses to simulate the change in the carbon sink capacity of the proposed Kunlun Mountain National Park ecosystem over the past two decades (2000–2020) and the change in the carbon sink capacity under different zoning controls and various climate change scenarios from 2020 to 2060. Our results show that the carbon sink capacity of the proposed Kunlun Mountain National Park area is increasing. Simultaneously, the carbon sink capacity will be improved with the implementation of park management and control measures; which will be increased by 2.04% to 2.13% by 2060 in the research area under multiple climate change scenarios. The research results provide a scientific basis for the establishment and final boundary determination of the proposed Kunlun Mountain National Park.
... It is important to evaluate the stability of Xinjiang's dryland in response to the rapid climate change. As a useful indicator of ecosystem function, Net Primary Productivity (NPP) is a key component of energy and matter transformation in terrestrial ecosystems (Zhang et al., 2007b;Chen and Zhang, 2017). Local and regional changes in NPP caused by climate change are critical in studying relationships between climate and vegetation (Zhang et al., 2010b). ...
Dryland in Xinjiang, China has been threatened in recent decades by rapid climate change. However, due to large uncertainties in spatial climate datasets, the impacts of climate change and rising CO2 on net primary productivity (NPP) in Xinjiang have remained unclear. These uncertainties in climate will inevitably lead to uncertainties in model estimated NPP in Xinjiang. The uncertainties can be assessed by running an Arid Ecosystem Model (AEM) using multiple climate datasets. Such an approach allows to disentangle the relative contributions of individual climate factors on NPP change using numerical simulations and factorial analysis. The average annual NPP for Xinjiang from 2000 to 2014 was 155.90±2.74 g·C/(m²·year), 189.99±1.80 g·C/(m²·year), 213.04±8.93 g·C/(m²·year) by using the MERRA, ERA-Interim and CFSR datasets respectively to drive AEM. Our multiple simulations show a consistent temporal pattern of the regional NPP during 2000–2014 that increased during 2008–2011, and decreased during 2005–2006 and 2013–2014. However, we found large uncertainties in the spatial pattern of NPP change during this time, particularly in the Taklimakan Desert and northern Xinjiang (except for the Tacheng where NPP increased). All simulations indicated that areas surrounding the Taklamakan Desert and ecosystems at high latitude (>47°) and northwestern Xinjiang were dominated by precipitation change. However, there are large uncertainties in the dominant climate driver in other areas. To assess uncertainties in ecosystem NPP assessment, efforts should be made to improve the confidence in climate data in the northern Xinjiang. This can be achieved by establishing more rain gauges.
... Remote sensing-based approaches can estimate NPP variations and patterns, but it can hardly analyze complicated ecosystem processes and underlying internal control mechanisms and thus cannot quantitatively separate the relative contributions of various environmental factors [Li et al., 2012]. Ecosystem process-based modeling is a powerful tool to estimate and predict dynamics in terrestrial biogeochemical cycles and associated control mechanisms in the context of global change [Ren et al., 2012;Tao et al., 2007;Zhang et al., 2007Zhang et al., , 2012. However, popular models such as Biome-BGC [Parton et al., 1988], CENTURY [Running and Coughlan, 1988], and OCHIDEE [Krinner et al., 2005] either oversimplifies or ignores the unique vegetation structure and ecophysiological processes in dryland [Yang et al., 2001;Zeng et al., 2008], while dryland ecosystem models, like PALS [Reynolds et al., 2000] and AEM , usually have not been adequately evaluated against observations from climate/CO 2 manipulative studies for their performances in predicting ecosystem's responses to climatic changes. ...
Central Asia covers a large land area of 5×106 km2 and has unique temperate dryland ecosystems, with over 80% of the world's temperate deserts, which has been experiencing dramatic warming and drought in the recent decades. How the temperate dryland responds to complex climate change, however, is still far from clear. This study quantitatively investigates terrestrial net primary productivity (NPP) in responses to temperature, precipitation, and atmospheric CO2 during 1980-2014, by using the Arid Ecosystem Model, which can realistically predicte ecosystems' responses to changes in climate and atmospheric CO2 according to model evaluation against 28 field experiments/obervations. The simulation results show that unlike other mid/high latitude regions, NPP in Central Asia declined 10% (0.12×1015 g C) since the 1980s in response to a warmer and drier climate The dryland's response to warming was weak while its cropland was sensitive to the CO2 fertilization effect (CFE). However, the CFE was inhibited by the long-term drought from 1998-2008 and the positive effect of warming on photosynthesis was largely offset by the enhanced water deficit. The complex interactive effects among climate drivers, unique responses from diverse ecosystem types, and intensive and heterogeneous climate changes led to highly complex NPP changing patterns in Central Asia, of which 69% was dominated by precipitation variation and 20% and 9% was dominated by CO2 and temperature, respectively. The Turgay Plateau in northern Kazakhstan and southern Xinjiang in China were hotspots of NPP degradation in response to climate changes during the past three decades and in the future.
... Ozone-induced reductions in plant growth can result in reduced C input to soil and therefore soil C content (Andersen, 2003). The simulations of most ecosystem models support this prediction (Ren et al., 2007b;Zhang et al., 2007a;Felzer et al., 2004). However, very few studies have directly measured soil C dynamics under elevated O 3 . ...
Chapter 9 presents the most policy-relevant information related to this review of the NAAQS for the welfare effects of ozone on vegetation and ecosystems. This section integrates the key findings from the disciplines evaluated in this assessment of the ozone scientific literature, which includes plant physiology, whole plant biology, ecosystems, and exposure-response.
... Modelled estimates of C sequestration by forests in the Great Smoky Mountains National Park (southeast-central USA) attribute a 50% loss to ambient O 3 between 1971 and 2001 (Zhang et al. 2007). Between 2001 and 2003, a maximum of 31% loss in productivity of aspen in parts of its North American range was estimated to be caused by O 3 (Percy et al. 2007). ...
... The eastern Tennessee deciduous forest site in this study had an estimated ANPP of 446 g C m −2 yr −1 (13 oz C yd −2 yr −1 ) (wood + leaves portion of their NPP estimate). Zhang et al. (2007) used the Dynamic Land Ecosystem Model to estimate carbon storage in the park. Their results revealed an NPP contribution of 748 g C m −2 yr −1 (22 oz C yd −2 yr −1 ). ...
Protected areas such as national parks are recognized as important providers of ecosystem services, the benefits nature conveys to humans. However, some threats to these services, such as air pollution, can derive from outside a park’s boundaries. Ground-level ozone (O3) is a human-made pollutant that at elevated levels can damage vegetation, resulting in decreased growth and increased water loss through evapotranspiration, which in turn results in decreased overall streamflow. Using studies conducted on similar ecosystems in and near Great Smoky Mountains National Park, we estimated the potential loss from O3 damage of two ecosystem services, climate regulation (through the intermediate service of carbon sequestration) and water provisioning (through streamflow), at this national park. These ecosystem functions directly benefit humans by providing a livable climate and by providing downstream beneficiaries with water for drinking, agriculture, recreation, and hydropower. We found the loss from impairment of these services could be significant when O3 levels are elevated. A 50% increase in O3 exposure is projected to result in a loss of carbon (C) sequestration of 500,000–960,000 t C yr−1 (metric tons of C per year) (551,000 sh t C yr−1), while a 25% reduction in O3 concentrations could result in an increase in streamflow of 109.6 M m3 (million cubic meters) (88,854 ac-ft) from the park during the critical dry August–October period. This highlights the important services provided by protected landscapes such as this national park and the need for more in-depth research on the effects air pollution can have on the benefits we receive from nature.
... The observational data used for model calibration are mainly from the Chinese Ecological Research Network, US Long Term Ecological Research (LTER) sites, and AmeriFlux network. After evaluating the model performance at the site level, DLEM has been widely used to simulate the effects of climate variability and change, atmospheric CO 2 , tropospheric O 3 , nitrogen deposition, and LCLUC on the pools and fluxes of carbon and water in China (Lu and Tian, 2013;Lu et al., 2012;Ren et al., 2007;Ren et al., 2011;Tian et al., 2011), the continental United States (Song et al., 2013;Tian et al., 2010a;Zhang et al., 2007;Zhang et al., 2012;Zhang et al., 2010) and North America (Tian et al., 2010c;Xu et al., 2010). ...
A wide variety of studies have estimated the magnitude of global terrestrial net primary production (NPP), but its variations, both spatially and temporally, still remain uncertain. By using an improved process-based terrestrial ecosystem model (DLEM, Dynamic Land Ecosystem Model), we provide an estimate of global terrestrial NPP induced by multiple environmental factors and examine the response of terrestrial NPP to climate variability at biome and global levels and along latitudes throughout the first decade of the 21st century. The model simulation estimates an average global terrestrial NPP of 54.6 Pg C yr–1 during 2000–2009, varying from 52.8 Pg C yr–1 in the dry year of 2002 to 56.4 Pg C yr–1 in the wet year of 2008. In wet years, a large increase in terrestrial NPP compared to the decadal mean was prevalent in Amazonia, Africa and Australia. In dry years, however, we found a 3.2% reduction in global terrestrial NPP compared to the decadal mean, primarily due to limited moisture supply in tropical regions. At a global level, precipitation explained approximately 63% of the variation in terrestrial NPP, while the rest was attributed to changes in temperature and other environmental factors. Precipitation was the major factor determining inter-annual variation in terrestrial NPP in low-latitude regions. However, in mid- and high-latitude regions, temperature variability largely controlled the magnitude of terrestrial NPP. Our results imply that projected climate warming and increasing climate extreme events would alter the magnitude and spatiotemporal patterns of global terrestrial NPP.
... Modelled estimates of C sequestration by forests in the Great Smoky Mountains National Park (southeast-central USA) attribute a 50% loss to ambient O 3 between 1971 and 2001 (Zhang et al. 2007). Between 2001 and 2003, a maximum of 31% loss in productivity of aspen in parts of its North American range was estimated to be caused by O 3 (Percy et al. 2007). ...
Growing awareness of air pollution effects on forests has, from the early 1980s on, led to intensive forest damage research and monitoring. This has fostered air pollution control, especially in Europe and North America, and to a smaller extent also in other parts of the world. At several forest sites in these regions, there are first indications of a recovery of forest soil and tree conditions that may be attributed to improved air quality. This caused a decrease in the attention paid by politicians and the public to air pollution effects on forests. But air pollution continues to affect the structure and functioning of forest ecosystems not only in Europe and North America but even more so in parts of Russia, Asia, Latin America, and Africa. At the political level, however, attention to climate change is focussed on questions of CO2 emission and carbon sequestration. But ecological interactions between air pollution including CO2 and O3 concentrations, extreme temperatures, drought, fire, insects, pathogens, and fire, as well as the impact of ecosystem management practices, are still poorly understood. Future research should focus on the interacting impacts on forest trees and ecosystems. The integrative effects of air pollution and climatic change, in particular elevated O3, altered nutrient, temperature, water availability, and elevated CO2, will be key issues for impact research. An important improvement in our understanding might be obtained by the combination of long-term multidisciplinary experiments with ecosystem-level monitoring, and the integration of the results with ecosystem modelling within a multiple-constraint framework.
... Aboveground inputs to belowground systems are comprised of simple, easilydecomposed substrates (labile C) as well as complex, structural molecules that are not easily degraded (recalcitrant C), and theoretical studies indicate that these substrates should have different temperature sensitivities (Sierra, 2012). The difference between labile and recalcitrant C pools has important C cycling implications, as many models partition soil C into two or three pools, with varying turnover times ranging from years to millennia (Paustian et al., 1997;Falloon et al., 1998;Tague and Band, 2004;Zhang et al., 2007). In natural systems there is a continuum of soil C recalcitrance, and the proportion of recalcitrant C increases over the course of decomposition during the process of humus formation (Berg, 2000). ...
Anthropogenic climate change is expected to increase global temperatures and potentially increase soil carbon (C) mineralization, which could lead to a positive feedback between global warming and soil respiration. However the magnitude and spatial variability of belowground responses to warming are not yet fully understood. Some of the variability may depend on the native temperature regimes of soils. Soils from low temperature climates may release more C than will soils from high temperature climates because soils in cold climates are often C-rich and may experience more warming. We investigated whether soils from low native temperatures respired more than did soils from high native temperatures. We collected intact soil cores from three elevational transects along a latitudinal gradient in the forests of southern Appalachian Mountains. Soil cores were incubated for 292 days at low, medium, and high temperatures (separated by 3 °C each) with diurnal temperature and light regimes that simulated realistic temperature changes likely to occur within the next century. The native temperature regimes of soils negatively influenced soil respiration, such that soils from cold climates respired more in response to experimental warming than did soils from warm climates. Conversely, soils from warm climates mineralized the largest proportion of available soil C and available soil nitrogen in response to warming. Across all soils, modest experimental warming increased soil respiration, the proportion of available soil C that was being respired (respiration/soil C), and the proportion of soil nitrogen that was mineralized (N min/soil N). Taken together, these data suggest that soils from low native temperatures have a greater potential to release C in response to climate warming because the C stocks are larger and respiration rates will be higher than those in soils from high native temperatures.
... To develop a baseline for the carbon and water pools, the AEM was initially run to an equilibrium state with the mean climate data from 1990–2004 and 2005–2007 from the FSDE1 and Kendell sites, respectively. Using long-term detrended climate data from the identical periods, a 90-year spin-up simulation was conducted for each site to eliminate the noise caused by simulation mode shifts (Zhang et al., 2007). The data-detrending approach subtracted the best-fit line from the transient climate dataset and only retained the fluctuations regarding the trend. ...
... January 2012 55 NITROGEN DEPOSITION AND CARBON UPTAKE leaching, N 2 O, NO emission, NH 3 volatilization). Since detailed information on DLEM is accessible in other published papers (Ren et al. 2007, 2011a, b, Zhang et al. 2007, Liu et al. 2008, Tian et al. 2010a, c, d, 2011a, b, c, Xu et al. 2010), we primarily introduce the model strategy related to N budget and N control on C cycling in this study (details can be found in the Appendix). ...
During the past decades, nitrogen deposition in China has been substantially increased, with mean values higher than those in the United States and Europe. The elevated nitrogen input, as important part of global changes, has greatly altered ecosystem structure and functioning, and thus exerted significant impacts on carbon cycling. In this study, three process-based ecosystem models (BIOME-BGC, DLEM and TEM) are used to explore the effects of increased nitrogen deposition and its combination with other environmental factors on C dynamic of China's terrestrial ecosystems. To illustrate the interactive effects of N deposition with ecosystem components, our simulations also consider multiple changes in climate, CO2, O3 concentration and land use & land cover (LULC) patterns. The model results indicate that raised nitrogen deposition level has enhanced net primary productivity and carbon storage in China during the year 1961 to 2005. Over the past 45 years, nitrogen amendment in China plays a vital role to accumulate carbon into ecosystems, considerably offsetting C release through climatic extremes, LULC changes and increased O3 concentration. It is proved that N deposition and its interaction with other driving forces accelerate carbon sequestration as the greatest contributor, although there apparently exist large uncertainties due to model assumptions and reliability of input dataset. Model inter-comparisons also reveal that N amendment imposes greater impacts on vegetation carbon pool than that on soil carbon pool. Among the models, the net effect of elevated nitrogen deposition on annual NPP ranges from 0.1 to 0.5 Pg C/yr in China from the year 1961 to 2005, while total carbon storage of terrestrial ecosystems has been enhanced by an average of about 5 Pg C in the same period. Key words: nitrogen deposition, carbon sequestration, China, process-based ecosystem models, model inter- comparison
... Whether constant or varying diurnally, seasonally, and developmentally (Massman and Grantz, 1995; Musselman and Massman, 1999; Massman et al., 2000; Panek and Goldstein, 2001; Danielsson et al., 2003; Grunhage et al., 2004; Massman, 2004; Musselman et al. 2006; Fares et al., 2010), defence capacity is a significant determinant of plant sensitivity to O 3 (S), and may be the principal determinant of injury (I) in cases such as genetic variation in O 3 sensitivity. This requires that [O 3 ] or F be assigned differential weights to reflect S. Weighting factors incorporated into the models MOSES- TRIFFID (Sitch et al., 2007) and DLEM (Zhang et al., 2007), while based on ozone flux, are time invariant and assigned according to plant functional type. The weighting factor, W126 (Lefohn and Runeckles, 1987; Lefohn et al., 1988), is cumulative and based on [O 3 ] , emphasizing higher concentrations without ignoring lower concentrations. ...
Plant injury by ozone (O3) occurs in three stages, O3 entrance through stomata, overcoming defences, and attack on bioreceptors. Concentration, deposition, and uptake of O3 are accessible by observation and modelling, while injury can be assessed visually or through remote sensing. However, the
relationship between O3 metrics and injury is confounded by variation in sensitivity to O3. Sensitivity weighting parameters have previously been assigned to different plant functional types and growth stages, or
by differentially weighting O3 concentrations, but diel and seasonal variability have not been addressed. Here a plant sensitivity parameter (S) is introduced,
relating injury to O3 dose (uptake) using three independent injury endpoints in the crop species, Pima cotton (Gossypium barbadense). The diel variability of S was determined by assessment at 2h intervals. Pulses of O3 (15min) were used to assess passive (constitutive) defence mechanisms and dose was used rather than concentration to avoid
genetic or environmental effects on stomatal regulation. A clear diel trend in S was apparent, with maximal sensitivity in
mid-afternoon, not closely related to gas exchange, whole leaf ascorbate, or total antioxidant capacity. This physiologically
based sensitivity parameter provides a novel weighting factor to improve modelled relationships between either flux or exposure
to O3, and O3 impacts. This represents a substantial improvement over concentration- or phenology-based weighting factors currently in
use. Future research will be required to characterize the variability and metabolic drivers of diel changes in S, and the
performance of this parameter in prediction of O3 injury.
... The DLEM model is a highly integrated process-based ecosystem model which couples biophysical characteristics, plant physiological processes, biogeochemical cycles, and vegetation dynamics and land use to make daily, spatially-explicit estimates of carbon, nitrogen and water fluxes and pool sizes in the terrestrial ecosystems at various scales from site to region and globe. It has been documented in previous studies through extensive applications to investigate the impacts of multiple environmental factors, including changes in climate, atmospheric composition (CO 2 , O 3 , reactive nitrogen), land use, and land management (harvest, rotation, fertilization, irrigation etc.), on the structure and functioning of terrestrial ecosystems over China, Monsoon Asia, the conterminous US, and North America (e.g., Ren et al., 2007;Zhang et al., 2007;Liu et al., 2008;Tian et al., 2010a,b;Xu et al., 2010;Zhang et al., 2010;Ren et al., 2011a,b;Tian et al., 2011a,b,c;Lu et al., 2012;Tian et al., 2012). ...
Tropical Asia has experienced dramatic cropland expansion and agricultural intensification to meet the increasing food demand and is likely to undergo further rapid development in the near future. Much concern has been raised about how cropland expansion and associated management practices (nitrogen fertilizer use, irrigation, etc.) have affected the terrestrial carbon cycle in this region. In this study, we used a process-based ecosystem model, the Dynamic Land Ecosystem Model (DLEM), to assess the magnitude, spatial and temporal patterns of terrestrial carbon fluxes and pools in Tropical Asia as resulted from cropland expansion and land management practices during 1901–2005. The results indicated that cropland expansion had resulted in a release of 19.12 ± 3.06 Pg C (0.18 ± 0.029 Pg C/yr) into the atmosphere in Tropical Asia over the study period. Of this amount, approximately 22% (4.18 ± 0.66 Pg C) was released from South Asia and 78% (14.94 ± 2.40 Pg C) from Southeast Asia. Larger land area was converted to cropland while less carbon was emitted from South Asia than from Southeast Asia, where forest biomass and soil carbon were significantly higher. Changes in vegetation, soil organic matter, and litter pools caused emissions of 15.58, 2.25, and 1.71 Pg C, respectively, from the entire region. Significant decreases in vegetation carbon occurred across most regions of Southeast Asia due to continuous cropland expansion and shrink of natural forests. When considering land management practices, however, less carbon was released into the atmosphere, especially in South Asia where land management practices contributed to an approximately 10% reduction in carbon emission. This implies that optimizing land management practices could greatly reduce the carbon emissions caused by cropland expansion and might be one of important climate mitigation options in Tropical Asia.
... Dynamic Land Ecosystem Model [7] The Dynamic Land Ecosystem Model (DLEM) is a highly integrated process‐based terrestrial ecosystem model that simulates daily carbon, water and nitrogen cycles as influenced by changes in atmospheric chemistry (CO 2 , ozone concentration and nitrogen deposition), climate, land cover and land use change and other disturbances (fire, hurricane, and harvest). The DLEM is well‐documented and has been extensively applied to study the terrestrial carbon, water and nitrogen cycles across the globe [e.g., Chen et al., 2006; Ren et al., 2007a, 2007b, 2011; Zhang et al., 2007; Liu et al., 2008; Tian et al., 2008, 2010a, 2010b]. [8] The DLEM includes five core components: (1) biophysics ; (2) plant physiology; (3) soil biogeochemistry; (4) dynamic vegetation; and (5) land use, disturbance and land management. ...
1] The magnitude, spatial, and temporal patterns of the terrestrial carbon sink and the underlying mechanisms remain uncertain and need to be investigated. China is important in determining the global carbon balance in terms of both carbon emission and carbon uptake. Of particular importance to climate‐change policy and carbon management is the ability to evaluate the relative contributions of multiple environmental factors to net carbon source and sink in China's terrestrial ecosystems. Here the effects of multiple environmental factors (climate, atmospheric CO 2 , ozone pollution, nitrogen deposition, nitrogen fertilizer application, and land cover/land use change) on net carbon balance in terrestrial ecosystems of China for the period 1961–2005 were modeled with newly developed, detailed historical information of these changes. For this period, results from two models indicated a mean land sink of 0.21 Pg C per year, with a multimodel range from 0.18 to 0.24 Pg C per year. The models' results are consistent with field observations and national inventory data and provide insights into the biogeochemical mechanisms responsible for the carbon sink in China's land ecosystems. In the simulations, nitrogen deposition and fertilizer applications together accounted for 61 percent of the net carbon storage in China's land ecosystems in recent decades, with atmospheric CO 2 increases and land use also functioning to stimulate carbon storage. The size of the modeled carbon sink over the period 1961–2005 was reduced by both ozone pollution and climate change. The modeled carbon sink in response to per unit nitrogen deposition shows a leveling off or a decline in some areas in recent years, although the nitrogen input levels have continued to increase.
... The DLEM operates at a daily time step and at a variety of spatial scales ranging from meters to kilometers, from regional to global. The detailed information for DLEM could be referred to our previous publications Liu et al., 2008;Ren et al., 2007;Zhang et al., 2007a;Ren, 2009;Zhang, 2008;Lu, 2009;Tian et al., 2010b;Xu, 2010), and the CH 4 module has been described in detail in Tian et al. (2010a). ...
The attribution of spatial and temporal variations in terrestrial methane (CH<sub>4</sub>) flux is essential for assessing and mitigating CH<sub>4</sub> emission from terrestrial ecosystems. In this study, we used a process-based model, the Dynamic Land Ecosystem Model (DLEM), in conjunction with spatial data of six major environmental factors to attribute the spatial and temporal variations in the terrestrial methane (CH<sub>4</sub>) flux over North America from 1979 to 2008 to six individual factors and their interaction. Over the past three decades, our simulation indicates that global change factors accumulatively contributed 43.05 Tg CH<sub>4</sub>-C (1 Tg = 10<sup>12</sup> g) emission over North America, among which ozone (O<sub>3</sub>) pollution led to a reduced CH<sub>4</sub> emission by 2.69 Tg CH<sub>4</sub>-C, all other factors including climate variability, nitrogen (N) deposition, rising atmospheric carbon dioxide (CO<sub>2</sub>), N fertilization, and land conversion increased terrestrial CH<sub>4</sub> emissions by 40.37 Tg CH<sub>4</sub>-C, 0.42 Tg CH<sub>4</sub>-C, 6.95 Tg CH<sub>4</sub>-C, 0.11 Tg CH<sub>4</sub>-C, and 3.70 Tg CH<sub>4</sub>-C, respectively, and interaction between/among these global change factors led to a decline of CH<sub>4</sub> emission by 5.80 Tg CH<sub>4</sub>-C. Climatic variability dominated the inter-annual variations in terrestrial CH<sub>4</sub> fluxes at both continental and country levels. The relative importance of each environmental factor in determining the magnitude of methane flux shows substantially spatial variation across North America. This factorial attribution of CH<sub>4</sub> fluxes over the North America might benefit policy makers who would like to curb climate warming by reducing CH<sub>4</sub> emission.
... The DLEM operates at a daily time step and at varied spatial resolutions, from meters to kilometers, from regional to global. The detailed information for DLEM has been described in our previous publications (Liu et al., 2008; Ren et al., , 2011a Zhang et al., 2007a; Tian et al., 2010a, b, 2011a, b; Xu et al., 2010; Lu et al., 2012 ), and the N 2 O module has been described in detail in Tian et al. (2010b). In the DLEM, the N 2 O module is incorporated into nitrogen cycling; it simulates the nitrification and denitrification processes. ...
Nitrous oxide (N 2 O) is a potent greenhouse gas which also contributes to the depletion of stratospheric ozone (O 3). However, the magnitude and underlying mechanisms for the spatiotemporal variations in the terrestrial sources of N 2 O are still far from certain. Using a process-based ecosystem model (DLEM – the Dynamic Land Ecosystem Model) driven by multiple global change factors, includ-ing climate variability, nitrogen (N) deposition, rising at-mospheric carbon dioxide (CO 2), tropospheric O 3 pollu-tion, N fertilizer application, and land conversion, this study examined the spatial and temporal variations in terrestrial N 2 O flux over North America and further attributed these variations to various driving factors. From 1979 to 2010, the North America cumulatively emitted 53.9 ± 0.9 Tg N 2 O-N (1 Tg = 10 12 g), of which global change factors con-tributed 2.4 ± 0.9 Tg N 2 O-N, and baseline emission con-tributed 51.5 ± 0.6 Tg N 2 O-N. Climate variability, N deposi-tion, O 3 pollution, N fertilizer application, and land conver-sion increased N 2 O emission while the elevated atmospheric CO 2 posed opposite effect at continental level; the interactive effect among multiple factors enhanced N 2 O emission over the past 32 yr. N input, including N fertilizer application in cropland and N deposition, and multi-factor interaction dom-inated the increases in N 2 O emission at continental level. At country level, N fertilizer application and multi-factor inter-action made large contribution to N 2 O emission increase in the United States of America (USA). The climate variabil-ity dominated the increase in N 2 O emission from Canada. N inputs and multiple factors interaction made large contribu-tion to the increases in N 2 O emission from Mexico. Cen-tral and southeastern parts of the North America – including central Canada, central USA, southeastern USA, and all of Mexico – experienced increases in N 2 O emission from 1979 to 2010. The fact that climate variability and multi-factor in-teraction largely controlled the inter-annual variations in ter-restrial N 2 O emission at both continental and country levels indicate that projected changes in the global climate system may substantially alter the regime of N 2 O emission from ter-restrial ecosystems during the 21st century. Our study also showed that the interactive effect among global change fac-tors may significantly affect N 2 O flux, and more field exper-iments involving multiple factors are urgently needed.
... January 2012 55 NITROGEN DEPOSITION AND CARBON UPTAKE leaching, N 2 O, NO emission, NH 3 volatilization). Since detailed information on DLEM is accessible in other published papers (Ren et al. 2007, 2011a, b, Zhang et al. 2007, Liu et al. 2008, Tian et al. 2010a, c, d, 2011a, b, c, Xu et al. 2010), we primarily introduce the model strategy related to N budget and N control on C cycling in this study (details can be found in the Appendix). ...
The amount of atmospheric nitrogen (N) deposited on the land surface has increased globally and by nearly five times in China from 1901 to 2005. Little is known about how elevated reactive N input has affected the carbon (C) sequestration capability of China's terrestrial ecosystems, largely due to the lack of reliable data on N deposition. Here we have used a newly developed data set of historical N deposition at a spatial resolution of 10 km x 10 km in combination with other gridded historical information on climate, atmospheric composition, land use, and land management practices to drive a process-based ecosystem model, the dynamic land ecosystem model (DLEM) for examining how increasing N deposition and its interactions with other environmental changes have affected C fluxes and storage in China's terrestrial ecosystems during 1901-2005. Our model simulations indicate that increased N deposition has resulted in a net C sink of 62 Tg C/yr (1 Tg = 1012 g) in China's terrestrial ecosystems, totaling up to 6.51 Pg C (1 Pg = 10(15) g) in the past 105 years. During the study period, the N-induced C sequestration can compensate for more than 25% of fossil-fuel CO2 emission from China. The largest C sink was found in southeast China, a region that experienced the most significant increase of N deposition in the period 1901-2005. However, the net primary productivity induced by per-unit N deposition (referred to as ecosystem N use efficiency, ENUE, in this paper) has leveled off or declined since the 1980s. This indicates that part of the deposited N may not be invested to stimulate plant growth, but instead leave the ecosystem by various pathways. Except shrubland and northwest/southwest China, signs of N saturation are apparent in the rest major biome types and regions, with ENUE peaking in the 1980s and leveling off or declining thereafter. Therefore, to minimize the excessive N pollution while keeping the N-stimulated C uptake in China's terrestrial ecosystems, optimized management practices should be taken to increase N use efficiency rather than to keep raising N input level in the near future.
... To address the complexity of the urban ecosystem, we have to rely on spatiallyexplicit, process-based ecosystem models driven by multiple environmental factors. The Dynamic Land Ecosystem Model (DLEM) is one of few spatial-explicit process-based models that can simultaneously simulate complex interactions among multiple biogeochemical processes (i.e., C, water, and nitrogen cycles) and multiple environmental controls (e.g., changes in land-use, climate, and atmosphere) (Chen et al., 2006a;Zhang et al., 2007Zhang et al., , 2010Ren et al., 2011a, b;Tian et al., 2008Tian et al., , 2010a2011a, b). In this study, we used DLEM to simulate the long-term (1945e2007) effects of urbanization on C dynamics in the SUS where about one-third of the developed area has been added in the last 15 years of the 20th century, making this region the most rapid urbanization rate in the USA (Alig et al., 2004). ...
Using a process-based Dynamic Land Ecosystem Model, we assessed carbon dynamics of urbanized/developed lands in the Southern United States during 1945-2007. The results indicated that approximately 1.72 (1.69-1.77) Pg (1P = 10(15)) carbon was stored in urban/developed lands, comparable to the storage of shrubland or cropland in the region. Urbanization resulted in a release of 0.21 Pg carbon to the atmosphere during 1945-2007. Pre-urbanization vegetation type and time since land conversion were two primary factors determining the extent of urbanization impacts on carbon dynamics. After a rapid decline of carbon storage during land conversion, an urban ecosystem gradually accumulates carbon and may compensate for the initial carbon loss in 70-100 years. The carbon sequestration rate of urban ecosystem diminishes with time, nearly disappearing in two centuries after land conversion. This study implied that it is important to take urbanization effect into account for assessing regional carbon balance.
Ozone (O3), oxides of nitrogen (NOx), and carbon monoxide (CO) were monitored at CSIR National Physical Laboratory, New Delhi, during May and October, 2014. One-hour O3 concentrations exceeding more than 120 ppb were observed for 9 days in May and 16 days in October months with concurrent high NOx and CO values. The higher ozone episodes in October 2014 as compared to May 2014 may be due to contribution of biomass burning. The nitrogen oxide (NO), nitrogen dioxide (NO2), O3, and the photolysis rate of NO2 (i.e., JNO2) values of May and October, 2014, were used to evaluate the photo-stationary state ratio (or Leighton ratio, ϕ) for clear sky conditions. Leighton ratio ranged from 0.4 to 2.6 (average values 0.89 ± 0.4) for May 2014 and from 0.3 to 3.6 (average value 1.01 ± 0.52) for October 2014. The observed NOy values in October 2014 indicated the production of O3 through hydrocarbon-sensitive O3 chemistry. The sum of reactive nitrogen species’ (i.e., NOy) mixing ratios recorded during October 2014 varied from 259.4 to 10.4 ppb with an average value of 80.5 ± 39 ppb. The averaged diurnal ratio of NOx–NOy of > 0.3 indicated the presence of freshly emitted pollutants.
Twenty-five percent of the world’s forests are in the temperate biome. They include a wide range of forest types, and the exact boundaries with boreal forests to the north and tropical forests to the south are not always clear. There is a great variety of species, soil types, and environmental conditions which lead to a diversity of factors affecting carbon storage and flux. Temperate forests have been severely impacted by human use – throughout history, all but about 1% have been logged-over, converted to agriculture, intensively managed, grazed, or fragmented by sprawling development. Nevertheless, they have proven to be resilient – mostly second growth forests now cover about 40–50% of the original extent of the biome. Although remaining intact temperate forests continue to be fragmented by development, particularly in North America, there is no large-scale deforestation at present, nor is there likely to be in the future. The status of the temperate biome as a carbon reservoir and atmospheric CO2 sink rests mainly on strong productivity and resilience in the face of disturbance. The small “sink” status of temperate forests could change to a “source” status if the balance between photosynthesis and respiration shifts.
Plant sensitivity to ozone (O3) is critical to modeling impacts of air pollution on vegetation. A diel timecourse of sensitivity (S) was recently determined in Pima cotton (Grantz et al., 2013). The sensitivity parameter serves as a weighting factor for stomatal uptake (ozone flux, F), or cumulative F (dose, D). Previous approaches used various weighting schemes to modify ozone concentration ([O3]) or cumulative [O3] (exposure, E). Use of the S parameter allows calculation of effective flux (Feff) and effective dose (Deff). Though theoretically sound, the practical significance of S has not been evaluated due to the previous lack of available data. Here, the newly available S parameter is used to explore the relationships between exposure- and flux-based O3 metrics in response to scenarios of contrasting stomatal conductance (gs) and ambient [O3].
Greenhouse gas (GHG)-induced climate change is among the most pressing sustainability challenges facing humanity today, posing serious risks for ecosystem health. Methane (CH4) and nitrous oxide (N2O) are the two most important GHGs after carbon dioxide (CO2), but their regional and global budgets are not well known. In this study, we applied a process-based, coupled, biogeochemical model to concurrently estimate the magnitude and spatial and temporal patterns of CH4 and N2O fluxes as driven by multiple environmental changes, including climate variability, rising atmospheric CO2, increasing nitrogen deposition, tropospheric ozone pollution, land use change, and nitrogen fertilizer use. The estimated CH4 and N2O emissions from global land ecosystems during 1981–2010 were 144.39 6 32.92 Tg C/yr and 12.52 6 1.89 Tg N/yr, respectively. Our simulations indicated a significant (P , 0.01) increasing trend for CH4 (0.43 6 0.06 Tg C�yr�1�yr�1 [mean 6 2SE]) and N2O (0.14 6 0.02 Tg N�yr�1�yr�1) in the study period. CH4 and N2O emissions increased significantly in most climatic zones and continents, especially in the tropical regions and Asia. The most rapid increase in CH4 emission was found in natural wetlands and rice fields due to increased rice cultivation area and climate warming. N2O emission increased substantially in all the biome types and the largest increase occurred in upland crops due to increasing air temperature and nitrogen fertilizer use. Clearly, the three major GHGs (CH4, N2O, and CO2) should be simultaneously considered when evaluating if a policy is effective to mitigate climate change.
Methane (CH4) and nitrous oxide (N2O) are two most important greenhouse gases after carbon dioxide, but their regional and global budgets are far from certain, which is largely owing to uncertainties in scaling up field measurements as well as the poor model representation of processes and factors governing CH4 and N2O exchange between the terrestrial biosphere and atmosphere. In this study, we applied a process-based, coupled biogeochemical model (DLEM – the Dynamic Land Ecosystem Model) to estimate the magnitudes, spatial and temporal patterns of CH4 and N2O fluxes as driven by multiple environmental changes including climate variability, rising atmospheric CO2, increasing nitrogen deposition, tropospheric ozone pollution, land use change and nitrogen fertilizer use. The estimated CH4 and N2O emissions from global land ecosystems were 169.43 ± 32.92 Tg C yr−1 and 12.52 ± 1.52 Tg N yr−1, respectively. Our simulations have indicated a significant (P
The Gulf of Mexico (GOM) is facing large pressures from environmental
changes since the beginning of the last century. However, the magnitude
and long-term trend of total water discharge to the GOM and the
underlying processes are not well understood. In this study, the dynamic
land ecosystem model (DLEM) has been improved and applied to investigate
spatial and temporal variations of evapotranspiration (ET) and runoff
(R) over drainage basins of the GOM during 1901-2008. Modeled ET and
discharge were evaluated against upscaled data sets and gauge
observations. Simulated results demonstrated a significant decrease in
ET at a rate of 15 mm yr-1 century-1 and an
insignificant trend in runoff/precipitation (R/P) and river discharge
over the whole region during 1901-2008. However, the trends in estimated
water fluxes show substantial spatial and temporal heterogeneities
across the study region. Generally, in the west arid area, ET, R, and
R/P decreased; while they increased in the eastern part of the study
area during the last 108 years. In the recent 30 years, this region
experienced a substantial decrease in R. Factorial simulation
experiments indicate that climate change, particularly P, was the
dominant factor controlling interannual variations of ET and R; while
land use change had the same magnitude of effects on long-term trends in
water fluxes as climate change did. To eliminate modeling uncertainties,
high-resolution historical meteorological data sets and model
parameterizations on anthropogenic effects, such as water use and dam
constructions, should be developed.
How terrestrial ecosystems respond to future environmental change in the 21st century is critically important for understanding the feedbacks of terrestrial ecosystems to global climate change. The southeastern United States (SEUS) has been one of the major regions acting as a carbon sink over the past century; yet it is unclear how its terrestrial ecosystems will respond to global environmental change in the 21st century. Applying a process-based ecosystem model (Dynamic Land Ecosystem Model, DLEM) in combination with three projected climate change scenarios (A1B, A2, and B1 from the IPCC report) and changes in atmospheric carbon dioxide, nitrogen deposition, and ozone pollution, we examined the potential changes of carbon storage and fluxes in the terrestrial ecosystems across the SEUS during 2000-2099. Simulation results indicate that SEUS's terrestrial ecosystems will likely continue to sequester carbon in the 21st century, resulting in an increase in total carbon density (i.e., litter, vegetation biomass and soil carbon) from 13.5 kg C/m2 in the 2000s to 16.8 kg C/m2 in the 2090s. The terrestrial gross primary production and net primary production will probably continuously increase, while the net carbon exchange (positive indicates sink and negative indicates source) will slightly decrease. The carbon sequestration is primarily attributed to elevated atmospheric carbon dioxide and nitrogen deposition. Forests, including both deciduous and evergreen, show the largest increase in carbon storage as compared with other biomes, while cropland carbon storage shows a small decrease. The sequestered carbon will be primarily stored in vegetation for deciduous forest and in soil for evergreen forest. The central and eastern SEUS will sequester more carbon, while the western portion of the SEUS will release carbon to the atmosphere. The combined effects of climate and atmospheric changes on carbon fluxes and storage vary among climate models and climate scenarios. The largest increase in carbon storage would occur under the A1B climate scenario simulated by the NCAR climate model. Generally, the A1B scenario would result in more carbon sequestration than A2 and B1 scenarios; and the projected climate condition by the NCAR model would result in more carbon sequestration than other climate models.
The Dynamic Land Ecosystem Model (DLEM) was developed to meet critical needs for understanding and predicting the large-scale patterns and processes of terrestrial ecosystems and continental margins, and complex interactions among climate, ecosystem and human in the context of multifactor global change. The DLEM couples major biophysical, biogeochemical, vegetation dynamical and land use processes, and works at multiple scales in time step ranging from daily to yearly and spatial resolution from meters to kilometers, from region to globe. The DLEM is characterized by the following features: 1) multiple factors driven; 2) fully-coupled cycles of carbon, nitrogen and water; 3) concurrently simulation of major greenhouse gases (CO 2, CH 4, N 2O, & H 2O); 4) dynamically tracking changes in land cover/use and vegetation distribution. The model has been validated against site-specific measurements across the globe and applied at various scales. In this paper, we have briefly addressed model structure, parameters, key processes and major input/output variables. As a case study, we presented the simulated global fluxes of net primary productivity, evapotranspiration and methane during 1948-2005 and their spatial patterns in the year 2000. We also identified major gaps in terrestrial ecosystem modeling and field observations, and further discussed some critical future research needs.
How terrestrial ecosystems respond to future environmental change in the 21st century is critically important for understanding the feedbacks of terrestrial ecosystems to global climate change. The southeastern United States (SEUS) has been one of the major regions acting as a carbon sink over the past century; yet it is unclear how its terrestrial ecosystems will respond to global environmental change in the 21st century. Applying a process-based ecosystem model (Dynamic Land Ecosystem Model, DLEM) in combination with three projected climate change scenarios (A1B, A2, and B1 from the IPCC report) and changes in atmospheric carbon dioxide, nitrogen deposition, and ozone pollution, we examined the potential changes of carbon storage and fluxes in the terrestrial ecosystems across the SEUS during 2000–2099. Simulation results indicate that SEUS's terrestrial ecosystems will likely continue to sequester carbon in the 21st century, resulting in an increase in total carbon density (i.e., litter, vegetation biomass and soil carbon) from 13.5 kg C/m2 in the 2000s to 16.8 kg C/m2 in the 2090s. The terrestrial gross primary production and net primary production will probably continuously increase, while the net carbon exchange (positive indicates sink and negative indicates source) will slightly decrease. The carbon sequestration is primarily attributed to elevated atmospheric carbon dioxide and nitrogen deposition. Forests, including both deciduous and evergreen, show the largest increase in carbon storage as compared with other biomes, while cropland carbon storage shows a small decrease. The sequestered carbon will be primarily stored in vegetation for deciduous forest and in soil for evergreen forest. The central and eastern SEUS will sequester more carbon, while the western portion of the SEUS will release carbon to the atmosphere. The combined effects of climate and atmospheric changes on carbon fluxes and storage vary among climate models and climate scenarios. The largest increase in carbon storage would occur under the A1B climate scenario simulated by the NCAR climate model. Generally, the A1B scenario would result in more carbon sequestration than A2 and B1 scenarios; and the projected climate condition by the NCAR model would result in more carbon sequestration than other climate models.
Continental-scale estimations of terrestrial methane (CH4) and nitrous oxide (N2O) fluxes over a long time period are crucial to accurately assess the global balance of greenhouse gases and enhance our understanding and prediction of global climate change and terrestrial ecosystem feedbacks. Using a process-based global biogeochemical model, the Dynamic Land Ecosystem Model (DLEM), we quantified simultaneously CH4 and N2O fluxes in North America's terrestrial ecosystems from 1979 to 2008. During the past 30 years, approximately 14.69 ± 1.64 T g C a-1 (1 T g = 1012 g) of CH4, and 1.94 ± 0.1 T g N a-1 of N2O were released from terrestrial ecosystems in North America. At the country level, both the US and Canada acted as CH4 sources to the atmosphere, but Mexico mainly oxidized and consumed CH4 from the atmosphere. Wetlands in North America contributed predominantly to the regional CH4 source, while all other ecosystems acted as sinks for atmospheric CH4, of which forests accounted for 36.8%. Regarding N2O emission in North America, the US, Canada, and Mexico contributed 56.19%, 18.23%, and 25.58%, respectively, to the continental source over the past 30 years. Forests and croplands were the two ecosystems that contributed most to continental N2O emission. The inter-annual variations of CH4 and N2O fluxes in North America were mainly attributed to year-to-year climatic variability. While only annual precipitation was found to have a significant effect on annual CH4 flux, both mean annual temperature and annual precipitation were significantly correlated to annual N2O flux. The regional estimates and spatiotemporal patterns of terrestrial ecosystem CH4 and N2O fluxes in North America generated in this study provide useful information for global change research and policy making.
A process-based ecosystem model, the Dynamic Land Ecosystem Model (DLEM), was applied to evaluate the effects of cropland expansion on terrestrial carbon fluxes and pools in South and Southeast Asia in the 20th century. The results indicated that cropland expansion in both regions has resulted in a release of 18.26 Pg C into the 5 atmosphere in the study period. Of this amount, approximately 23 % (4.19 Pg C) was released from South Asia and 77 % (14.07 Pg C) from Southeast Asia. More land area was converted to cropland but less carbon was emitted in South Asia than in Southeast Asia, where forest biomass and soil carbon are significantly higher. Carbon losses in vegetation, soil organic matter, and litter carbon pools accounted for 15.09, 2.01, and 10 1.60 Pg C, respectively. Significant decreases in vegetation carbon occurred across most regions of Southeast Asia due to continuous cropland expansion and depletion of natural forests. Our study also indicated that it is important to take into account the land use legacy effect when evaluating the contemporary carbon balance in terrestrial ecosystems.
Terrestrial ecosystems in the southern United States (SUS) have experienced a complex set of changes in climate, atmospheric CO2 concentration, tropospheric ozone (O3), nitrogen (N) deposition, and land-use and land-cover change (LULCC) during the past century. Although each of these factors has received attention for its alterations on ecosystem carbon (C) dynamics, their combined effects and relative contributions are still not well understood. By using the Dynamic Land Ecosystem Model (DLEM) in combination with spatially explicit, long-term historical data series on multiple environmental factors, we examined the century-scale responses of ecosystem C storage and flux to multiple environmental changes in the SUS. The results indicated that multiple environmental changes shifted SUS ecosystems from a C source of 1.20 ± 0.56 Pg (1 Pg = 1015 g) during the period 1895 to 1950, to a C sink of 2.00 ± 0.94 Pg during the period 1951 to 2007. Over the entire period spanning 1895–2007, SUS ecosystems were a net C sink of 0.80 ± 0.38 Pg. The C sink was primarily due to an increase in the vegetation C pool, whereas the soil C pool decreased during the study period. The spatiotemporal changes of C storage were caused by changes in multiple environmental factors. Among the five factors examined (climate, LULCC, N deposition, atmospheric CO2, and tropospheric O3), elevated atmospheric CO2 concentration was the largest contributor to C sequestration, followed by N deposition. LULCC, climate, and tropospheric O3 concentration contributed to C losses during the study period. The SUS ecosystem C sink was largely the result of interactive effects among multiple environmental factors, particularly atmospheric N input and atmospheric CO2.
The Earth’s terrestrial ecosystems have experienced a complex set of global changes, occurring on large spatial-temporal scales
and interactively affecting individual organisms and ecological systems, most of which are not amenable to direct experimentation.
To understand, predict, and assess the large-scale and long-term impacts of global changes on the Earth’s terrestrial ecosystems,
we need such a new approach for extrapolating the growth of plants, animals, or ecosystems into the future when climate, CO2, and other factors may be different, and extrapolating individual plant or site studies onto a regional or global scale.
In this chapter, we present such a newly developed approach called the Regional Integration System for Earth’s ecosystem (RISE),
which builds upon improved knowledge of the fundamental mechanisms of ecological systems, and supported by rapidly developing
technology from high-speed computer systems to high-resolution remote sensing sources with global coverage. Then we apply
the RISE to address our common understanding of perhaps the most important issue facing humankind in the twenty-first century,
our disruption of the global carbon cycle. We use two case studies to illustrate the overall merits and applications of the
RISE in terrestrial ecosystem research. In the first case study, the RISE has been used to predict and assess the impacts
of global change on net primary productivity and ecosystem carbon storage in southeastern U.S. under current climatic conditions
and future climate scenarios. In the second case study, we have used the RISE to assess changes in ecosystem carbon storage
and fluxes induced by multiple environmental stresses including climate variability/change, land-use and land-cover change,
elevated carbon dioxide, and air pollution in China.
The attribution of spatial and temporal variations in terrestrial methane (CH4) flux is essential for assessing and mitigating CH4 emission from terrestrial ecosystems. In this study, we used a process-based model, the Dynamic Land Ecosystem Model (DLEM), in conjunction with spatial data of six major environmental factors to attribute the spatial and temporal variations in the terrestrial methane (CH4) flux over North America from 1979 to 2008 to six individual factors and their interaction. Over the past three decades, our simulation indicates that global change factors accumulatively contributed 43.05 Tg CH4-C (1 Tg = 1012 g) emission over North America, among which ozone (O3) pollution led to a reduced CH4 emission by 2.69 Tg CH4-C, all other factors including climate variability, nitrogen (N) deposition, rising atmospheric carbon dioxide (CO2), N fertilization, and land conversion increased terrestrial CH4 emissions by 40.37 Tg CH4-C, 0.42 Tg CH4-C, 6.95 Tg CH4-C, 0.11 Tg CH4-C, and 3.70 Tg CH4-C, respectively, and interaction between/among these global change factors led to a decline of CH4 emission by 5.80 Tg CH4-C. Climatic variability dominated the inter-annual variations in terrestrial CH4 fluxes at both continental and country levels. The relative importance of each environmental factor in determining the magnitude of methane flux shows substantially spatial variation across North America. This factorial attribution of CH4 fluxes over the North America might benefit policy makers who would like to curb climate warming by reducing CH4 emission.
Hourly ozone (O3) data from one rural and four urban sites throughout West Virginia were analyzed for a three-year period (1987-1989) focusing on seasonal and diurnal patterns of O3 concentrations. Based on maximum hourly O3 concentrations (highest 1-hr maximum value per month), there were definite seasonal patterns with highest values from May to August and lowest values from December to February for all years and sites. High O3 exposures (defined in this study as concentrations ≥0.080 ppm) for the study period were greater in Greenbrier County (a distinctly rural region) and Wood County (an industrialized area). Of these two sites, O3 concentrations remained high with little diurnal variation in June and July (1988) in the rural area; in contrast, there was large diurnal fluctuation in the urban area. Considering the high O3 concentrations found at our rural site, the seasonal coincidence of high O3 with the growing season, and a low diurnal fluctuation of high summer O3 concentrations at the rural high-elevation site, tropospheric O3 pollution could represent a distinct threat to the high-elevation hardwood forests of West Virginia. Ozone concentrations presented in this study are well within the range of values which were found by studies in the literature to be damaging to physiological processes and growth parameters in trees.
Little is known about the characteristics of undisturbed soils in the eastern USA, since few exist. Eight sites with virgin forest soils formed under southern Appalachian hardwood vegetation were stud-ied in the Joyce Kilmer Memorial Forest, an unlogged watershed in western North Carolina. Sites ranged in elevation from 720 to 1200 m with only two on <50% slopes. Generally, soils were quite deep and highly weathered because of high rainfall (>200 cm), weath-erable feldspathic parent material, and no accelerated erosion. Av-erage solum depth was 90 cm, while depth to metasandstone bedrock was typically >1.3 m. Deeply weathered saprolites were commonly encountered. Soils on northerly aspects had thick umbric epipedons and more organic matter than soils on south-facing slopes. Organic matter contents of Al horizons ranged from 45 to 170 g/kg and surface horizons contained moderate coarse and medium crumb structure. Most soils had cambic horizons and clay contents that decreased with depth. Argillic horizons were present only at low elevations on south-facing slopes. While the majority of soils were formed in colluvium, significant amounts of deep soils in residuum occurred on sideslopes and appeared stable. The present-day land-forms appear to be significantly influenced by periglacial activity. Windthrow appears to have mixed the surfaces of these soils. Due to their decreasing clay content with depth, oxidic mineralogy, and low CEC, these soils resemble tropical forest soils.
As global population increases and industrialization expands, carbon dioxide (CO2) and toxic air pollutants can be expected to be injected into the atmosphere at increasing rates. This analysis reviews a wide range of direct plant responses to rising CO2, increasing levels of gaseous pollutants, and climate change, and to potential interactions among the factors. Although several environmental interactions on stomata and foliage temperatures are reviewed briefly, a comprehensive review of effects of potential climatic change on plants is not a major objective of this analysis. Research shows that elevated CO2 increases photosynthetic rates, leaf area, biomass, and yield. Elevated CO2 also reduces transpiration rate per unit leaf area, but not in proportion to reduction of stomatal conductance, because foliage temperature tends to rise. With increasing leaf area and foliage temperature, water use per unit land area is scarcely reduced by elevated CO2. Increases in photosynthetic water-use efficiency are caused primarily by increased photosynthesis rather than reduced transpiration. Gaseous pollutants (O3, SO2, NO(x), H2S) affect plants adversely primarily by entry through the stomata. An example calculation showed that reduction in stomatal conductance by doubled CO2 could potentially reduce the effects of ambient O3 and SO2 by 15%. However, information on the interaction of CO2 and air pollutants is scanty. More research is needed on these interactions, because regional changes in air pollutants are occurring concurrently with global changes in CO2.
Regional-scale forest ecosystem models can provide insight into how forest hydrology and productivity differ in response to variations in climate and soil conditions. However, the databases necessary to define and validate these models are difficult to compile and utilize. The use of a Geographic Information System (GIS) can greatly simplify problems in database management. As an example of regional-scal e ecosystem model database development and model validation, the Arc-Info GIS and the PnET-IIS forest ecosystem model were used to predict and validate annual drainage and net primary productivity (NPP) on a 0.5° x 0.5° grid (approximately 50 x 75 km) for southern pine forests in the state of Georgia, USA. PnET-IIS is a lump-sum physiological model which used historic climate data from 1951 to 1984, along with soil water-holding capacity and species-specific vegetation charac- teristics. Annual predictions of drainage were well correlated with measured United States Geological Survey (USGS) drainage (r = 0.87, p < 0.0001), and predicted NPP was related with Forest Inventory Assessment (FIA) growth data collected across the state (r = 0.84, p < 0.0001). This study demonstrated the utility of a GIS in broad-scale ecosystem modeling and suggests that the need for model/GIS inter- facing in future research will continue to increase.
Aboveground biomass and aboveground net primary production (ANPP) were determined for leaf, branch, and bole compartments of cove forests in the Great Smoky Mountains, Tennessee. The sample plots included young stands (42-63 years following agricultural abandonment) and old stands with no history of logging or catastrophic fire. Tree species, diameter at breast height (DBH), and 10-year radial growth increment data were collected on plots of 0.4–1.0 ha. Biomass was estimated with species-specific allometric equations for the Great Smoky Mountains and eastern Tennessee. ANPP was estimated using diameter growth measurements to determine biomass accumulation over the preceding 10-year interval. Biomass estimates for the predominantly deciduous old-growth stands ranged from 326 to 394 Mg•ha−1 on plots ≥ 0.4 ha. These were consistently greater than the corresponding estimates of 216–277 Mg•ha−1 for young stands. The old Tsuga-dominated stands had the highest biomass estimates of 415–471 Mg•ha−1 for 1.0-ha plots. Annual ANPP estimates were high (11.7–13.1 Mg•ha−1) among the young stands. These stands had particularly high bolewood production. ANPP of the old-growth plots ≥ 0.4 ha ranged from 6.3 to 8.6 Mg•ha−1•year−1 for the deciduous stands and 8.0–10.1 Mg•ha−1•year−1 for the coniferous–deciduous stands. Previous biomass estimates for primeval cove forests were well above temperate forest means of 300–350 Mg•ha−1. Our estimates based on larger plots were lower than previous estimates of 500–610 Mg•ha−1, but they still exceeded temperate forest means. Our deciduous values were 26–94 Mg•ha−1 above the temperate deciduous forest mean of 300 Mg•ha−1, and our Tsuga–deciduous values were 65–121 Mg•ha−1 above the temperate coniferous forest mean of 350 Mg•ha−1.
On a global basis, plants and soils may hold more than twice the amount of carbon present in the atmosphere [Geider et al., 2001]. Under increasing atmospheric carbon dioxide (CO2) concentrations and subsequently warming temperatures, these large biogenic pools may change in size [Cox et al., 2000]. Due to a lack of long-term field studies, there is uncertainty as to whether vegetation and soils will act as a net sink or a source of atmospheric CO2 in coming years. It is certain, however, that no retrospective analysis of the U.S. carbon balance will be possible without a comprehensive historical baseline of the sizes of various ecosystem carbon pools and the variability in their net annual increments. This article provides one of the first spatially detailed terrestrial carbon budgets for the regions of the continental United States in the 1980s and 1990s. At a resolution of less than 10 kilometers, this carbon accounting estimation includes major vegetation and surface soil pools and is based on remote sensing and vegetation-soil modeling for ecosystems.
Limited information is available on soil organic C (SOC) sequestered in frigid soils of the southern Appalachians, although the soils in frigid areas probably hold more SOC than warmer soils in the region. This study determined SOC in high-elevation (>1300 m) soils across northeast (N) and southwest (S) aspects and across three slope classes (7–15, 15–35, and 35–55%) per aspect in the Ridge and Valley of southwest Virginia. Overall, rock fragment content and bulk densities were lower, B-horizon SOC was higher, and sola were shallower than reported in similar regional studies. The A horizons were thicker on N versus S aspects, as were the sola (57 vs. 47 cm). Sola were 50% thicker on N35 to N55% sites than on S35 to S55% sites (65 vs. 44 cm). The S35 to S55% sites had the thickest litter but the thinnest A horizons, and were characterized by the highest C/N ratio and least incorporation of leaf litter. The study area average mass SOC was 112 Mg ha−1 More mass SOC was retained on S than N aspects in litter layers but less was retained in the A horizons and whole solum (99 vs. 127 Mg ha−1). There was less solum mass SOC on S7 to S15% and S35 to S55% sites than on N15 to N35% or N35 to N55% sites. The results reinforce the importance of using mass SOC rather than SOC concentration in regional SOC studies and demonstrate that steep northeast-facing slopes in frigid Appalachian landscapes have the highest mass SOC and highest potential for sequestering organic C in the soil
The Amazon Basin contains almost one-half of the world's undisturbed
tropical evergreen forest as well as large areas of tropical savanna,.
The forests account for about 10 per cent of the world's terrestrial
primary productivity and for a similar fraction of the carbon stored in
land ecosystems,, and short-term field measurements suggest that these
ecosystems are globally important carbon sinks. But tropical land
ecosystems have experienced substantial interannual climate variability
owing to frequent El Niño episodes in recent decades. Of
particular importance to climate change policy is how such climate
variations, coupled with increases in atmospheric CO2
concentration, affect terrestrial carbon storage. Previous model
analyses have demonstrated the importance of temperature in controlling
carbon storage,. Here we use a transient process-based biogeochemical
model of terrestrial ecosystems, to investigate interannual variations
of carbon storage in undisturbed Amazonian ecosystems in response to
climate variability and increasing atmospheric CO2
concentration during the period 1980 to 1994. In El Niño years,
which bring hot, dry weather to much of the Amazon region, the
ecosystems act as a source of carbon to the atmosphere (up to 0.2
petagrams of carbon in 1987 and 1992). In other years, these ecosystems
act as a carbon sink (up to 0.7Pg C in 1981 and 1993). These fluxes are
large; they compare to a 0.3Pg C per year source to the atmosphere
associated with deforestation inthe Amazon Basin in the early 1990s.
Soil moisture, which is affected by both precipitation and temperature,
and which affects both plant and soil processes, appears to be an
important control on carbon storage.
Tropospheric ozone can affect crop yield and has been reported to cause reductions in growth and biomass of forest tree species in laboratory and glasshouse studies. However, linkages between growth and ambient ozone concentrations in the field are not well established for forest trees. Ambient ozone concentrations have been shown to cause foliar injury on a number of tree species throughout much of the eastern USA. Symptom expression is influenced by endogenous and exogenous factors and, therefore, ozone-exposure/tree-response relationships have been difficult to confirm. Clearly defined, cause-effect relationships between visible injury and growth losses due to ozone have not been validated. Generalizations of sensitivity of forest trees to ozone are complicated by tree development stage, microclimate, leaf phenology, compensatory processes, within-species variation and other interacting stresses. In general, decreases in above-ground growth at ambient ozone levels in the eastern USA appear to be in the range of 0–10% per year. However, these conclusions are based on a small number of tree species, with the vast majority of studies involving individual tree seedlings in a non-competitive environment. Comparative studies of small and large trees indicate that seedlings are not suitable surrogates for predicting responses of mature trees to ozone. Process-level modelling is a promising methodology that has been recently utilized to assess ozone effects on a stand to regional scale, indicating that ozone is affecting forest growth in the eastern USA. The extent and magnitude of the response is variable and depends on many edaphic and climatic factors. It is imperative when conducting assessment exercises, however, that forest biologists constantly keep in mind the tremendous variability that exists within natural systems. Scaling of single site/physiological response phenomena from an individual tree to an ecosystem and/or region necessitates further research.
Ozone pollution in the lower atmosphere is known to have adverse effects on forest vegetation, but the degree to which mature forests are impacted has been very difficult to assess directly. In this study, we combined leaf-level ozone response data from independent ozone fumigation studies with a forest ecosystem model in order simulate the effects of ambient ozone on mature hardwood forests. Reductions in leaf carbon gain were determined as a linear function of ozone flux to the leaf interior, calculated as the product of ozone concentration and leaf stomatal conductance. This relationship was applied to individual canopy layers within the model in order to allow interaction with stand-and canopy-level factors such as light attenuation, leaf morphology, soil water limitations, and vertical ozone gradients. The resulting model was applied to 64 locations across the northeastern United States using ambient ozone data from 1987 to 1992. Predicted declines in annual net primary production ranged from 3 to 16% with greatest reductions in southern portions of the region where ozone levels were highest, and on soils with high water-holding capacity where drought stress was absent. Reductions in predicted wood growth were slightly greater (3-22%) because wood is a lower carbon allocation priority in the model than leaf and root growth. Interannual variation in predicted ozone effects was small due to concurrent fluctuations in ozone and climate, Periods of high ozone often coincided with hot, dry weather conditions, causing reduced stomatal conductance and ozone uptake. Within-canopy ozone concentration gradients had little effect on predicted growth reductions because concentrations remained high through upper canopy layers where net carbon assimilation and ozone uptake were greatest. Sensitivity analyses indicate a trade-off between model sensitivity to available soil water and foliar nitrogen and demonstrate uncertainties regarding several assumptions used in the model. Uncertainties surrounding ozone effects on stomatal function and plant water use efficiency were found to have important implications on current predictions, Field measurements of ozone effects on mature forests will be needed before the accuracy of model predictions can be fully assessed.
In 1985, when the Southern Commercial Forest Research Cooperative was initiated, regional, national, and international concerns about the vitality of forests were emerging. Beginning in the late 1970’s in Europe there had been reports of unhealthy forests, and by the early 1980s, 20 to 25% of these forests were classified as moderately or severely damaged from unknown causes (Schulze, 1989). Perhaps no European forests were so severely impacted as in Germany, where Waldsterben or “forest decline”became a focal point of international concern. In 1986, visible symptoms of damage were identified in over one half of the West German forests. These symptoms were primarily in older trees and consisted mostly of chlorotic leaves and premature leaf senescence (Krzak et al., 1988). As the intensity of forest surveys increased, damage appeared even more widespread and varied (Blank et al., 1988). Eventually in Germany and elsewhere in Europe, the amount of reported damage would stabilize (Schulze, 1989), but at the time of the initiation of the Southern Commercial Forest Research Cooperative, the health of European forests appeared to be declining rapidly. The European scientific community focused its attention on the possibility that air pollutants were the causal agents responsible for the damage, which by then was being called neuartige Waldschaden (i.e., new types of forest damage).
China has very complex geographical and geological environments, and is strongly influenced by monsoon climate, which results in substantial temporal and spatial climate variability. China's climate has experienced enormous changes in the past 40 years (1961-2000). We found air temperature and precipitation increased 0.21°C and 5.88 mm (3.8% increase) per decade, respectively in China from 1961 to 2000. During the same time period, global average temperature and precipitation only increased about 0.1°C and 0.05-1.0% per decade, respectively. To assess climate impacts on China's terrestrial ecosystems, we used the Dynamic Land Ecosystem Model (DLEM), a daily time-step and carbon, nitrogen, and water coupled biogeochemical model, to simulate effects of seasonal, interannual and spatial climate variability on net primary production (NPP) and net carbon exchange (NCE). Based on DLEM model simulation results, we found climate change has caused CO2 release into the atmosphere, while NPP decreased slightly from 1961 to 2000. In the last 10 years (1990s) climate change has caused about 50% of the total released CO2 in the 40-year period, while NPP only slightly decreased. From seasonal comparisons, we found summer was the major contributor to CO 2 release, while spring was the major contributor to CO2 sink. Mean annual NCE increased greatly with increased monsoon intensity and was higher in the years of El Niño.
Samples based on stand measurements and borings of trees, and clippings of undergrowth, were taken from 24 forests and forest heaths in the Great Smoky Mountains. Samples were taken also from heath balds and grassy balds and a California coastal redwood forest. Available information on ratios and regressions of forest tree and shrub dimensions were used to estimate stand biomass and net annual production above ground. Mature climax forests of mesic environments below 1,400 m are characterized by: wood basal areas of 50-64 m^2/ha and basal area increments of 0.3-0.6 m^2/ha/yr, stem wood volumes (parabolic estimate) of 750-900 m^3/ha and estimated wood volume increments of 530-590 cm^3/m^2/yr, aboveground biomasses of 500-610 t/ha and aboveground net annual productions of 1,000-1,200 g/m^2, and biomass accumulation ratios of 40-50. These and other stand dimensions decrease along the moisture gradient to xeric sites and decrease toward higher elevations. Above-ground net annual productions of forest heaths of xeric slopes and forests of highest elevations are 420-650 g/m^2. Evergreen spruce-fir forests are more productive than deciduous forests above 1,400 m. Among mesic high-elevation beech and fir forests, production is higher on south slopes than on north slopes. Production and biomass of steady-state forests were estimated from multiple correlations using elevation and weighted-average index of site moisture as independent variables. Apart from some high-elevation stands, production is not significantly correlated with evergreen vs. deciduous foliage or with direction of exposure affecting incident sunlight. When unstable stands are excluded, production and biomass are strongly correlated with each other and with mean tree height and other stand dimensions. A wide range of temperate-zone climax forests of relatively favorable environments have net annual productions above and below ground of 1,200-1,500 g/m^2. Productions of stable closed heath balds are lower, 700-1,200 g/m^2, but productions of heath balds and less productive forests overlap broadly. Production of unstable forests, both successional stands and those opened by removal of large trees, exceeds that of steady-state forests of similar environments. Canopy coverage and light penetration are not strongly correlated with forest production. Light penetration to the herb stratum ranges from 1.4-7.0% of incident sunlight in more open forests and forest heaths to 0.3-0.9% in cove forests. Foliage live/dry ratios decrease along the moisture gradient from mesic to xeric stands--from about 5.0 to 2.8 in shrub clippings, 7.6 to 2.8 in herb clippings. Undergrowth production and biomass are trivial compared with the tree stratum in many forests. Shrub production is generally higher in xeric environments and is 20-145% of tree production in forest heaths. Herb production is higher at the extremes of the moisture gradient (exceeding 3% of total aboveground production in mesic and in open xeric stands) than in intermediate stands (below 1%). Apart from such differences, her and thallophyte biomass and production increase with elevation to maximum values in fir forests of the highest summits.
Ozone pollution in the lower atmosphere is known to have adverse effects on forest vegetation, but the degree to which mature forests are impacted has been very difficult to assess directly. In this study, we combined leaf-level ozone response data from independent ozone fumigation studies with a forest ecosystem model in order simulate the effects of ambient ozone on mature hardwood forests. Reductions in leaf carbon gain were determined as a linear function of ozone flux to the leaf interior, calculated as the product of ozone concentration and leaf stomatal conductance. This relationship was applied to individual canopy layers within the model in order to allow interaction with stand- and canopy-level factors such as light attenuation, leaf morphology, soil water limitations, and vertical ozone gradients.
The resulting model was applied to 64 locations across the northeastern United States using ambient ozone data from 1987 to 1992. Predicted declines in annual net primary production ranged from 3 to 16% with greatest reductions in southern portions of the region where ozone levels were highest, and on soils with high water-holding capacity where drought stress was absent. Reductions in predicted wood growth were slightly greater (3–22%) because wood is a lower carbon allocation priority in the model than leaf and root growth. Interannual variation in predicted ozone effects was small due to concurrent fluctuations in ozone and climate. Periods of high ozone often coincided with hot, dry weather conditions, causing reduced stomatal conductance and ozone uptake. Within-canopy ozone concentration gradients had little effect on predicted growth reductions because concentrations remained high through upper canopy layers where net carbon assimilation and ozone uptake were greatest.
Sensitivity analyses indicate a trade-off between model sensitivity to available soil water and foliar nitrogen and demonstrate uncertainties regarding several assumptions used in the model. Uncertainties surrounding ozone effects on stomatal function and plant water use efficiency were found to have important implications on current predictions. Field measurements of ozone effects on mature forests will be needed before the accuracy of model predictions can be fully assessed.
The relationships between above- and belowground net primary production and soil nitrogen availability were studied at a temperate forest sites. Annual allocations of N and net primary production to leaf litter, perennial tissues (wood + bark), and aboveground biomass all increased significantly in relation to apparent N uptake by vegetation (Nu) as calculated using field measures of net N mineralization (Nm) and other major N fluxes to and from available N pools. Mean annual N content and biomass of fine roots (= or <3.0mm diameter) were both negatively correlated with Nu, but only approx 50% of Nm at each site could be accounted for by allocation to aboveground litter and perennial tissues. Assuming that mineralized N not accounted for by allocation of these components was taken up by vegetation and allocated to fine roots, annual N allocation to fine roots (Nfr) was a constant fraction of N uptake. Therefore, Nfr increased in absolute terms with both Nm and apparent N uptake. Fine-root N turnover rates (or Nfr/fine-root N content) also increased as Nm and Nu increased. Provided that fine-root biomass and N turnover rates were similar within individual sites, allocation of production to belowground biomass also increased relative to increases in soil N availability. The proportion of total net primary production allocated to belowground biomass did not decrease with increased N availability.-from Authors
Temperate forests are affected by a wide variety of environmental factors that stem from human industrial and agricultural activities. In the north‐eastern US, important change agents include tropospheric ozone, atmospheric nitrogen deposition, elevated CO 2 , and historical human land use. Although each of these has received attention for its effects on forest carbon dynamics, integrated analyses that examine their combined effects are rare. To examine the relative importance of all of these factors on current forest growth and carbon balances, we included them individually and in combination in a forest ecosystem model that was applied over the period of 1700–2000 under different scenarios of air pollution and land use history.
Results suggest that historical increases in CO 2 and N deposition have stimulated forest growth and carbon uptake, but to different degrees following agriculture and timber harvesting. These differences resulted from the effects of each land use scenario on soil C and N pools and on the resulting degree of growth limitations by carbon vs. nitrogen. Including tropospheric ozone in the simulations offset a substantial portion of the increases caused by CO 2 and N deposition. This result is particularly relevant given that ozone pollution is widespread across much of the world and because broad‐scale spatial patterns of ozone are coupled with patterns of nitrogen oxide emissions. This was demonstrated across the study region by a significant correlation between ozone exposure and rates of N deposition and suggests that the reduction of N‐induced carbon sinks by ozone may be a common phenomenon in other regions.
Collectively, the combined effects of all physical and chemical factors we addressed produced growth estimates that were surprisingly similar to estimates obtained in the absence of any form of disturbance. The implication of this result is that intact forests may show relatively little evidence of altered growth since preindustrial times despite substantial changes in their physical and chemical environment.
Observed responses of upland‐oak vegetation of the eastern deciduous hardwood forest to changing CO 2 , temperature, precipitation and tropospheric ozone (O 3 ) were derived from field studies and interpreted with a stand‐level model for an 11‐year range of environmental variation upon which scenarios of future environmental change were imposed. Scenarios for the year 2100 included elevated [CO 2 ] and [O 3 ] (+385 ppm and +20 ppb, respectively), warming (+4°C), and increased winter precipitation (+20% November–March). Simulations were run with and without adjustments for experimentally observed physiological and biomass adjustments.
Initial simplistic model runs for single‐factor changes in CO 2 and temperature predicted substantial increases (+191% or 508 g C m ⁻² yr ⁻¹ ) or decreases (−206% or −549 g C m ⁻² yr ⁻¹ ), respectively, in mean annual net ecosystem carbon exchange (NEE a ≈266±23 g C m ⁻² yr ⁻¹ from 1993 to 2003). Conversely, single‐factor changes in precipitation or O 3 had comparatively small effects on NEE a (0% and −35%, respectively). The combined influence of all four environmental changes yielded a 29% reduction in mean annual NEE a . These results suggested that future CO 2 ‐induced enhancements of gross photosynthesis would be largely offset by temperature‐induced increases in respiration, exacerbation of water deficits, and O 3 ‐induced reductions in photosynthesis. However, when experimentally observed physiological adjustments were included in the simulations (e.g. acclimation of leaf respiration to warming), the combined influence of the year 2100 scenario resulted in a 20% increase in NEE a not a decrease. Consistent with the annual model's predictions, simulations with a forest succession model run for gradually changing conditions from 2000 to 2100 indicated an 11% increase in stand wood biomass in the future compared with current conditions.
These model‐based analyses identify critical areas of uncertainty for multivariate predictions of future ecosystem response, and underscore the importance of long term field experiments for the evaluation of acclimation and growth under complex environmental scenarios.
Dose-response relationship were developed for ozone and four full-sib families of Pinus taeda L. Seedlings were planted in field plots in open-top chambers near Raleigh, North Carolina, and exposed daily during three growing seasons (1985, 1986, and 1987) to ozone at concentrations from 0.022 to 0.092 μL/L of air (seasonal mean concentrations for the daily exposure period 08:00 to 20:00 eastern standard time). Plants were harvested on five dates during the study to quantify effects of ozone on growth. Ozone suppressed stem height, root collar diameter, total branch length, and (or) dry weights of above-ground parts of plants in three families, but response to ozone depended upon dose, family, and the plant part measured. One family exposed during 1985 and 1986 did not exhibit significant growth responses to ozone. The family that exhibited the greatest growth suppression in 1985 and 1986 was exposed during the 3rd year, and ozone continued to reduce growth. Suppression of root dry weight was observed after three seasons of exposure. Root collar diameter and dry weight of stem + branches (without needles) may be the most useful measurements of growth response in multiple-year experiments. Dose-response models predicted that ambient levels of ozone could reduce growth relative to the growth predicted for chronic exposure to half-ambient levels (charcoal-filtered air). For aboveground woody tissue, this suppression ranged from 0 to 19% among the four families after two seasons of exposure and was 13% for the most sensitive family after three seasons.
The adverse effects of ozone and other air pollutants on crop yields are well documented. This paper reports on an assessment of the benefits to agriculture arising from reductions in ambient ozone pollution. Estimates are derived using recent plant science data as input for a spatial equilibrium model of U.S. agriculture. Sensitivity of benefit estimates to biological and economic sources of uncertainty is also investigated. Results suggest that the benefits of a 25 percent reduction in ambient ozone are substantial, amounting to $@@‐@@1.7 billion. The robustness of these estimates varies across alternative assumptions concerning response data and export markets.
The potential need for national-level comparisons of greenhouse gas emissions, and the desirability of understanding terrestrial sources and sinks of carbon, has prompted interest in quantifying national forest carbon budgets. In this study, we link a forest inventory database, a set of stand-level carbon budgets, and information on harvest levels in order to estimate the current pools and flux of carbon in forests of the conterminous United States. The forest inventory specifies the region, forest type, age class, productivity class, management intensity, and ownership of all timberland. The stand-level carbon budgets are based on growth and yield tables, in combination with additional information on carbon in soils, the forest floor, woody debris, and the understory. Total carbon in forests of the conterminous U.S. is estimated at 36.7 Pg, with half of that in the soil compartment. Tree carbon represents 33% of the total, followed by woody debris (10%), the forest floor (6%), and the understory (1%). The carbon uptake associated with net annual growth is 331 Tg, however, much of that is balanced by harvest-related mortality (266 Tg) and decomposition of woody debris. The forest land base at the national level is accumulating 79 Tg/yr, with the largest carbon gain in the Northeast region. The similarity in the magnitude of the biologically driven flux and the harvest-related flux indicates the importance of employing an age-class-based inventory, and of including effects associated with forest harvest and harvest residue, when modeling national carbon budgets in the temperate zone. 76 refs., 12 figs., 4 tabs.
A small watershed in the White Moutains of New Hampshire bearing mesophytic, cool-temperate, broadleaf-deciduous forests was studied. Acer saccharum, Betula lutea, and Fagus grandifolia are dominant, but toward higher elevations Picea rubens and Abies balsamea also occur and indicate the transition toward subalpine climate. The stands are young (following cutting in 1909-17) but contain older trees; stand composition is thought reasonably representative of the climax. For application of the Brookhaven system of forest dimension analysis, 93 sample trees, of major species were cut and roots excavated. Mean dimensions of sample trees, and the constants for the system of logarithmic regressions relating volume, surface, mass, and growth to diameter at breast height and other independent variables, show decrease in tree sizes and height/diameter ratios toward higher elevations. Stand characteristics, based on application of the regressions to forest samples, show trends of decrease for the elevation belts from low to high. Biomass (and, presumably, production) of root systems is 18 to 21% of that aboveground. Different estimations suggest that a mean climax biomass for the watershed may be around 350 t/ha, aboveground. Net ecosystem production is estimated as 350 g/m²/yr aboveground and 85 belowground for 1956-60, 238 and 52 g/m²/yr for 1961-65. Analysis of stem wood volume increments reveals an abrupt and striking (18%) decrease in volume growth and productivity from 1956-60 to 1961-65. Both drought and effects of increasing air pollution (notably increasing acidity of rainfall) may be responsible for the recent decrease in productivity.
Great Smoky Mountains National Park, located in the southern Appalachians on the border of Tennessee and North Carolina, was designated by Congress in 1977 as a Class I area, where resources are to be protected from damage due to air pollution. The National Park Service, Air Quality Division, collected and then used data on levels of pollutants and response of sensitive resources in the park and found that forests, soils, surface waters, and visibility in the park were experiencing adverse impacts from air pollution. We reviewed the data and process that supported this finding as well as the legal tools available to federal land managers confronted with evidence that sources of air pollution outside the boundaries of Class I areas are affecting resources. We prepared this case study to describe how the National Park Service has used scientific information in regulatory arenas to address air pollution problems at Great Smoky Mountains National Park. Based on this case study we show that the requirement for Federal Land Manager review of permits for new stationary sources of air pollution is not sufficient to protect the parks from what has become a regional air quality problem.
Terrestrial ecosystem models are built, among several reasons, to explore how the Earth's biosphere responds to climate change and to the projected continual increase of atmospheric CO2 concentration. Many of these models adopt the Farquhar et al. approach, in which leaf carbon assimilation of C3 plants is regulated by two limitations depending on the rate of Rubisco activity and ribulose-1, 5-bisphosphate regeneration (RuBP). This approach was expanded upon by others to include a third limitation that expresses the occurrence, in some plant species, of a photosynthetic downregulation under high concentrations of ambient CO2. Several ecosystem models, however, constrain leaf photosynthesis using only two limitations according to the original formulation of Farquhar et al. and thus neglect the limitation that represents the downregulation of photosynthesis under elevated atmospheric CO2. In this study, the authors first reviewed the effect of elevated CO2 on photosynthesis of C 3 plants, which illustrated that short-term observations are likely to considerably underestimate the number of plant species that exhibit a photosynthetic downregulation. Several recent long-term field observations have shown that such downregulation starts to be effective only after several seasons/years of plant exposure to elevated CO2. Second, an ecosystem model was used to illustrate that neglecting the photosynthetic downregulation may significantly bias predictions of net primary production of the middle and high latitudes under high atmospheric CO2 concentrations. Based on both review of field observations and results of simulations, the authors conclude that a more appropriate representation of plant physiology and choice of plant functional types may be required in ecosystem models in order to accurately simulate plant responses to changing environmental conditions.
Seasonal growth patterns of mature loblolly pine (Pinustaeda L.) trees over the interval 1988–1993 have been analyzed to evaluate the effects of ambient ozone on growth of large forest trees. Patterns of stem expansion and contraction of 34 trees were examined using serial measurements with sensitive dendrometer band systems. Study sites, located in eastern Tennessee, varied significantly in soil moisture, soil fertility, and stand density. Levels of ozone, rainfall, and temperature varied widely over the 6-year study interval. Regression analysis identified statistically significant influences of ozone on stem growth patterns, with responses differing widely among trees and across years. Ozone interacted with both soil moisture stress and high temperatures, explaining 63% of the high frequency, climatic variance in stem expansion identified by stepwise regression of the 5-year data set. Observed responses to ozone were rapid, typically occurring within 1–3 days of exposure to ozone at ≥40 ppb and were significantly amplified by low soil moisture and high air temperatures. Both short-term responses, apparently tied to ozone-induced increases in whole-tree water stress, and longer term cumulative responses were identified. These data indicate that relatively low levels of ambient ozone can significantly reduce growth of mature forest trees and that interactions between ambient ozone and climate are likely to be important modifiers of future forest growth and function. Additional studies of mechanisms of short-term response and interspecies comparisons are clearly needed.
Soil information is now widely required by many climate and hydrology models and soil-vegetation-atmosphere transfer schemes. This pa- per describes the development of a multilayer soil characteristics dataset for the conterminous United States (CONUS-SOIL) that specifically addresses the need for soil physical and hydraulic property information over large areas. The State Soil Geographic Database (STATSGO) developed by the U.S. De- partment of Agriculture-Natural Resources Conservation Service served as the starting point for CONUS-SOIL. Geographic information system and Perl computer programming language tools were used to create map coverages of soil properties including soil texture and rock fragment classes, depth-to-bed- rock, bulk density, porosity, rock fragment volume, particle-size (sand, silt, and clay) fractions, available water capacity, and hydrologic soil group. In- terpolation procedures for the continuous and categorical variables describing these soil properties were developed and applied to the original STATSGO data. In addition to any interpolation errors, the CONUS-SOIL dataset reflects the limitations of the procedures used to generate detailed county-level soil
Ambient ozone data collected at two sites in the Great Smoky Mountains
National Park (GSMNP) are summarized and compared with data from an
urban and a low-elevation rural site. The ozone climatology in the park
is found to be similar to that of other remote sites in the southern
Appalachian Mountain region. As expected, terrain elevation is
identified as a major factor influencing local ozone levels. Episodes of
high ozone concentrations (90 ppb) in the park are shown to be primarily
attributable to the transport of ozone into the park from outside.
Backward air trajectories computed for high-ozone episodes in the GSMNP
reveal that no preferred source regions exist, although some episodes
appear to be associated with transport from urban areas.
We used an individual tree physiology model, TREGRO, and a stand succession model, ZELIG, to extrapolate from the direct response of leaf photosynthesis to ozone exposure in individual plants to an estimate of the possible future ozone effects on a forest located in the Twin Creeks region in Great Smoky Mountains National Park (GSMNP). This forest is dominated by yellow-poplar (Liriodendron tulipifera L.). With TREGRO, we estimated whether the reduction in the supply of carbon caused by ozone exposure would prevent an individual tree from meeting its demands for carbon for growth. With ZELIG, we analyzed the effect that changes in individual tree growth have in altering the ability of trees to successfully compete during forest succession. Current ambient levels of ozone measured at mid-elevations on the west side of GSMNP were predicted to reduce the abundance of yellow-poplar by 10% over the next 100 yr. This reduction would accelerate declines that are expected because of natural successional change. An increase of ozone of 50% above current ambient conditions was predicted to decrease yellow-poplar abundance by 30%. A reduction of ozone to halfambient levels was insufficient to prevent yellow-poplar abundance decreases. Increases in abundance in two other canopy dominants, red maple (Acer rubrum L.) and black cherry (Prunus serotina Ehrh.) were predicted from the release from competition as yellow-poplar was suppressed by ozone exposure. These increases, however, were predicted to be short-lived, with large decreases of up to 30% eventually occurring for both species. Overall basal area of the forest was predicted to be relatively unchanged by ozone exposure. FOR. Sci. 47(1)2942.
Soil organic carbon in active exchange with the atmosphere constitutes approximately two-thirds of the carbon in terrestrial ecosystems1,2. The relatively large size and long residence time of this pool (of the order of 1,200 yr) make it a potentially important sink for carbon released to the atmosphere by fossil fuel combustion; however, in many cases, human disturbance has caused a decrease in soil carbon storage3,4. Various recent estimates place the global total of soil carbon between 700 (ref. 2) and 2,946 × 1015 g (ref. 5) with several intermediate estimates: 1,080 (ref. 1), 1,392 (ref. 6), 1,456 (ref. 3), and 2,070 × 1015g (ref. 7). Schlesinger's3 estimate seems to be based on the most extensive data base (~200 observations, some of which are mean values derived from large studies in particular areas) and is widely cited in carbon cycle studies. In addition to estimating the world soil carbon pool, it is important to establish the relationships between the geographical distribution of soil carbon and climate, vegetation, human development and other factors as a basis for assessing the influence of changes in any of these factors on the global carbon cycle. Our analysis of 2,700 soil profiles, organized on a climate basis using the Holdridge life-zone classification system8, indicates relationships between soil carbon density and climate, a major soil forming factor. Soil carbon density generally increases with increasing precipitation, and there is an increase in soil carbon with decreasing temperature for any particular level of precipitation. When the potential evapotranspiration equals annual precipitation, soil carbon density9 is ~10 kg m−2, exceptions to this being warm temperate and subtropical soils. Based on recent estimates of the areal extent of major ecosystem complexes9,10 which correspond well with climatic life zones, the global soil organic carbon pool is estimated to be ~1,395 × 1015g.
GLOBAL increases in concentrations of tropospheric ozone are of worldwide concern because of its potential to affect both human and ecological health1. Ozone has been implicated in the declining health of some forest tree species2,3 because of its effects on many growth-related processes in controlled studies3–5. Despite strong evidence that growth of forest tree seedlings can be reduced by ambient levels of ozone6, there has been little basis for evaluating the threshold and magnitude of direct effects on growth of mature, economically important forest trees. We describe here a five-year study of serial changes in stem circumference of 28 mature loblolly pine (Pinus taeda L.) trees. This study has defined a rough ozone response threshold and quantified short- and longer-term components of growth responses to varying ozone and climate variables. Ozone exposures at ⪈40 nl 1−1 interacted with low soil moisture and high air temperatures to reduce short-term rates of stem expansion. Annual growth was also inversely related to seasonal ozone exposure and soil moisture stress. Effects varied widely between individual trees and years. Future ozone effects on forests are likely to be influenced by climate change and by projected increases in regional ozone pollution in industrialized countries.
A max , maximum CO 2 assimilation rate
CAB , genes encoding chlorophyll a / b binding proteins
C i , intercellular CO 2 concentration
PGK , the gene encoding 3‐phosphoglycerate kinase
PRK , the gene encoding phosphoribulokinase
PSAB , the gene encoding the 83 kDa apoprotein of the PSI reaction centre
PSBA, the gene encoding the D1 protein of photosystem II
RBCS , genes encoding the Rubisco small subunit protein
RBCL , the gene encoding the Rubisco large subunit protein
Rubisco, ribulose‐1,5‐bisphosphate carboxylase/ oxygenase
SBP , the gene encoding sedoheptulose‐1,5‐bisphosphatase
There have been many recent exciting advances in our understanding of the cellular processes that underlie photosynthetic acclimation to rising atmospheric CO 2 concentration. Of particular interest have been the molecular processes that modulate photosynthetic gene expression in response to elevated CO 2 and the biochemical processes that link changes in atmospheric CO 2 concentration to the production of a metabolic signal. Central to this acclimation response is a reduction in ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) protein content. Studies indicate that this reduction results from species‐dependent variation in the differential use and temporal control of molecular processes. We present a model for the control of Rubisco protein accumulation that emphasizes the role of subunit message translation as well as the abundance of subunit messages as components of the acclimation response. Many studies indicate that photosynthetic acclimation to elevated CO 2 results from adjustments in leaf carbohydrate signalling. The repression of photosynthetic gene expression is considered to occur primarily by hexokinase functioning as a hexose flux sensor that ultimately affects transcription. Leaf hexoses may be produced as potential sources of signals primarily by sucrose cycling and secondarily by starch hydrolysis. An increased rate of sucrose cycling is suggested to occur at elevated CO 2 by enhanced provision of sucrose to leaf acid invertases. Additionally, sink limitations that accentuate photosynthetic acclimation may result from a relative decrease in the export of leaf sucrose and subsequent increase in cellular sucrose levels and sucrose cycling.
Differences in radial growth at breast height of yellow-poplar (Liriodendron tulipifera L.) and black cherry (Prunus serotina Ehrh.) were tested between individual trees with a history of visible foliar ozone injury and those not expressing foliar injury to ozone at three sites in Great Smoky Mountains National Park, USA. No significant differences in growth for black cherry were found although there was a 12% reduction in radial growth over 5 yrs (1990–1994) (p-level 0.4) and 8% over 10 yrs (1985–1994) (p-level 0.6). There was a significant difference in radial growth for yellow-poplar of 43% over 5 yrs (p-level 0.001) and 30% over 10 yrs (p-level 0.005). Even though the trees of both species were selected to balance the diameter distribution of each species between the two groups at each site, it was still possible that the differences were due to some other factors than sensitivity to ozone exposure. Therefore, a series of multiple linear regressions were used to identify the most explanatory model based on principal components derived from the following independent variables: Diameter at breast height, total height, height to the live crown, percent slope, and a number of competition indices based on the diameter and distance to competitors. These regressions were then tested for different intercepts and slopes between the sensitive and nonsensitive trees. Once again, no significant differences occurred for black cherry (p-levels of 0.4 and 0.7 for five-year and ten-year radial growth, respectively) and some differences for yellow-poplar (p-levels of 0.04 and 0.1 for five-year and ten-year radial growth, respectively). Although the conclusions did not change, the importance of proper balancing of the diameter distribution and accounting for the effects of uncontrollable independent variables are discussed.
The role of undisturbed tropical land ecosystems in the global carbon budget is not well understood. It has been suggested that interannual climate variability can affect the capacity of these ecosystems to store carbon in the short term. In this paper, we use a transient version of the Terrestrial Ecosystem Model (TEM) to estimate annual carbon storage in undisturbed Amazonian ecosystems during the period 1980–94, and to understand the underlying causes of the year‐to‐year variations in net carbon storage for this region.
Changes to forest production drivers (light, water, temperature, and site nutrient) over the last 55 years have been documented in peer-reviewed literature. The main objective of this paper is to review documented evidence of the impacts of climate change trends on forest productivity since the middle of the 20th century. We first present a concise overview of the climate controls of forest production, provide evidence of how the main controls have changed in the last 55 years, followed by a core section outlining our findings of observed and documented impacts on forest productivity and a brief discussion of the complications of interpreting trends in net primary production (NPP). At finer spatial scales, a trend is difficult to decipher, but globally, based on both satellite and ground-based data, climatic changes seemed to have a generally positive impact on forest productivity when water was not limiting. Of the 49 papers reporting forest production levels we reviewed, 37 showed a positive growth trend, five a negative trend, three reported both a positive and a negative trend for different time periods, one reported a positive and no trend for different geographic areas, and two reported no trend. Forests occupy ≈52% of the Earth's land surface and tend to occupy more temperature and radiation-limited environments. Less than 7% of forests are in strongly water-limited systems. The combined and interacting effects of temperature, radiation, and precipitation changes with the positive effect of CO2, the negative effects of O3 and other pollutants, and the presently positive effects of N will not be elucidated with experimental manipulation of one or a few factors at a time. Assessments of the greening of the biosphere depend on both accurate measurements of rates (net ecosystem exchange, NPP), how much is stored at the ecosystem level (net ecosystem production) and quantification of disturbances rates on final net biome production.
The effects of air pollution on vegetation may provide an important control on the carbon cycle that has not yet been widely considered. Prolonged exposure to high levels of ozone, in particular, has been observed to inhibit photosynthesis by direct cellular damage within the leaves and through possible changes in stomatal conductance. We have incorporated empirical equations derived for trees (hardwoods and pines) and crops into the Terrestrial Ecosystem Model to explore the effects of ozone on net primary production (NPP) and carbon sequestration across the conterminous United States. Our results show a 2.6–6.8% mean reduction for the United States in annual NPP in response to modelled historical ozone levels during the late 1980s-early 1990s. The largest decreases (over 13% in some locations) occur in the Midwest agricultural lands, during the mid-summer when ozone levels are highest. Carbon sequestration since the 1950s has been reduced by 18–38 Tg C yr−1 with the presence of ozone. Thus the effects of ozone on NPP and carbon sequestration should be factored into future calculations of the United States' carbon budget.
We use the Terrestrial Ecosystem Model (TEM, Version 4.1) and the land cover data set of the international geosphere–biosphere program to investigate how increasing atmospheric CO2 concentration and climate variability during 1900–1994 affect the carbon storage of terrestrial ecosystems in the conterminous USA, and how carbon storage has been affected by land-use change. The estimates of TEM indicate that over the past 95 years a combination of increasing atmospheric CO2 with historical temperature and precipitation variability causes a 4.2% (4.3 Pg C) decrease in total carbon storage of potential vegetation in the conterminous US, with vegetation carbon decreasing by 7.2% (3.2 Pg C) and soil organic carbon decreasing by 1.9% (1.1 Pg C). Several dry periods including the 1930s and 1950s are responsible for the loss of carbon storage. Our factorial experiments indicate that precipitation variability alone decreases total carbon storage by 9.5%. Temperature variability alone does not significantly affect carbon storage. The effect of CO2 fertilization alone increases total carbon storage by 4.4%. The effects of increasing atmospheric CO2 and climate variability are not additive. Interactions among CO2, temperature and precipitation increase total carbon storage by 1.1%. Our study also shows substantial year-to-year variations in net carbon exchange between the atmosphere and terrestrial ecosystems due to climate variability. Since the 1960s, we estimate these terrestrial ecosystems have acted primarily as a sink of atmospheric CO2 as a result of wetter weather and higher atmospheric CO2 concentrations. For the 1980s, we estimate the natural terrestrial ecosystems, excluding cropland and urban areas, of the conterminous US have accumulated 78.2 Tg C yr−1 because of the combined effect of increasing atmospheric CO2 and climate variability. For the conterminous US, we estimate that the conversion of natural ecosystems to cropland and urban areas has caused a 18.2% (17.7 Pg C) reduction in total carbon storage from that estimated for potential vegetation. The carbon sink capacity of natural terrestrial ecosystems in the conterminous US is about 69% of that estimated for potential vegetation.
Many uncertainties remain regarding how climate change will alter the structure and function of forest ecosystems. At the Aspen FACE experiment in northern Wisconsin, we are attempting to understand how an aspen/birch/maple forest ecosystem responds to long-term exposure to elevated carbon dioxide (CO2) and ozone (O3), alone and in combination, from establishment onward. We examine how O3 affects the flow of carbon through the ecosystem from the leaf level through to the roots and into the soil micro-organisms in present and future atmospheric CO2 conditions. We provide evidence of adverse effects of O3, with or without co-occurring elevated CO2, that cascade through the entire ecosystem impacting complex trophic interactions and food webs on all three species in the study: trembling aspen (Populus tremuloides Michx.), paper birch (Betula papyrifera Marsh), and sugar maple (Acer saccharum Marsh). Interestingly, the negative effect of O3 on the growth of sugar maple did not become evident until 3 years into the study. The negative effect of O3 effect was most noticeable on paper birch trees growing under elevated CO2. Our results demonstrate the importance of long-term studies to detect subtle effects of atmospheric change and of the need for studies of interacting stresses whose responses could not be predicted by studies of single factors. In biologically complex forest ecosystems, effects at one scale can be very different from those at another scale. For scaling purposes, then, linking process with canopy level models is essential if O3 impacts are to be accurately predicted. Finally, we describe how outputs from our long-term multispecies Aspen FACE experiment are being used to develop simple, coupled models to estimate productivity gain/loss from changing O3.
Incidence and severity of foliar symptoms due to ambient ozone exposures were documented on mature black cherry (Prunus serotina) in two National Parks [Great Smoky Mountains National Park (GRSM) and Shenandoah National Park (SHEN)] in the Appalachian Mountains of the eastern USA during the summer of 1991-1993. Three plots in each park containing 30 trees each (Big Meadows in SHEN had 60 trees) with 90 and 120 trees total trees were evaluated in GRSM and SHEN, respectively. Plots were established at different elevations adjacent to ozone monitoring stations. Samples of foliage were collected and three exposed branches from the upper- crown and three branches from the mid-to-lower crown were examined for symptoms of foliar ozone injury. Incidence was greatest in 1991 at both locations; 60% and 45% for GRSM and SHEN, respectively. In 1992 and 1993, incidence was very similar in both parks, with approximately 33% of the trees affected. Black cherry at the highest elevations exhibited the greatest amount of symptoms in both parks all three years of the study. These sites also exhibited the highest ozone concentrations. In addition, the percent of trees injured by ozone was positively correlated with SUM06 and W126. These results along with forest surveys and open-top chamber studies indicate that black cherry may be a reliable bioindicator of foliar injury due to ambient ozone.
Mountain regions are of special importance for global change research. Due to the strong altitudinal gradients many mountain
regions provide unique opportunities to detect and analyse global change processes and phenomena. Therefore, integrated interdisciplinary
collaborative research activities are suggested to be implemented globally in a well coordinated way to understand, model
and predict environmental change processes in mountain regions and, where needed, make proposals towards sustainable land,
water and resources management. The required research is suggested to be structured around four activities and a number of
specific tasks to be briefly described in the following. Moreover, suggestions will be made for the implementation and international
coordination of the research.
Detailed vegetation databases and associated maps of the Great Smoky Mountains National Park, a rugged, forested area of more than 2000 km2, were constructed to support resource management activities of the U.S. National Park Service (NPS). These detailed vegetation databases and associated maps have a terrain relief exceeding 1700 m and a continuous forest cover over 95% of the Park. The requirement to use 1:12,000- and 1:40,000-scale color infrared aerial photographs as the primary data source for mapping overstory and understory vegetation, respectively, necessitated the integration of analog photointerpretation with both digital softcopy photogrammetry and geographic information system (GIS) procedures to overcome problems associated with excessive terrain relief and a lack of ground control. Applications of the vegetation database and associated large-scale maps include assessments of vegetation patterns related to management activities and quantification of forest fire fuels.
A method for generating daily surfaces of temperature, precipitation, humidity, and radiation over large regions of complex terrain is presented. Required inputs include digital elevation data and observations of maximum temperature, minimum temperature and precipitation from ground-based meteorological stations. Our method is based on the spatial convolution of a truncated Gaussian weighting filter with the set of station locations. Sensitivity to the typical heterogeneous distribution of stations in complex terrain is accomplished with an iterative station density algorithm. Spatially and temporally explicit empirical analyses of the relationships of temperature and precipitation to elevation were performed, and the characteristic spatial and temporal scales of these relationships were explored. A daily precipitation occurrence algorithm is introduced, as a precursor to the prediction of daily precipitation amount. Surfaces of humidity (vapor pressure deficit) are generated as a function of the predicted daily minimum temperature and the predicted daily average daylight temperature. Daily surfaces of incident solar radiation are generated as a function of Sun-slope geometry and interpolated diurnal temperature range. The application of these methods is demonstrated over an area of approximately 400 000 detailed illustration of the parameterization process. A cross-validation analysis was performed, comparing predicted and observed daily and annual average values. Mean absolute errors (MAE) for predicted annual average maximum and minimum temperature were 0.7°C and 1.2°C, with biases of +0.1°C and −0.1°C, respectively. MAE for predicted annual total precipitation was 13.4 cm, or, expressed as a percentage of the observed annual totals, 19.3%. The success rate for predictions of daily precipitation occurrence was 83.3%. Particular attention was given to the predicted and observed relationships between precipitation frequency and intensity, and they were shown to be similar. We tested the sensitivity of these methods to prediction grid-point spacing, and found that areal averages were unchanged for grids ranging in spacing from 500 m to 32 km. We tested the dependence of the results on timestep, and found that the temperature prediction algorithms scale perfectly in this respect. Temporal scaling of precipitation predictions was complicated by the daily occurrence predictions, but very nearly the same predictions were obtained at daily and annual timesteps.
Data on three major determinants of the carbon storage in terrestrial ecosystems are used with the process-based Terrestrial Ecosystem Model (TEM) to simulate the combined effect of climate variability, increasing atmospheric CO2 concentration, and cropland establishment and abandonment on the exchange of CO2 between the atmosphere and monsoon Asian ecosystems. During 1860–1990, modeled results suggest that monsoon Asia as a whole released 29.0 Pg C, which represents 50% of the global carbon release for this period. Carbon release varied across three subregions: East Asia (4.3 Pg C), South Asia (6.6 Pg C), and Southeast Asia (18.1 Pg C). For the entire region, the simulations indicate that land-use change alone has led to a loss of 42.6 Pg C. However, increasing CO2 and climate variability have added carbon to terrestrial ecosystems to compensate for 23% and 8% of the losses due to land-use change, respectively. During 1980–1989, monsoon Asia as a whole acted as a source of carbon to the atmosphere, releasing an average of 0.158 Pg C per year. Two of the subregions acted as net carbon source and one acted as a net carbon sink. Southeast Asia and South Asia were sources of 0.288 and 0.02 Pg C per year, respectively, while East Asia was a sink of 0.149 Pg C per year. Substantial interannual and decadal variations occur in the annual net carbon storage estimated by TEM due to comparable variations in summer precipitation and its effect on net primary production (NPP). At longer time scales, land-use change appears to be the important control on carbon dynamics in this region.
Data from 25 experiments on seedlings of 43 tree species and hybrids show that ozone (O3) can reduce growth and photosynthesis at concentrations common in many areas of the USA. Seedlings have been primarily employed for such studies for logistic reasons, and will likely provide the greatest breadth of information for some time to come. However, a number of impediments limit application of seedling response studies to assessment of impacts on regional timber production. Large trees differ from seedlings in a number of ways, including C allocation and canopy structure, and methods must be developed to account for these differences if information from seedling studies is to prove useful to forest impact assessment. Understanding how competition mediates individual tree responses will require investigation of whether systematic differences of microclimate leaf morphology that exist across canopies affects foliage sensitivity to O3, and whether the maximum growth rates of genotypes are correlated with susceptibility to O3. Definitive information on these factors is necessary to assess impacts of O3 on stand development and diameter distributions in both multi- and single species stands. Of critical economic importance is whether O3 preferentially damages taller, more valuable individuals within stands and more valuable, faster growing stand types.
Contribution of the Pest Impact Assessment Technol Res. Work Unit, USDA Forest Service, Southeastern Forest Exp. Stn.
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Anet, leaf net CO2 assimilation
ca, CO2 concentration of air surrounding a leaf
ci, leaf intercellular CO2 concentration
Δ, ¹³C isotope discrimination
δ¹³C, relative stable carbon isotope content
ɛ, ratio of Anet at ca = 560μmol mol–1 to Anet at ca = 360 μmol mol–1
FACE, free-air CO2 enrichment
gw, stomatal conductance to water vapour
Πi, initial leaf osmotic potential
Rt, relative water content at incipient turgor loss
Ψl, xylem water potential of leaves
Ψm, soil matric potential
Elevated CO2 is expected to reduce forest water use as a result of CO2-induced stomatal closure, which has implications for ecosystem-scale phenomena controlled by water availability. Leaf-level CO2 and H2O exchange responses and plant and soil water relations were examined in a maturing loblolly pine (Pinus taeda L.) stand in a free-air CO2 enrichment (FACE) experiment in North Carolina, USA to test if these parameters were affected by elevated CO2. Current-year foliage in the canopy was continuously exposed to elevated CO2 (ambient CO2+200μmol mol–1) in free-air during needle growth and development for up to 400 d. Photosynthesis in upper canopy foliage was stimulated by 50–60% by elevated CO2 compared with ambient controls. This enhancement was similar in current-year, ambient-grown foliage temporarily measured at elevated CO2 compared with long-term elevated CO2 grown foliage. Significant photosynthetic enhancement by CO2 was maintained over a range of conditions except during peak drought.