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Issues in Modelling Plant Ecosystem Responses to Elevated CO2: Interactions with Soil Nitrogen
Habitat for wildlife species that depend on sagebrush ecosystems is of great management concern. Evaluating how management activities and climate change may affect the abundance of moderate and high-quality habitat necessitates the development of models that examine vegetation dynamics, but modeling tools for rangeland systems are limited. I developed state-and-transition models using a combination of scientific literature and data for climate, soils, and wildfire to examine how different types of natural events, management activities, changing climate, and potential future vegetation dynamics may interact and affect the abundance of habitat for the greater sage-grouse (Centrocercus urophasianus). Specific periods examined include the era prior to 1850, the current era, and late in the 21 st century in southeastern Oregon. A primary purpose of this study was to evaluate the use of climate data to define most event probabilities and, subsequently, the relative mix of ecological states, community phases, and sage-grouse habitat with an eye towards a modeling approach that was objective, repeatable, and transferrable to other locations. Contrary to expectations, model results of the conditions prior to 1850 indicated fire may not have been the most important disturbance factor influencing sage-grouse habitat abundance, merely the most visible. Other, more subtle disturbances that thinned sagebrush density,
Model–data fusion is defined as matching model prediction and observations by varying model parameters or states using statistical estimation. In this paper, we review the history of applications of various model–data fusion techniques in studies of terrestrial carbon fluxes in two approaches: top-down approaches that use measurements of global CO2 concentration and sometimes other atmospheric constituents to infer carbon fluxes from the land surface, and bottom-up approaches that estimate carbon fluxes using process-based models. We consider applications of model–data fusion in flux estimation, parameter estimation, model error analysis, experimental design and forecasting. Significant progress has been made by systematically studying the discrepancies between the predictions by different models and observations. As a result, some major controversies in global carbon cycle studies have been resolved, robust estimates of continental and global carbon fluxes over the last two decades have been obtained, and major deficiencies in the atmospheric models for tracer transport have been identified. In the bottom-up approaches, various optimization techniques have been used for a range of process-based models. Model–data fusion techniques have been successfully used to improve model predictions, and quantify the information content of carbon flux measurements and identify what other measurements are needed to further constrain model predictions. However, we found that very few studies in both top-down and bottom-up approaches have quantified the errors in the observations, model parameters and model structure systematically and consistently. We therefore suggest that future research will focus on developing an integrated Bayesian framework to study both model and measurement errors systematically.
We used a daily time step ecosystem model (DAYCENT) to simulate ecosystem processes at a daily, biweekly, monthly, and annual time step. The model effectively represented variability of ecosystem processes at each of these timescales. Evolution of CO2 and N2O, NPP, and net N mineralization were more responsive to variation in precipitation than temperature, while a combined temperature-moisture decomposition factor (DEFAC) was a better predictor than either component alone. Having established the efficacy of CENTURY at representing ecosystem processes at multiple timescales, we used the model to explore interannual variability over the period 1949-1996 using actual daily climate data. Precipitation was more variable than temperature over this period, and our most variable responses were in CO2 flux and NEP. Net ecosystem production averaged 6 g C m-2 yr and varied by 100% over the simulation period. We found no reliable predictors of NEP when compared directly, but when we considered NEP to be lagged by 1 year, predictive power improved. It is clear from our study that NEP is highly variable and difficult to predict. The emerging availability of system-level C balance data from a network of flux towers will not only be an invaluable source of information for assessments of global carbon balance but also a rigorous test for ecosystem models.
Within the last decade the study of phenology has taken on a new legitimacy in the area of climate change research. A growing literature reveals that a change in the timing of natural events is occurring in a wide range of locations and affecting a wide range of species. Changes in spring have been those most commonly reported, with the emphasis on an advance in spring linked to an increase in temperature. Detection of change in autumn is hampered by a smaller pool of available data, events that are harder to define (such as leaf coloration), and various influencing environmental factors triggering autumnal phases. Despite this, the general pattern may be towards a delay in autumn. Plant, animal and abiotic responses, especially in spring, are quite similar. Thus, it would appear that winter is being squeezed at both ends, and this effect, of increasing the growing season, should become more pronounced in the face of predicted global warming. Copyright © 2002 Royal Meteorological Society.
Changes in the global water cycle can cause major environmental and socioeconomic impacts. As the average global temperature increases, it is generally expected that the air will become drier and that evaporation from terrestrial water bodies will increase. Paradoxically, terrestrial observations over the past 50 years show the reverse. Here, we show that the decrease in evaporation is consistent with what one would expect from the observed large and widespread decreases in sunlight resulting from increasing cloud coverage and aerosol concentration.
Plant respiratory regulation is too complex for a mechanistic representation in current terrestrial productivity models for carbon accounting and global change research. Accordingly, simpler approaches that attempt to capture the essence of respiration are commonly adopted. Several approaches have been used in the literature: respiration may be embedded implicitly in growth algorithms; assumed values for specific respiration rates may be adopted; respiration may be calculated in terms of growth and maintenance components; conservatism in the ratio of respiration to photosynthesis (R : P) may be assumed; or a more complex process or residual approach may be adopted. Review of this literature suggests that the assumption of conservative R : P ratio is an effective and practicable approach in the context of C-cycle modelling for global change research and documentation, requiring minimal ecosystem-specific data on respiration.Some long-standing controversies in respiration are now becoming resolved. The apparently wasteful process of cyanide-resistant respiration by the alternative oxidase may not be wasteful, as it is thought to be involved in protecting the plant from 'reactive oxygen species'. It is now clear that short-term respiratory response coefficients of plants (e.g. the Q10) do not predict their long-term temperature response. A new experimental approach suggests that leaf respiration is not suppressed by light as previously thought. Careful experiments, taking account of several proposed measurement artefacts, indicate that plant respiration is not suppressed by elevated CO2 concentration in a short-term reversible way.
Although environmental factors such as precipitation and temperature are
recognized as influencing pollen production, the impact of rising atmospheric
carbon dioxide concentration ([CO2]) on the
potential growth and pollen production of hay-fever-inducing plants is
unknown. Here we present measurements of growth and pollen production of
common ragweed (Ambrosia artemisiifolia L.) from
pre-industrial [CO2] (280 mol
mol–1) to current concentrations (370 mol
mol–1) to a projected 21st century concentration
(600 mol mol–1). We found that exposure to current
and elevated [CO2] increased ragweed pollen
production by 131 and 320%, respectively, compared to plants grown at
pre-industrial [CO2]. The observed
stimulations of pollen production from the pre-industrial
[CO2] were due to an increase in the number
(at 370 mol mol–1) and number and size (at 600 mol
mol–1) of floral spikes. Overall, floral weight as
a percentage of total plant weight decreased (from 21% to 13%),
while investment in pollen increased (from 3.6 to 6%) between 280 and
600 mol mol–1 CO2. Our
results suggest that the continuing increase in atmospheric
[CO2] could directly influence public health
by stimulating the growth and pollen production of allergy-inducing species
such as ragweed.
The mechanism by which the model-simulated North Atlantic thermohaline circulation (THC) weakens in response to increasing greenhouse gas (GHG) forcing is investigated through the use of a set of five multi-century experiments. Using a coarse resolution version of the GFDL coupled climate model, the role of various surface fluxes in weakening the THC is assessed. Changes in net surface freshwater fluxes (precipitation, evaporation, and runoff from land) are found to be the dominant cause for the model's THC weakening. Surface heat flux changes brought about by rising GHG levels also contribute to THC weakening, but are of secondary importance. Wind stress variations have negligible impact on the THC's strength in the transient GHG experiment.
In this chapter respiration as an integrated part of the metabolism of the intact plant will be considered; its aim is to discuss the qualitative and, wherever possible, quantitative links of respiration with various other aspects of metabolism.
The results of published and unpublished experiments investigating the impacts of elevated [CO2] on the chemistry of leaf litter and decomposition of plant tissues are summarized. The data do not support the hypothesis that changes in leaf litter chemistry often associated with growing plants under elevated [CO2] have an impact on decomposition processes. A meta-analysis of data from naturally senesced leaves in field experiments showed that the nitrogen (N) concentration in leaf litter was 7.1% lower in elevated [CO2] compared to that in ambient [CO2]. This statistically significant difference was: (1) usually not significant in individual experiments, (2) much less than that often observed in green leaves, and (3) less in leaves with an N concentration indicative of complete N resorption. Under ideal conditions, the efficiency with which N is resorbed during leaf senescence was found not to be altered by CO2 enrichment, but other environmental influences on resorption inevitably increase the variability in litter N concentration. Nevertheless, the small but consistent decline in leaf litter N concentration in many experiments, coupled with a 6.5% increase in lignin concentration, would be predicted to result in a slower decomposition rate in CO2-enriched litter. However, across the assembled data base, neither mass loss nor respiration rates from litter produced in elevated [CO2] showed any consistent pattern or differences from litter grown in ambient [CO2]. The effects of [CO2] on litter chemistry or decomposition were usually smallest under experimental conditions similar to natural field conditions, including open-field exposure, plants free-rooted in the ground, and complete senescence. It is concluded that any changes in decomposition rates resulting from exposure of plants to elevated [CO2] are small when compared to other potential impacts of elevated [CO2] on carbon and N cycling. Reasons for experimental differences are considered, and recommendations for the design and execution of decomposition experiments using materials from CO2-enrichment experiments are outlined.
Various aspects of the biochemistry of photosynthetic carbon assimilation in C3 plants are integrated into a form compatible with studies of gas exchange in leaves. These aspects include the kinetic properties of ribulose bisphosphate carboxylase-oxygenase; the requirements of the photosynthetic carbon reduction and photorespiratory carbon oxidation cycles for reduced pyridine nucleotides; the dependence of electron transport on photon flux and the presence of a temperature dependent upper limit to electron transport. The measurements of gas exchange with which the model outputs may be compared include those of the temperature and partial pressure of CO2(p(CO2)) dependencies of quantum yield, the variation of compensation point with temperature and partial pressure of O2(p(O2)), the dependence of net CO2 assimilation rate on p(CO2) and irradiance, and the influence of p(CO2) and irradiance on the temperature dependence of assimilation rate.
The expansion growth of plant organs is inhibited at low water potentials (Ψ w), but the inhibition has not been compared in different organs of the same plant. Therefore, we determined elongation rates of the roots, stems, leaves, and styles (silks) of maize (Zea mays L.) as soil water was depleted. The Ψ w was measured in the region of cell expansion of each organ. The complicating effects of transpiration were avoided by making measurements at the end of the dark period when the air had been saturated with water vapor for 10 h and transpiration was less than 1% of the rate in the light. Growth was inhibited as the Ψ w in the region of cell expansion decreased in each organ. The Ψ w required to stop growth was-0.50,-0.75, and-1.00 MPa, in this order, in the stem, silks, and leaves. However, the roots grew at these Ψ w and ceased only when Ψ w was lower than-1.4 MPa. The osmotic potential decreased in each region of cell expansion and, in leaves, roots and stems, the decrease was sufficient to maintain turgor fully. In the silks, the decrease was less and turgor fell. In the mature tissue, the Ψ w of the stem, leaves and roots was similar to that of the soil when adequate water was supplied. This indicated that an equilibrium existed between these tissues, the vascular system, and the soil. At the same time, the Ψ w was lower in the expanding regions than in the mature tissues, indicating that there was a Ψ w disequilibrium between the growing tissue and the vascular system. The disequilibrium was interpreted as a Ψ w gradient for supplying water to the enlarging cells. When water was withheld, this gradient disappeared in the leaf because Ψ w decreased more in the xylem than in the soil, indicating that a high flow resistance had developed in the xylem. In the roots, the gradient did not decrease because vascular Ψ w changed about the same amount as the soil Ψ w. Therefore, the gradient in Ψ w favored water uptake by roots but not leaves at low Ψ w. The data show that expansion growth responds to low Ψ w differently in different growing regions of the plant. Because growth depends on the maintenance of turgor for extending the cell walls and the presence of Ψ w gradients for supplying water to the expanding cells, several factors could have been responsible for these differences. The decrease of turgor in the silks and the loss of the Ψ w gradient in the leaves probably contributed to the high sensitivity of these organs. In the leaves, the gradient loss was so complete that it would have prevented growth regardless of other changes. In the roots, the maintenance of turgor and Ψ w gradients probably allowed growth to continue. This difference in turgor and gradient maintenance could contribute to the increase in root/shoot ratios generally observed in water-limited conditions.
Normalized difference vegetation index (NDVI) data processed from measurements of advanced very high resolution radiometers (AVHRR) onboard the afternoon-viewing NOAA series satellites (NOAA 7, 9, and 11) were analyzed for spatial and temporal patterns comparable to those observed in atmospheric CO2, near-surface air temperature, and sea surface temperature (SST) data during the 1981-1991 time period. Two global data sets of NDVI were analyzed for consistency: (1) the land segment of the joint NOAA/NASA Earth Observing System AVHRR Pathfinder data set and (2) the Global Inventory Monitoring and Modeling Studies AVHRR NDVI data set. The impact of SST events was found to be confined mostly to the tropical latitudes but was generally dominant enough to be manifest in the global NDVI anomaly. The vegetation index anomalies at latitudes north of 45 degrees N were found to exhibit an increasing trend. This linear trend corresponds to a 10% increase in seasonal NDVI amplitude over a 9 year period (1981-1990). During the same time period, annual amplitude in the record of atmosphere CO2 measured at Point Barrow, Alaska, was reported to have increased by about 14%. The increase in vegetation index data between years was especially consistent through the spring and early summer time periods. When this increase was translated into an advance in the:timing of spring green-up, the measure (8 +/- 3 days) was similar to the recently published estimate of about 7 days in the advance of the midpoint of CO2 drawdown between spring and summer at Point Barrow, Alaska. The geographical distribution of the increase in vegetation activity was consistent with the reported patterns in springtime warming and decline of snow cover extent over the northern hemisphere land area.
1 The work examines the growth and photosynthesis of saplings and mature trees of Pinus sylvestris L. along an altitudinal gradient in the Cairngorm Mountains of Scotland. 2 Meristem temperatures of seedlings declined with altitude, but an especially sharp decline was sometimes observed at the tree-line. 3 Growth, measured as shoot length, needle length and needle (fascicle) number, declined with altitude. 4 Extension growth could be related to temperature using regression analysis. However, the number of fascicles formed in a given year was related to the temperature of the previous summer. 5 Substantial losses of first-year needles were observed over the winter. 6 In the early part of the year, photosynthetic performance in the valley population was superior to the tree-line population, both in the current- and previous-year's needles. 7 The results are discussed in relation to the likely impact of increasing temperatures on the productivity of the species.
Responses to elevated CO[sub 2] have not been measured for natural grassland ecosystems. Global carbon budgets will likely be affected by changes in biomass production and allocation in major terrestrial ecosystems. Whether ecosystems sequester or release excess carbon to the atmosphere will partly determine the extent and rate that atmospheric CO[sub 2] concentration rises. Elevated CO[sub 2] also may change plant community species composition and water status. We determined above- and belowground biomass production, plant community species composition, and water status of a tallgrass prairie ecosystem in Kansas exposed to ambient and twice-ambient CO[sub 2] concentrations in open-top chambers during the growing season 1989 through 1991. Dominant species were Andropogon gerardii, A. scoparius, and Sorghastrum nutans (C[sub 4] metabolism) and Poa pratensis (C[sub 3]). Aboveground biomass and leaf area were estimated by periodic sampling throughout the growing season in 1989 and 1990. In 1991, peak biomass and leaf area were estimated by an early August harvest. Relative root production was estimated using root ingrowth bags which remained in place through the growing season. Latent heat flux was simulated with and without water stress. Botanical composition was estimated annually. Compared to ambient CO[sub 2] levels, elevated CO[sub 2] increased production of C[sub 4] grass species, but not of C[sub 3] grass species. Species composition of C[sub 4] grasses did not change, but Poa pratensis (C[sub 3]) declined and C[sub 3] forbs increased in the stand with elevated CO[sub 2] compared to ambient. Open-top chambers appeared to reduce latent heat flux and increase water-use efficiency similar to the elevated CO[sub 2] treatment when water stress was not severe, but under severe water stress, the chamber effect on water-use efficiency under elevated CO[sub 2] apparently has greater impact on productivity irrespective of photosynthetic pathway. 34 refs., 8 figs., 1 tab.
In a survey of several plant species found at Churchill, Manitoba, in the transition zone between the low and subarctic regions, we measured leaf respiration in terms of total respiration and alternative pathway respiration rates. Leaves of arctic plants exhibit higher rates of total respiration and alternative (cyanide insensitive) respiration than temperate species. There is a negative correlation between plant height and alternative pathway activity. Shorter plants have higher rates of alternative pathway respiration. More alternative pathway activity may mean that there is less energy in the form of ATP available for growth. A shorter growth habit keeps these plants in the still air close to the ground. This prevents cooling, water loss and physical damage due to wind abrasion. Thus plants with high rates of alternative pathway respiration may be better adapted to the arctic environment. The alternative pathway respiration of Orchis roturuiijolia was shown to be under the influence of the biological clock.
Variations in the amplitude and timing of the seasonal cycle of atmospheric CO2 have shown an association with surface air temperature consistent with the hypothesis that warmer temperatures have promoted increases in plant growth during summer1 and/or plant respiration during winter2 in the northern high latitudes. Here we present evidence from satellite data that the photosynthetic activity of terrestrial vegetation increased from 1981 to 1991 in a manner that is suggestive of an increase in plant growth associated with a lengthening of the active growing season. The regions exhibiting the greatest increase lie between 45°N and 70°N, where marked warming has occurred in the spring time3 due to an early disappearance of snow4. The satellite data are concordant with an increase in the amplitude of the seasonal cycle of atmospheric carbon dioxide exceeding 20% since the early 1970s, and an advance of up to seven days in the timing of the drawdown of CO2 in spring and early summer1. Thus, both the satellite data and the CO2 record indicate that the global carbon cycle has responded to interannual fluctuations in surface air temperature which, although small at the global scale, are regionally highly significant.
In the absence of human activities, biotic fixation is the primary source of reactive N, providing about 90-130 Tg N yr-1 (Tg=1012 g) on the continents. Human activities have resulted in the fixation of an additional ~140 Tg N yr-1 by energy production (~20 Tg N yr-1), fertilizer production (~80 Tg N yr-1), and cultivation of crops (e.g., legumes, rice) (~40 Tg N yr-1). We can only account for part of this anthropogenic N. N2O is accumulating in the atmosphere at a rate of 3 Tg N yr-1. Coastal oceans receive another 41 Tg N yr-1 via rivers, much of which is buried or denitrified. Open oceans receive 18 Tg N yr-1 by atmospheric deposition, which is incorporated into oceanic N pools (e.g., NO-3, N2). The remaining 80 Tg N yr-1 are either retained on continents in groundwater, soils, or vegetation or denitrified to N2. Field studies and calculations indicate that uncertainties about the size of each sink can account for the remaining anthropogenic N. Thus although anthropogenic N is clearly accumulating on continents, we do not know rates of individual processes. We predict the anthropogenic N-fixation rate will increase by about 60% by the year 2020, primarily due to increased fertilizer use and fossil-fuel combustion. About two-thirds of the increase will occur in Asia, which by 2020 will account for over half of the globe anthropogenic N fixation.
From the 1950s to the 1980s, a significant decrease of surface solar radiation has been observed at different locations throughout the world. Here we show that this phenomenon, widely termed global dimming, is dominated by the large urban sites. The global-scale analysis of year-to-year variations of solar radiation fluxes shows a decline of 0.41 W/m2/yr for highly populated sites compared to only 0.16 W/m2/yr for sparsely populated sites (<0.1 million). Since most of the globe has sparse population, this suggests that solar dimming is of local or regional nature. The dimming is sharpest for the sites at 10°N to 40°N with great industrial activity. In the equatorial regions even the opposite trend to dimming is observed for sparsely populated sites.
Many researchers have proposed that the stimulus of plant growth under elevated [CO2] observed in short-term experiments will be moderated in the longer term by a reduction in soil nitrogen (N) availability
linked to decreased litter quality and/or increased litter production. However, these negative feedbacks may be offset to
some extent by a stimulus in N fixation linked to increased root exudation. The aim of this modelling study is to examine
how changes in litter quality/quantity and root exudation –- if they occur –- will affect the CO2 responses of net primary productivity and ecosystem carbon (C) storage on different timescales. We apply a model of C and
N cycling in forest ecosystems (G’DAY) to stands of Norway spruce (Picea abies, L. Cast) growing at a N-limited experimental site at Flakaliden, Sweden, and draw the following conclusions: (1) in the
absence of changes in litter quality and root exudation, the short-term CO2 stimulus of litter quantity leads to only a minimal CO2 stimulus of productivity or C storage in the medium term (≈ 20 years) and long term (≈ 200 years), because of constraints
on soil N availability; (2) increasing plant nitrogen use efficiency (via a decrease in the N:C ratio of new litter) makes
little impact on these results; (3) a significant CO2 response in the medium term requires a substantial decrease in the N:C ratio of older litter, when it is approaching stabilisation
as soil organic matter, although the long-term CO2 response remains small; and (4) an increase in N fixation leads to a small effect on productivity in the short term, but
a very large effect on both productivity and C storage in the long term. These results suggest that soil N constraints on
the long-term CO2-fertilisation effect can be overcome to a significant extent only by increases in N acquisition, although only modest increases
may be required.
Based on about 20 years of NOAA/CMDL's atmospheric CO2 concentration data and a global atmospheric tracer transport model, we estimate interannual variations and spatial patterns of surface CO2 fluxes in the period 01/1982-12/2000, by using a time-dependent Bayesian inversion technique. To increase the reliability of the estimated temporal features, particular care is exerted towards the selection of data records that are homogeneous in time. Fluxes are estimated on a grid-scale resolution (≈8° latitude ×10°longitude), constrained by a-priori spatial correlations, and then integrated over different sets of regions. The transport model is driven by interannually varying re-analyzed meteorological fields. We make consistent use of unsmoothed measurements. In agreement with previous studies, land fluxes are estimated to be the main driver of interannual variations in the global CO2 fluxes, with the pace predominantly being set by the El Niño/La Niña contrast. An exception is a 2- 3 year period of increased sink of atmospheric carbon after Mt. Pinatubo's volcanic eruption in 1991. The largest differences in fluxes between El Niño and La Niña are found in the tropical land regions, the main share being due to the Amazon basin. The flux variations for the Post-Pinatubo period, the 1997/1998 El Niño, and the 1999 La Niña events are exploited to investigate relations between CO2 fluxes and climate forcing. A rough comparison points to anomalies in precipitation as a prominent climate factor for shortterm variability of tropical land fluxes, both through their role on NPP and through promoting fire in case of droughts. Some large flux anomalies seem to be directly related to large biomass burning events recorded by satellite observation. Global ocean carbon uptake shows a trend similar to the one expected if ocean uptake scales proportional to the anthropogenic atmospheric CO2 perturbation. In contrast to temporal variations, the longterm spatial flux distribution can be inferred with lesser robustness only. The tentative pattern estimated by the present inversion exhibits a northern hemisphere land sink on the order of 0.4 PgC/yr (for 01/1996- 12/1999, non-fossil fuel carbon only) that is mainly confined to North America. Southern hemisphere land regions are carbon neutral, while the tropical land regions are taking up carbon (e.g., at a rate of 0.8 PgC/yr during 01/1996-12/1999). Ocean fluxes show larger uptake in the Northern mid to high latitudes than in the Southern mid latitude regions, in contrast to the estimates by Takahashi et al. (1999) based on in-situ measurements. On a regional basis, results that differ the most from previous estimates are large carbon uptake of 1 to 1.5 PgC/yr by the Southern temperate Pacific ocean region, weak outgassing from the Southern ocean, and a carbon source from eastern Europe.
The annual timing of leaf colouring of deciduous trees in temperate regions is not predictable using phenological models. In this analysis we show that commonly applied hypotheses of leaf colouring triggers have neither satisfactory explanatory power nor significant statistical proof. We tested meteorological parameters, such as monthly mean temperatures, threshold temperatures, monthly sums of precipitation and number of dry days per month of the year of the phenological event and of the previous year and the length of the vegetation period. Their influence on leaf colouring dates for 4 deciduous tree species (horse chestnut, beech, birch, oak) in Germany (1951-2003) was tested by Pearson's correlations. We created 3 different datasets: (A) phenological observations for single stations within 25 km of meteorological stations, (B) means of phenological observations around (< 25 km) meteorological stations, and (C) phenological means for Germany. Only the mean temperature of September had a slight influence on the onset of leaf colouring (mean: r(chestnut) = 0.45; rbeech 0.56; roak = 0.51; ri,i,,h = 0.45). Taking all correlation coefficients r > 10.31 into account, we can deduce that a warm September (all species) and August (oak, birch) delayed leaf colouring while a warm June (horse chestnut, oak) and May (horse chestnut) advanced leaf colouring.
This study characterizes the influence of the El Nino-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) on the most important crops in Spain using statistical analysis of the response of historical yields to ENSO and NAO phases. The response to ENSO phases is weak, being only significant for lemon yields, whereas the response to NAO phases is stronger, being significant for lemon, wheat, rye and olive. The results could be useful for producers, allowing them to adjust crop management, and for politicians, allowing better quantification of national crops limits.
Leaf photosynthesis (Ps), nitrogen (N) and light environment were measured on Populus tremuloides trees in a developing canopy under free-air CO 2 enrichment in Wis-consin, USA. After 2 years of growth, the trees averaged 1·5 and 1·6 m tall under ambient and elevated CO 2 , respectively , at the beginning of the study period in 1999. They grew to 2·6 and 2·9 m, respectively, by the end of the 1999 growing season. Daily integrated photon flux from cloud-free days (PPFD day,sat) around the lowermost branches was 16·8 ± ± ± ± 0·8 and 8·7 ± ± ± ± 0·2% of values at the top for the ambient and elevated CO 2 canopies, respectively. Elevated CO 2 significantly decreased leaf N on a mass, but not on an area, basis. N per unit leaf area was related linearly to PPFD day,sat throughout the canopies, and elevated CO 2 did not affect that relationship. Leaf Ps light-response curves responded differently to elevated CO 2 , depending upon canopy position. Elevated CO 2 increased Ps sat only in the upper (unshaded) canopy, whereas characteristics that would favour photosynthesis in shade were unaffected by elevated CO 2. Consequently, estimated daily integrated Ps on cloud-free days (Ps day,sat) was stimulated by elevated CO 2 only in the upper canopy. Ps day,sat of the lowermost branches was actually lower with elevated CO 2 because of the darker light environment. The lack of CO 2 stimulation at the mid-and lower canopy was probably related to significant down-regulation of photosynthetic capacity; there was no down-regulation of Ps in the upper canopy. The relationship between Ps day,sat and leaf N indicated that N was not optimally allocated within the canopy in a manner that would maximize whole-canopy Ps or photosynthetic N use efficiency. Elevated CO 2 had no effect on the optimization of canopy N allocation.
If future CO2-induced warming of 2oC increased the incidence of warm springs, of the type that have occurred in Britain during this century, then warming would induce earlier blossoming and budburst in many temperate trees, with an increase in the risk of subsequent damaging frosts. Empirical thermal time-chilling models suggested that, on average, Cox's apple Malus pumila in Kent would blossom 18-24 days earlier than at present following 2oC warming, but that Picea sitchensis in the Scottish uplands would burst its buds only 5 days earlier than at present. -from Authors
The distribution, seasonal and diurnal patterns of stomatal conductance, and bulk tissue water relations characteristics were examined in three co-occurring species of alpine Salix spp. in the C Canadian Rocky Mountains. The species differ in their distribution along a topographic-soil moisture gradient. Correlated to the differences in spatial distribution were differences in water relations among the species. Salix reticulata inhabited the most xeric end of the gradient and exhibited the lowest osmotic potentials near full-tissue hydration and turgor loss, had relatively inelastic tissues, was able to adjust osmotically, had low rates of stomatal conductance, and a relatively high tolerance to an increased leaf-to-air vapour pressure gradient (ΔW). S. barrattiana grew at the wettest end of the gradient and demonstrated comparatively high osmotic potentials near full-tissue hydration and turgor loss, highly elastic tissues, no ability to adjust osmotically, had high rates of stomatal conductance, and a very marked stomatal sensitivity to increased ΔW. S. arctica inhabited intermediate sites along the gradient and its tissues demonstrated intermediate osmotic and elastic properties, ability to adjust osmotically, rates of stomatal conductance, and stomatal sensitivity to increased ΔW. A measure of spatial and physiological overlap was calculated using Pianka's (1974) symmetrical index. -from Author
(1) The thermal times (T) (day ⚬C > 5 ⚬C since 1 January) to vegetative budburst were measured on fifteen woody perennials in Britain after different durations of chilling (C) (number of days ≤ 5 ⚬C since 1 November). (2) Late-flushing species, like Fagus sylvatica, had high values of T, even after 145 chill days, and T increased greatly with decreased chilling. Early-flushing species, like Crataegus monogyna, had small values of T, which did not increase much until there were fewer than 80-100 chill days. (3) Relationships of the form T = a + b exp (r C) were fitted for each species, which were then classified into five groups. Past meteorological records were then used to estimate the mean dates of budburst of each group of species at Edinburgh (26 m altitude) and Braemar (339 m) with 0-3 ⚬C uniform climatic warming. (4) Climatic warming did not markedly shift the date of budburst of any group of species at Edinburgh, so they flushed in warmer conditions and failed to exploit the earlier springs. However, at Braemar, the species with small thermal time and chilling requirements, like C. monogyna, flushed much earlier following climatic warming.
1. Passive greenhouse apparatus is commonly used to investigate the in situ biological response of terrestrial communities to global warming. 2. Although close conformity of greenhouse treatment effects to general circulation model (GCM) scenarios is widely claimed, no proof of such a relationship has yet been published. 3. Here, the relationship between passive greenhouse thermal environment and future climate conditions is considered using temperature data collected from within and without greenhouses deployed in the maritime Antarctic. It is revealed that in terms of thermal extremes, diel and annual variation, and overall distribution across the temperature spectrum, such apparatus achieves only poor simulation of GCM forecasts. 4. During summer, greenhouses induce an amplified daily range of temperatures, elevated maxima and accelerated rates of change. 5. During spring and autumn, diel temperature variation continues inside the greenhouses while snow cover protects the controls. 6. During winter, an inverse treatment effect occurs, in which the relative depth of snow cover causes lower temperatures in greenhouses than in controls. 7. These treatment effects differ significantly from GCM climate predictions. Changes recorded in the composition, structure and function of greenhouse biota may thus be artefacts of the methodology. 8. Thorough a priori testing of greenhouse treatment effects is recommended for future climate change studies that are to be conducted in environments subject to seasonal snowfall, solar elevation and day length.
It is hypothesized that the principal features of higher plant distributions at continental scales are determined by the macroclimate. Bioclimate data have been computed on a 50 km grid across Europe. Along with published maps of higher plant distributions based upon the same grid, these data have been used to derive climate response surfaces that model the relationship between a species' distribution and the present climate. Eight species representative of a variety of phytogeographic patterns have been investigated. The results support the hypothesis that the European distributions of all eight species are principally determined by macroclimate and illustrate the nature of the climatic constraints upon each species. Simulated future distributions in equilibrium with 2× CO2 climate scenarios derived from two alternative GCMs show that all of the species are likely to experience major shifts in their potential range if such climatic changes take place. Some species may suffer substantial range and population reductions and others may face the threat of extinction. The rate of the forecast climate changes is such that few, if any, species may be able to maintain their ranges in equilibrium with the changing climate. In consequence, the transient impacts upon ecosystems will be varied but often may lead to a period of dominance by opportunist, early-successional species. Our simulations of potential ranges take no account of such factors as photoperiod or the direct effects of CO2, both of which may substantially alter the realized future equilibrium.
(1) A simple model is used to calculate transpiration and assimilation rates for leaves or canopies. This is then used to investigate interactions between the various crop and environmental parameters that determine the `assimilation ratio' (the ratio between assimilation and transpiration). (2) The output from the model is discussed in relation to breeding for drought tolerance. It is concluded that the plant breeders' ideotypes will depend on the particular climate in which a crop is grown. (3) Selection for specific characters, including high photosynthetic activity, low respiration rates, and certain morphological attributes such as leaf size or disposition, could all lead to increased assimilation ratios. The greatest scope for maximizing the assimilation ratio, particularly when measured over the whole growth period of a crop, may be offered by the selection for stomatal behaviour appropriate to the expected environment. (4) Figures are presented which provide estimates of the relative effects of various plant attributes on the assimilation ratio in particular situations.
Alfalfa (Medicago sativa L.) and orchard grass (Dactylis glomerata L.) plots were exposed to ambient or ambient plus 350 cm3 m-3 carbon dioxide concentrations at Beltsville, Maryland, U.S.A. Replicate plots were established in different years and fertilized annually. We report here data for the second and third years after establishment. There has been no increase in the yearly production of either species at the elevated carbon dioxide concentration after the first season. In orchard grass, reduced growth at the high carbon dioxide concentration in the spring offset growth stimulation in the summer. Weed growth was consistently increased by carbon dioxide enrichment, but weed species composition was unaffected. Leaf photosynthetic capacity was reduced by the high carbon dioxide concentration in both crop species, as was leaf nitrogen content. Canopy carbon dioxide uptake was slightly higher in the elevated carbon dioxide treatments, consistent with the increased weed growth. In alfalfa, elevated carbon dioxide significantly reduced canopy carbon dioxide efflux at night for the same daytime uptake rate and temperature. The growth conversion efficiency estimated from elemental composition of the tissue was not substantially altered by carbon dioxide treatment in either crop species, indicating little effect of carbon dioxide treatment on the respiratory cost of tissue synthesis. Canopy conductance to water vapour averaged 23% less at high than at low carbon dioxide in the orchard grass plots, and 14% less in the alfalfa plots. This was consistent with the smaller short-term response of conductance to carbon dioxide concentration in the alfalfa plots. It is concluded that a warm climate and fertile soil does not guarantee a persistent response of production to elevated carbon dioxide concentration in these hebaceous perennial species.
Sunflower canopies were grown in mesocosom gas exchange chambers at ambient and elevated CO2 concentrations (360 and 700 ppm) and leaf photosynthetic capacities measured at several depths within each canopy. Elevated [CO2] had little effect on whole-canopy photosynthetic capacity and total leaf area, but had marked effects on the distribution of photosynthetic capacity and leaf area within the canopy. Elevated [CO2] did not significantly reduce the photosynthetic capacities per unit leaf area of young leaves at the top of the canopy, but it did reduce the photosynthetic capacities of older leaves by as much as 40%. This effect was not dependent on the canopy light environment since elevated [CO2] also reduced the photosynthetic capacities of older leaves exposed to full sun on the south edge of the canopy. In addition to the effects on leaf photosynthetic capacity, elevated [CO2] shifted the distribution of leaf area within the canopy so that more leaf area was concentrated near the top of the canopy. This change resulted in as much as a 50% reduction in photon flux density in the upper portions of the elevated [CO2] canopy relative to the ambient [CO2] canopy, even though there was no significant difference in the total canopy leaf area. This reduction in PFD appeared to account for leaf carbohydrate contents that were actually lower for many of the shaded leaves in the elevated as opposed to the ambient [CO2] canopy. Photosynthetic capacities were not significantly correlated with any of the individual leaf carbohydrate contents. However, there was a strong negative correlation between photosynthetic capacity and the ratio of hexose sugars to sucrose, consistent with the hypothesis that sucrose cycling is a component of the biochemical signalling pathway controlling photosynthetic acclimation to elevated [CO2].
One of the key questions in climate change research relates to the future dynamics of the large amount of C that is currently stored in soil organic matter. Will the amount of C in this pool increase or decrease with global warming? The future trend in amounts of soil organic C will depend on the relative temperature sensitivities of net primary productivity and soil organic matter decomposition rate. Equations for the temperature dependence of net primary productivity have been widely used, but the temperature dependence of decomposition rate is less clear. The literature was surveyed to obtain the temperature dependencies of soil respiration and N dynamics reported in different studies. Only laboratory-based measurements were used to avoid confounding effects with differences in litter input rates, litter quality, soil moisture or other environmental factors. A considerable range of values has been reported, with the greatest relative sensitivity of decomposition processes to temperature having been observed at low temperatures. A relationship fitted to the literature data indicated that the rate of decomposition increases with temperature at 0°C with a Q10 of almost 8. The temperature sensitivity of organic matter decomposition decreases with increasing temperature, indicated by the Q10 decreasing with temperature to be about 4.5 at 10°C and 2.5 at 20°C. At low temperatures, the temperature sensitivity of decomposition was consequently much greater than the temperature sensitivity of net primary productivity, whereas the temperature sensitivities became more similar at higher temperatures. The much higher temperature sensitivity of decomposition than for net primary productivity has important implications for the store of soil organic C in the soil. The data suggest that a 1°C increase in temperature could ultimately lead to a loss of over 10% of soil organic C in regions of the world with an annual mean temperature of 5°C, whereas the same temperature increase would lead to a loss of only 3% of soil organic C for a soil at 30°C. These differences are even greater in absolute amounts as cooler soils contain greater amounts of soil organic C. This analysis supports the conclusion of previous studies which indicated that soil organic C contents may decrease greatly with global warming and thereby provide a positive feed-back in the global C cycle.
We investigated the concentration and delivery of
1-aminocyclopropane-1-carboxylic acid (ACC) in the transpiration stream of
flooded and well-drained 1-month-old tomato plants
(Lycopersicon esculentum Mill. cv. Ailsa Craig) over
time in parallel with foliar ethylene production and petiole epinasty. ACC was
measured by gas chromatography using a nitrogen–phosphorus detector.
Before analysis, roots of freshly detopped plants were pressurised
pneumatically to make xylem sap flow at rates similar to those of whole plant
transpiration. Delivery of ACC from roots to shoots of well-drained plants was
sufficient to support basal ethylene production in shoots of unstressed
plants. Delivery from flooded, oxygen-deficient, roots increased after 6 h and
coincided with the onset of epinastic leaf curvature. Further increases in ACC
delivery and epinastic curvature occurred later in the photoperiod. After 24 h
flooding, ACC delivery in xylem sap was 28 times more than in well-drained
plants. This increased export of ACC from flooded roots was more than
sufficient to account for the extra ethylene production in the shoots and
coincided with ACC accumulation in the leaves. Removing the shoot before
flooding did not reduce ACC export from oxygen-deficient roots indicating that
the ACC originated in roots and not the shoot. Increased ethylene production
in petioles of flooded plants lagged 18 h behind epinasty.
Many of the most troublesome weeds in agricultural systems are C4 plants. As atmospheric CO2 increases it is conceivable that competitive ability of these weeds could be reduced relative to C3 crops such as rice. At the International Rice Research Institute (IRRI) in the Philippines, rice (IR72) and one of its associated C4 weeds, Echinochloa glabrescens, were grown from seeding to maturity using replacement series mixtures (100:0, 75:25, 50:50, 25:75, and 0:100, % rice:%weed) at two different CO2 concentrations (393 and 594 μL L-1) in naturally sunlit glasshouses. Since increasing CO2 may also result in elevated growth temperatures, the response of rice to each CO2 concentration was also examined at daylnight temperatures of 27/21 and 37/29ºC. At 27/21ºC, increasing the CO2 concentration resulted in a significant increase in above ground biomass (+47%) and seed yield (+55%) of rice when averaged over all mixtures. For E. glabrescens, the C4 species, no significant effect of CO2 concentration on biomass or yield was observed. When grown in mixture, the proportion of rice biomass increased significantly relative to that of the C4 weed at all mixtures at elevated CO2. Evaluation of changes in competitiveness (by calculation of plant relative yield (PRY) and replacement series diagrams) of the two species demonstrated that, at elevated CO2, the competitiveness of rice was increased relative to that of E. glabrescens. However, at the higher growth temperature (37/29ºC), growth and reproductive stimulation of rice by elevated CO2 was reduced compared to the lower growth temperature. This resulted in a reduction in the proportion of rice:weed biomass present in all mixtures relative to 27/21ºC and a greater reduction in PRY in rice relative to E. glabrescens. Data from this experiment suggest that competitiveness could be enhanced in a C3 crop (rice) relative to a C4 weed (E. glabrescens) with elevated CO2 alone, but that simultaneous increases in CO2 and temperature could still favour a C4 species.
Seedlings of rice cv. IR 36 were grown in soil in small pots with a horizontally divided root system: after 6-7 weeks, about 20% of the entire root system had protruded through the holes at the base of the pots and was kept in contact with nutrient solution. At this stage the plants were exposed to three different treatments: (a) the soil was kept watered and the protruding free roots were dried in air; (b) the free roots were kept moist and the soil left unwatered; (c) both soil and protruding roots were left unwatered for 30 h and then rewatered. During the first hours of treatment a and b, a decline in stomatal conductance was observed, whereas the stem water potential remained unchanged. The concentration of abscisic acid (ABA) in the xylem, however, increased. At later stages of treatment a and b, the stem water potential began to decrease with a parallel further increase of xylem ABA. Xylem sap contained considerable amounts of bound ABA, the level of which increased during total root drying and decreased again after rewatering. Level of cytokinins, zeatin (t-Z)+zeatin riboside (t-ZR) and isopentenyladenine (2iP) + isopentenyladenosine (2iPA), on the contrary, decreased during root drying and increased again after rewatering. The results are discussed with regard to a possible function of ABA and cytokinins as root-to-shoot signals.
Much research on the role of roots as ‘sensors’ of soil water
deficits (SWD) has been with plants growing in small volumes of soil. We
examined adaptive responses to SWD in processing tomatoes
(Lycopersicon esculentum Mill.) growing in the field.
The cv. Cannery Row was grown next to a rhizotron and trickle irrigated daily,
except when water was withheld for 7 or 14 days during flowering in two
deficit treatments. Rainfall was excluded.
Within 7 days of withholding irrigation, there was a substantial increase in
root production, particularly in the subsoil. However, predawn and midday leaf
water potentials did not differ between non-irrigated and control treatments
until around day 9. Even by day 14 the SWD had not affected stomatal
conductances, evapotranspiration, or plant dry mass. A rapid increase in root
death rate followed the re- irrigation of each deficit treatment. New root
production in the non-irrigated plots appeared sufficient to maintain an
adequate supply of water to the shoots. If so, this could be an even more
effective means of stress avoidance than reducing leaf and stem expansion
rates. Our results appear to be the first practical demonstration that root
systems may play such a feed-forward role.
Publisher Summary The study of leaf anatomy and of the mechanisms of the opening and closing of stomatal guard cells leads one to suppose that the stomata constitute the main or even the sole regulating system in leaf transpiration. Meteorologists have developed a wide variety of formulae for estimating evaporation from vegetation that are based entirely on weather variables and take no account at all of the species composition or stomatal properties of the transpiring vegetation. These “potential evaporation” formulae are widely and, to a large degree, successfully used for estimating evaporation from vegetation that is not water-stressed. Transpiration depends on stomatal conductance, net radiation receipt and upon air saturation deficit, temperature, and wind speed. Saturation deficit and wind speed vary through leaf boundary layers, through canopies, and through the atmosphere above the canopies. The sensitivity of saturation deficit to changes in stomatal conductance depends on where the saturation deficit is measured. If all of the stomata on a single leaf change aperture in unison, there may be a substantial change in saturation deficit measured at the leaf surface but a negligible change in saturation deficit measured a centimetre or two away, outside the leaf boundary layer.
The effects of drought on growth of red oak group species were studied by examining basal area increment and ring width index patterns of dominant Quercuscoccinea Muenchh. (scarlet oak) and Quercusvelutina Lam. (black oak) trees sampled in 1990–1991 on 62 continuous forest inventory plots located across the southeastern Missouri Ozark Mountains. Trees of both species were older on plots that had suffered high mortality and showed post-1979 reductions in growth rate compared with trees growing on low-mortality plots. Quercuscoccinea trees from high-mortality plots that were dead at the time of sampling exhibited a distinct flattening in growth rate after the mid-1930s, although death did not occur for many years. Severe droughts in 1980 and 1986–1988 were associated with further accentuated reductions in growth rate in dead trees. Dead Q. coccinea that had grown on plots with lower mortality showed comparable reductions in basal area index and similar post-1979 growth patterns, but the departure in basal area index between living and dead trees occurred 2 decades later and was associated with a severe drought during 1953–1956. Additionally, dead trees on lower mortality plots grew faster than living trees for many years before the 1953–1956 drought, suggesting that rapid early growth rates may predispose trees to early death under certain conditions. The ring width index chronologies of both species growing on high- and low-mortality plots were significantly correlated with Palmer drought severity index values, further emphasizing that drought has an important influence on growth of red oak group species in the Missouri Ozarks. Analysis of first differences of ring width index chronologies indicated that severe drought had an additional persistent effect involving long-term reductions in the sensitivity of growth to climate. The results are consistent with previously hypothesized mechanisms of stand dieback and emphasize the role of severe droughts in predisposing trees to eventual death.
A monthly dataset of Palmer Drought Severity Index (PDSI) from 1870 to 2002 is derived using historical precipitation and temperature data for global land areas on a 2.58 grid. Over Illinois, Mongolia, and parts of China and the former Soviet Union, where soil moisture data are available, the PDSI is significantly correlated (r 5 0.5 to 0.7) with observed soil moisture content within the top 1-m depth during warm-season months. The strongest correlation is in late summer and autumn, and the weakest correlation is in spring, when snowmelt plays an important role. Basin-averaged annual PDSI covary closely (r 5 0.6 to 0.8) with streamflow for seven of world's largest rivers and several smaller rivers examined. The results suggest that the PDSI is a good proxy of both surface moisture conditions and streamflow. An empirical orthogonal function (EOF) analysis of the PDSI reveals a fairly linear trend resulting from trends in precipitation and surface temperature and an El Nino- Southern Oscillation (ENSO)-induced mode of mostly interannual variations as the two leading patterns. The global very dry areas, defined as PDSI ,2 3.0, have more than doubled since the 1970s, with a large jump in the early 1980s due to an ENSO-induced precipitation decrease and a subsequent expansion primarily due to surface warming, while global very wet areas (PDSI .1 3.0) declined slightly during the 1980s. Together, the global land areas in either very dry or very wet conditions have increased from ;20% to 38% since 1972, with surface warming as the primary cause after the mid-1980s. These results provide observational evidence for the increasing risk of droughts as anthropogenic global warming progresses and produces both increased temperatures and increased drying.
Responses of individual leaves to short-term changes in CO2 partial pressure have been relatively well studied. Whole-plant and plant community responses to elevated CO2 are less well understood and scaling up from leaves to canopies will be complicated if feedbacks at the small scale differ from feedbacks at the large scale. Mathematical models of leaf, canopy, and ecosystem processes are important tools in the study of effects on plants and ecosystems of global environmental change, and in particular increasing atmospheric CO2, and might be used to scale from leaves to canopies. Models are also important in assessing effects of the biosphere on the atmosphere. Presently, multilayer and big leaf models of canopy photosynthesis and energy exchange exist. Big leaf models - which are advocated here as being applicable to the evaluation of impacts of 'global change' on the biosphere - simplify much of the underlying leaf-level physics, physiology, and biochemistry, yet can retain the important features of plant-environment interactions with respect to leaf CO2 exchange processes and are able to make useful, quantitative predictions of canopy and community responses to environmental change. The basis of some big leaf models of photosynthesis, including a new model described herein, is that photosynthetic capacity and activity are scaled vertically within a canopy (by plants themselves) to match approximately the vertical profile of PPFD. The new big leaf model combines physically based models of leaf and canopy level transport processes with a biochemically based model of CO2 assimilation. Predictions made by the model are consistent with canopy CO2 exchange measurements, although a need exists for further testing of this and other canopy physiology models with independent measurements of canopy mass and energy exchange at the time scale of 1 h or less.
Potted maize seedlings were subjected to a single period of water stress. As the severity of water stress increased, measurements were made of leaf and root solute and water potentials, leaf diffusive conductance and leaf and root growth. After day four of the drying cycle, the rate of leaf extension and the development of leaf area were reduced. This reduction correlated well with a reduction in leaf turgor which occurred at this time. A significant accumulation of solutes in the root tips of the unwatered plants resulted in the maintenance of root turgor for the duration of the water stress treatment. Root growth of the unwatered plants was also maintained as the severity of water stress increased. A mild degree of water stress resulted in a net increase in root growth compared to the situation in well-watered plants. The significance of solute regulation and continued root growth for plants growing in drying soil is discussed.
The growth and photosynethetic responses to atmospheric CO2 enrichment of 4 species of C4 grasses grown at two levels of irradiance were studied. We sought to determine whether CO2 enrichment would yield proportionally greater growth enhancement in the C4 grasses when they were grown at low irradiance than when grown at high irradiance. The species studied were Echinochloa crusgalli, Digitaria sanguinalis, Eleusine indica, and Setaria faberi. Plants were grown in controlled environment chambers at 350, 675 and 1,000 µl 1-1 CO2 and 1,000 or 150 µmol m-2 s-1 photosynthetic photon flux density (PPFD). An increase in CO2 concentration and PPFD significantly affected net photosynthesis and total biomass production of all plants. Plants grown at low PPFD had significantly lower rates of photosynthesis, produced less biomass, and had reduced responses to increases in CO2. Plants grown in CO2-enriched atmosphere had lower photosynthetic capacity relative to the low CO2 grown plants when exposed to lower CO2 concentration at the time of measurement, but had greater rate of photosynthesis when exposed to increasing PPFD. The light level under which the plants were growing did not influence the CO2 compensation point for photosynthesis.
Respiration is poorly represented in whole plant or ecosystem models relative to photosynthesis. This paper reviews the principles underlying the development of a more mechanistic approach to modelling plant respiration and the criteria by which model behaviour might be judged. The main conclusions are as follows: (1) Models should separate C substrate from structure so that direct or indirect C substrate dependence of the components of respiration can be represented. (2) Account should be taken of the fact that some of the energy for leaf respiration is drawn from the light reactions of photosynthesis. (3) It is possible to estimate respiration associated with growth, nitrate reduction, symbiotic N2fixation, N-uptake, other ion uptake and phloem loading, because reasonable estimates are available of average specific unit respiratory costs and the rates of these processes can be quantified. (4) At present, it is less easy to estimate respiration associated with protein turnover, maintenance of cell ion concentrations and gradients and all forms of respiration involving the alternative pathway and futile cycles. (5) The growth-maintenance paradigm is valuable but ‘maintenance ' is an approximate concept and there is no rigorous division between growth and maintenance energy-requiring processes. (6) An alternative ‘process-residual’ approach would be to estimate explicitly respiratory fluxes associated with the six processes listed in (3) above and treat the remainder as a residual with a phenomenological ‘ residual maintenance’ coefficient. (7) Maintenance or ‘residual maintenance’ respiration rates are often more closely related to tissue N content than biomass, volume or surface area. (8) Respiratory fluxes associated with different processes vary independently, seasonally and during plant development, and so should be represented separately if possible. (9) An unforced outcome of mechanistic models should be a constrained, but non-constant, ratio between whole plant gross photosynthesis and respiration.
Respiration rates of Lemna gibba fronds and Orobanche aegyptiaca and Lactuca sativa seedlings, were measured with a Clark type oxygen electrode in the presence or absence of a carbon-dioxide absorber (KOH) in the gas phase. Measured respiration rates in the presence of KOH were 17-34% higher than in its absence. The suppression of respiration by high CO 2 concentrations, [CO 2 ], was confirmed by parallel studies of CO 2 efflux, made by infrared gas spectrometry. These results are consistent with other reports of reduced rates of respiration at high [CO 2 ].
Measurements of respiration quotients of Lemna and Lactuca were made at 0 and 100 Pa [CO 2 ]. Results did not support the possibility of induced dark fixation of CO 2 at the ambient atmospheric [CO 2 ] predicted for the next century (35-100 Pa).
It is concluded that the numerous reports of respiration measurements made with O 2 electrodes, in the absence of a CO 2 absorber, may contain a significant error Copyright 1993, 1999 Academic Press
A severe drought in northern Arizona caused widespread pinyon (Pinus edulis) mortality, exceeding 40% in some populations. We measured tree-ring widths of pinyons that survived and that died in three sites designated as 'high,' 'medium,' and 'low' stress. Growth characteristics during the previous 10-15 years can be used to predict the likelihood of drought-induced death; dead trees exhibited 1.5 times greater variation in growth than live trees. A model of ring-width deviations vs. drought severity showed a loss of 'climatic sensitivity' with age in dead trees. These differences were independent of site. We found two distinct tree types that are predisposed to die during drought; highly sensitive young trees, and insensitive older trees. As the Southwest has a dynamic climate typified by severe droughts, it is important to understand how droughts act as bottleneck events to affect a dominant tree in a major vegetation type of the United States.
Established process-based models of forest biomass production in relation to atmospheric CO[sub 2] concentration (McMurtrie 1991) and soil carbon/nutrient dynamics (Parton et al. 1987) are integrated to derive the [open quotes]Generic Decomposition and Yield[close quotes] model (G'DAY). The model is used to describe how photosynthesis and nutritional factors interact to determine the productivity of forest growing under nitrogen-limited conditions. A simulated instantaneous doubling of atmospheric CO[sub 2] concentration lead to a growth response that is initially large (27% above productivity at current CO[sub 2]) but declines to
Past carbon (C) storage trends were estimated using dendroecological methods in a beech chronosequence in central Germany. Raw-ring-width chronologies, sensitivity curves, and carbon uptake trends were developed for 70-, 110-, and 150-year-old (S70, S110, and S150), even-aged stands. Ecosystem C stock and net ecosystem productivity (NEPC) were computed as the sum of the C stock and fluxes of the soil, the aboveground compartment, and the esti- mated belowground compartment. The ecosystem C stock ranged from 216 t C·ha-1 in S150, to 265 t C·ha-1 in S70, to 272 in S110. NEPC values followed ecosystem C stocks, ranging from 1.7, to 2.4, to 5.1 t C·ha-1·year-1 for S150, S70, and S110, respectively. Stem C-stock uptake rate in S110 showed an increase in growth rate over the first 110 years of S150. We estimate that this increase in stem C stock was 6.2%. Given the constancy of forest management among the stands of the chronosequence, we hypothesize that the increase in C stock shown by S110 is due to indirect human- induced effects. We conclude that managed young forests can take advantage of increased resources and counteract the C losses at harvest that are seen in the old forests.
Three soybean (Glycine max L. Merr.) cultivars (Maple Glen, Clark and CNS) were exposed to three CO2 concentrations (370, 555 and 740 μmol mol−1) and three growth temperatures (20/15°, 25/20° and 31/26°C, day/night) to determine intraspecific differences in single leaf/whole plant photosynthesis, growth and partitioning, phenology and final biomass. Based on known carboxylation kinetics, a synergistic effect between temperature and CO2 on growth and photosynthesis was predicted since elevated CO2 increases photosynthesis by reducing photorespiration and photorespiration increases with temperature. Increasing CO2 concentrations resulted in a stimulation of single leaf photosynthesis for 40–60 days after emergence (DAE) at 20/15°C in all cultivars and for Maple Glen and CNS at all temperatures. For Clark, however, the onset of flowering at warmer temperatures coincided with the loss of stimulation in single leaf photosynthesis at elevated CO2 concentrations. Despite the season-long stimulation of single leaf photosynthesis, elevated CO2 concentrations did not increase whole plant photosynthesis except at the highest growth temperature in Maple Glen and CNS, and there was no synergistic effect on final biomass. Instead, the stimulatory effect of CO2 on growth was delayed by higher temperatures. Data from this experiment suggest that: (1) intraspecific variation could be used to select for optimum soybean cultivars with future climate change; and (2) the relationship between temperature and CO2 concentration may be expressed differently at the leaf and whole plant levels and may not solely reflect known changes in carboxylation kinetics.
Bayesian synthesis inversion was applied to in-situ hourly
CO2 concentrations measured at Cape Grim, Australia to refine
the estimates of monthly mean gross photosynthesis, total ecosystem
respiration and net ecosystem production by the CSIRO Biospheric Model
(CBM) for eight regions in Australia for the period 1990-1998. It was
found that in-situ measurements of hourly CO2 concentrations
at Cape Grim could provide significant information about the carbon
fluxes from Tasmania, central-south and south-east Australia only. The
process-based model, CBM, overestimates the ecosystem respiration during
summer in south-east Australia, but underestimates ecosystem respiration
in Tasmania and central-south Australia. It was concluded that the
respiration submodel of CBM should be improved to account for the
seasonal variation in the plant and soil respiration parameters in
south-east Australia. For the whole period of 1990 to 1998, the mean net
ecosystem productions of terrestrial ecosystems in Tasmania,
central-south Australia and south-east Australia were estimated to be,
respectively, 6 ± 10, 7 ± 27 and -64 ± 18 Mt C
yr-1. The yearly uptake rate (being negative) of the
terrestrial ecosystems in south-east Australia was smallest (-42
± 55 Mt C yr-1) in 1998 and largest (-91 ± 52
Mt C yr-1) in 1992.
In the context of potential global warming, it is critical that ecologists bridge the typically local spatial scale of ecology to the regional scale of climatology by linking ecosystem responses to variations in the large-scale synoptic controls of regional climates. In northern Patagonia, Argentina, we related regional-scale tree mortality events over the past -100 years to annual and decadal-scale climatic variations associated with changes in the major synoptic climatic controls of the southeastern Pacific region, including the El Nifio-Southern Oscillation (ENSO). In nine stands of Austrocedrus chilensis, a xeric conifer, we used dendrochronological techniques to date the outermost tree ring on dead-standing and fallen trees to estimate the dates of tree death for 336 trees. To evaluate climatic conditions during periods of high tree mortality, we used regional records of precipitation and temperature from six climate stations and also used a regional set of 24 tree ring chronologies from Austrocedrus. Good preservation of the resinous wood of Austrocedrus allowed relatively precise dating of tree deaths over the past -90 years. Episodes of massive tree mortality coincide with exceptionally dry springs and summers during the 1910s, 1942-1943, and the 1950s. Although there is a general regional synchroneity of tree death associated with drought, intra-regional variations in the intensity of droughts, as interpolated and mapped from the regional network of tree ring chronologies, are also reflected by north-to-south variations in tree mortality patterns. Periods of drought and associated tree mortality during the 20th century in northern Patagonia are strongly associated with above average sea level atmospheric pressure off the coast of Chile at the same latitudes. Temperature and precipitation in northern Patagonia are highly influenced by the intensity and latitudinal position of the southeastern Pacific anticyclone, which, in turn, are greatly affected by ENSO. Tree mortality in northern Patagonia appears to be intensified by extreme events of the Southern Oscillation and is more strongly coincident with El Nifio events along the coast of northern Peru. These results, in combination with previously established climatic influences on fire occurrence and tree seedling establishment, strongly link stand-level and regional-scale forest dynamic processes in northern Patagonia with variations in large-scale atmospheric conditions.