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Responses of Tree Populations to Climatic Change
Abstract
The influence of climate on the population dynamics of trees must be inferred from indirect sources of information because the long lifespans of trees preclude direct observation of population growth and decline. Important insights about these processes come from 1) observations of the life histories and ecologies of trees in contemporary forests, 2) evidence of recent treeline movements in remote areas unaffected by human disturbance, and 3) results of experiments performed on forest simulation models. Each line of evidence indicates that tree population responses are influenced by many factors: including lifespans, seed productivity and dispersibility, phenotypic plasticity, genetic variability, competition, and disturbance. Some population characteristics should allow rapid changes in population sizes, while others should confer stability in times of environmental fluctuation. Interactions between controlling factors should result in a wide array of possible responses to climatic change. Interpretations of late-Quaternary forest dynamics must be based on an understanding of the biological processes involved in population responses to environmental variations.
... However, there are factors that limit the possibilities for such migration. These include seed dispersal limitations, competition pressure from resident species in new habitats, and local soil conditions (Brubaker, 1986;Stephenson, 1990;Peterson & Peterson, 1994;Korner & Paulsen, 2004). ...
... Recruitment and tree growth are particularly sensitive to climate change at species' distribution limits due to an increased frequency of sub-optimal environmental conditions causing seedling and sapling mortality, growth declines, and tree mortality (Brubaker, 1986;Lescop-Sinclair & Payette, 1995;Wang, Zhang & Ma, 2006). Responses of recruitment and growth to climate vary along the elevational gradient (Gworek, Wall & Bruss, 2007;Yu et al., 2011). ...
... All of these studies have suggested that temperature, precipitation, and soil moisture limit tree growth on Changbai Mountain, and that climate change may affect forest structure in this mountain region. However, growth data alone are inadequate to identify factors behind changes in a species distribution or forest composition (Brubaker, 1986;Wang, Zhang & Ma, 2006), since such dynamics may also be related to the recruitment patterns (Silva et al., 2012;Fisichelli, Frelich & Reich, 2013) and natural disturbance regimes (Flannigan & Bergeron, 1998;Vila-Cabrera et al., 2012). ...
To improve our understanding of climate-driven long-term dynamics of eastern Asian mountain forests, we used field surveys and dendrochronological techniques to examine regeneration density, growth rate of mature trees, and growth sensitivity to climate of 3 common coniferous tree species at their respective altitudinal distribution limits on Changbai Mountain, northeastern China. The studied species were Manchurian fir (Abies nephrolepis, distributed between 780 and 1750 m asl), Korean pine (Pinus koraiensis, 780 and 1300 m asl), and Jezo spruce (Picea jezoensis var. komarovii, 1000 and 1750 m asl). Regeneration densities did not differ significantly among the elevations except for Jezo spruce, which showed a significantly lower regeneration density at 1000 m asl as compared to 1300 and 1750 m asl. All 3 species showed a significantly higher basal area increment (BAI) at the middle part of their distribution ranges than at their limits. The growth of Manchurian fir and Jezo spruce exhibited higher sensitivity to precipitation than to temperature at their lower distribution limits, and the inverse pattern was observed at the upper limit. In all cases the correlations between growth and the respective climate variable were positive, except for the correlation between Jezo spruce growth and precipitation. Growth of Korean pine was positively correlated with spring temperature and summer moisture at its lower distribution limit and with summer temperature at its upper limit. Our study suggests that elevational limits of forest vegetation were likely constrained by climate factors affecting growth of dominant species rather than those controlling regeneration density.
... Production of pollen profiles for an individual site, the correlation of pollen profiles from several sites within a region and the construction of pollen isochrone maps based on the pollen stratigraphic changes, are all techniques that have been used to investigate the pattern and timing of tree spreading across Europe during the earlier Holocene [1,2]. Maps based on ages for pollen zone boundaries [3,4], particularly the first arrival (the empirical limit) or the point at which a pollen curve rises to sustained higher values (the rational limit), have proven invaluable in understanding the progressive reforestation of the landscape after glaciation, as tree populations responded to climate amelioration [5][6][7][8][9][10][11] and expanded from refugia [12][13][14] to which they had withdrawn during the glacial period. From the context of Britain and Ireland, these refugia were mostly in southern Europe and distant [4], but it is possible that small refugia existed in southern England [15,16] and so of immediate relevance to the Irish Sea area and therefore to the subject of this paper, the Isle of Man. ...
... Their effects were apparently not felt on the island, as is the case for many pollen diagrams in Britain and Ireland, perhaps because of its oceanic location [156,160]. There would have been a time-lag before tree populations responded to sudden climate deterioration [5] and severe but very brief cold phases might not have time to register given the sampling interval of most pollen diagrams. ...
The Isle of Man is a large island which lies in the middle of the northern Irish Sea between Britain and Ireland and, because of its insularity and size, has an impoverished flora compared with the two main islands. This has been the case throughout the postglacial and warrants the island’s description as a separate phytogeographic province. We have considered Holocene tree pollen data from seventeen sites on the island which together preserve a vegetation history that spans the six thousand years of the early and mid-postglacial from the end of the Lateglacial at 11,700 cal. BP to the mid-Holocene Ulmus decline at ca. 5800 cal. BP. Radiocarbon dating of the rational limits of the pollen curves for the main tree taxa has allowed an appraisal of the timing of each one’s expansion to become a significant component of the island’s woodland, and comparison with the dates of their expansion on the adjacent regions of Britain and Ireland. The radiocarbon dates show that, although some variability exists probably due to local factors, there is considerable concordance between the timings of major pollen zone boundaries in Britain and Ireland around the northern Irish Sea. On the Isle of Man the expansions of both Juniperus and Betula were delayed by several centuries compared to the British/Irish data, however the timing of the expansions of Corylus, Ulmus, Quercus, Pinus and Alnus on the Isle of Man all appear closely comparable to the ages for these pollen stratigraphic events in north Wales, northwest England, southwest Scotland and eastern Ireland, as are those for the Ulmus decline. It is likely that local pedological and edaphic factors on the island account for the differences in the first Holocene millennium, while regional climatic factors governed the timings for the rest of the expansions of tree taxa across the wider region, including the Isle of Man. Disturbance, including by human agency, was important at the site scale and perhaps triggered early tree expansion in some places, including Quercus, Ulmus and Alnus. Insularity seems not to have been a significant factor in the expansion of the major forest trees.
... Our ability to predict climate-induced shifts of tree populations is limited by an insufficient understanding of the effects of climate variability on tree recruitment (Brubaker, 1986). The effects of climate variability on tree recruitment may differ greatly depending on the spatiotemporal scale of study (Daniels and Veblen, 2004). ...
... The research on recruitment dynamics is a priority in the case of isolated populations located at the limit of the species' geographical range because these stands may be more sensitive to recent climate warming if they are located near the limit of the species' climatic tolerance (Brubaker, 1986). Isolated tree populations of pine species restricted to high elevations become isolated from each other on different mountains separated by unsuitable habitat, with consequent reduced gene flow and genetic differentiation (Oline et al., 2000). ...
To infer future changes in the distribution of tree species in response to climatic variability, we need an understanding of the recruitment dynamics and their climatic controls at the species’ distribution limit. We studied the recruitment processes in an isolated population of Pinus uncinata Ram. located at the southwestern limit of the species’ distribution in Europe (Iberian System, NE Spain). We assessed (1) the temporal patterns of pine recruitment, and (2) how climate influenced recruitment. To reconstruct the recent recruitment episodes and to assess the climatic influence on recruitment and radial growth we employed dendrochronological methods. We mapped, measured the size, and estimated the age of all P. uncinata individuals located within a 50 m × 40 m plot. Additional age data were obtained from individuals located in four nearby 20 m × 20 m plots. The main episodes of tree establishment (early 1960s, late 1980s) coincided with low radial growth during a period with reduced grazing pressure. Grazing pressure and tree recruitment were not related at the spatiotemporal scale of this study. High May, August, and September minimum temperatures and high April precipitation were positively associated with recruitment, whereas high maximum April and June temperatures were negatively associated with recruitment. The studied population was in equilibrium with climate until the late 1990s, one of the warmest decades in the 20th century, when recruitment decreased despite the availability of suitable sites for establishment and the presence of reproductive individuals. We suggest that late-summer temperatures might have a non-linear negative threshold effect on recruitment rather than a linear effect. Despite increasing evidence of climate-induced recruitment episodes in isolated cold mountain forests, threshold effects of temperature on recruitment may imply limited range shifts of these populations in response to climate warming.
... Climate variables are major drivers of species occurrences and assemblages at a given location. Changes in climate at a local and regional scale will result in varying changes to forests and associated tree populations (Brubaker 1986;Dynesius and Jansson 2000;Rehfeldt et al. 2014b, a). Species ranges have historically shifted with past changes in climate (Prentice et al. 1991;Dynesius and Jansson 2000) but rapid shifts brought about by anthropogenic climate change will likely outpace the ability of tree species to expand their ranges or migrate in many areas, particularly in northeastern forests (Burrows et al. 2011;Sittaro et al. 2017). ...
... • Mature trees in a given species can survive centuries past loss of suitable climatic conditions (Brubaker 1986;Noss 2001). Preserve legacy trees following harvest to allow for maintenance of locally adapted seed source during periods of less than ideal climate and to preserve essential forest network structure (retention, root structure, memory) ...
A review of the literature reveals the strengths and limitations of various climate adaptation frameworks and illuminates a general path by which a type of adaptation can be achieved. A number of useful frameworks exist but the number of independent case studies demonstrating the adaptation process in a detailed manner is much more limited. Additionally, components of the various adaptation processes can often seem vague and concepts such as adaptability ill-defined. For land managers approaching climate adaptation independently can be difficult, particularly in the areas of goal creation and vulnerability assessment. Within frameworks where user-defined adaptation goals dictate whether or not adaptability will be achieved, providing guidance on definition of these concepts is particularly important. To explore and improve the usability of climate adaptation concepts we applied them on Michigan Technological University’s Ford Forest. We reviewed the literature on climate change adaptation and applied the knowledge gained to the creation of a climate change adaptation-focused management plan. We assessed the difference between business-as-usual management and climate adaptation and identified where either had occurred in order to 1) better define climate change adaptation operationally, and 2) demonstrate how and where it had occurred within our plan. In doing so we hope to demonstrate explicitly methods for climate change adaptation planning and expand the definition of climate adapted systems. Through application of the principles of these frameworks, we have found that identifying priority values early in the management planning process while recognizing future climate uncertainty will improve the ability to generate meaningful, effective management actions. Recognition of organizational limitations and potential flaws in the decision-making process can help to improve planning outcomes. We have proposed a logical way to assess decision-making outcomes in a climate adaptation planning context. Vulnerability indices are useful for identifying areas of risk in a forest, but a general focus on adaptability is still necessary to respond to future climate uncertainty. Operationally, climate change adaptation refers to the broad category of planning and management measures undertaken to protect specific values from the negative effects of anthropogenic climate change.
... In the foothills region of Alberta, anthropogenic changes to climate and fire are two such dynamics. Warming is predicted to alter the mortality and establishment rates of trees (Brubaker, 1986;Allen et al., 2010;Luo & Chen, 2013), which may explain a recently observed demographic shift in Alberta's forests (Zhang et al., 2015). While regeneration rates are likely to decline in some areas, a number of models predict that fire frequency, size, intensity, and thus tree mortality rates (severity), will likely increase under warming (Flannigan et al., 2001;Ali et al., 2012;de Groot et al., 2013). ...
... An increasing magnitude of climatic disequilibrium is likely to reduce forest regeneration, which may be initiated through climate-driven changes to disturbance regimes (Magnani et al., 2007). Some studies indicate that forest regeneration conditions should be improving at higher elevations and latitudes (Brubaker, 1986;Lenoir et al., 2009) and declining in lower elevation forests (Loarie et al., 2009;Bertrand et al., 2011b), while others provide mixed results potentially related to changes in human activity (Boisvert-Marsh et al., 2014). Here, the findings support a relative improvement in regeneration conditions in lowelevation northern forests (Crimmins et al., 2011;Zhu et al., 2012Zhu et al., , 2014Dobrowski et al., toward the foothills and parkland regions, indicating that changes to soil water balance may drive future species migrations under warming (Crimmins et al., 2011;Piedallu et al., 2013). ...
In western Canada, the effects of warming and increasing human activity may alter the structure, composition, and function of forests, producing quantitatively and qualitatively different understory light conditions. While difficult to measure directly, process-based models may facilitate inference of historical forest states. Yet, existing models are limited in the dynamics they represent. A promising new approach in hybrid modeling, first demonstrated here, is the fusion of machine learning and process-based models to simulate pattern-based processes. The objective of this dissertation was to simulate the effects of past-century climate and fire conditions on understory global solar irradiation trajectories across a 25.2 million ha landscape in Alberta, Canada. The LANDIS-II forest landscape model was applied to simulate past-century changes to competition, fire, and regeneration. Simulated tree species and age maps were classified into landcover types. A regression model of canopy light transmission as a function of landcover and site index showed good fit with field observations (R2 = 0.94) and was applied to a classification of LANDIS-II outputs. Canopy light transmission was multiplied by mean annual bare-earth global solar irradiation to produce understory light maps. Empirical and semi- mechanistic fire models were also applied. A variant of stochastic gradient descent was applied for parameter optimization, improving fire model performance (R2 = 0.96; ΔR2= +0.14). Simulations showed a mild decline in forested area across the 1923-2012 period, attributable to a velocity of warming three times faster than migration. Migration was primarily controlled by fire and secondarily by regeneration. Simulated understory light levels declined across the period due to reduced mortality rates, preceding a likely long-term increase in light attributable to reduced regeneration rates. The key innovations of this work are as follows: characterization of human-dominated fire regimes in western Alberta (Chapter 4); advancement of the TACA-GEM regeneration model (Chapter 5); development of an algorithm for fire model parameter optimization (Chapter 6); development of new LiDAR models of canopy light transmission (Chapter 7); demonstration of a new hybrid modeling approach to simulating pattern-based processes, applied to understory light (Chapter 8); demonstration of long-term climatic regulation of understory solar irradiation through forest regeneration (Chapter 8).
... Research on treeline dynamics is useful for understanding responses of trees and ecotones to past and present climate changes (LaMarche, 1973;Payette et aI., 1985;Brubaker, 1986;Payette and Lavoie, 1994). Treeline ecotones are dynamic zones potentially sensitive to climate change (Hansen and di Castri, 1992;Johnston et aI., 1992). ...
... In this last ecotone, smaller and less consistent ecological boundaries appear because the spatial gradients are more subtle, forming fine-grained spatial patterns (van der Maarel, 1990). According to van der Maarel (1990), gradual ecotones are theoretically more sensitive to environmental changes, responding more rapidly to abiotic factors, mainly in limiting situations like treeline environments (Brubaker, 1986). Different ecological factors influence spatial structure and boundary properties in the ecotone, Conversely, boundary characteristics may determine tree population response in forestgrassland ecotones. ...
Spatial identifcation and description of ecological boundaries is fundamental to better understanding of treeline dynamics. Ecological boundaries across two contrasting subalpine Pinus uncinata forest-alpine grassland ecotones were delineated within the Central Pyrenees (Ordesa and Tessó sites). Boundaries were delineated using an edge detection algorithm for two-dimensional data (lattice-wombling). Tree density, size-structure, growth-form, and estimated age were used to reveal spatial location of boundaries for several size and growth-form tree classes. Overlap statistics were applied to quantify spatial relationships among boundaries determined for different sets of variables. The most significant and consistent boundaries were those for structural variables at the Ordesa site. At this site, the sequential disposition of bigger and unistemmed trees descending across the ecotone produced boundaries for size-structure and growth-form variables. These boundaries were located along an ordered spatial pattern (altitudinal diagonal). At the Tessó site, there were few consistent boundaries, most of which were developed along the slope. Overlap statistics showed that boundaries at the Ordesa site were more spatially related than were those at the Tessó site. This result held when any set of variables was considered. The studied ecotones describe sharp (Ordesa site) and gradual (Tessó site) structural changes in tree populations, related to situations similar to the ecotone and ecocline concepts, respectively. The possible environmental driving factors producing these patterns are the strong winds and reduced snow cover at higher altitudes at the Ordesa site, and snow avalanches at the Tessó site. Boundary detection through time in permanent plots might be a better tool for monitoring climate-change impact in the forest-alpine grassland ecotone than the subjective location of treelines.
... rasgos de tolerancia a frío). Este compromiso está fuertemente ligado a la fenología de brotación y cierre de yemas y la consecuente duración del periodo de crecimiento, en un balance que busca por un lado escapar a las heladas tempranas o tardías protegiendo los meristemas en estructuras estancas, y por otro maximizar el crecimiento para ganar en la competencia con sus congéneres, pudiendo este balance variar en diferentes estadíos ontogénicos (Brubaker 1986, Stettler and Bradshaw 1994, Hurme y col. 1997, Dunne y col. ...
... 2001). El estadío de plantín en árboles es especialmente sensible (Campbell 1979, Brubaker 1986, Rehfeldt y col. 1999) por la gran variación microclimática cerca del piso del bosque debida a la inversión térmica, pero también por grandes presiones ambientales (por ejemplo nieve o avalanchas) y por menor resistencia intrínseca a ambientes estresantes en estadíos tempranos. ...
La diversidad genética es un requisito para el cambio evolutivo. Por consiguiente, la conservación de la diversidad genética dentro de las especies es importante para asegurar el potencial de adaptación en un medio ambiente cambiante. La variación genética de los rasgos de importancia adaptativa se puede medir mediante la observación de la variación fenotípica en los ensayos de ambiente común.
De acuerdo a su área de distribución y productividad potencial, Nothofagus pumilio (Poepp. et Endl.) es la especie arbórea nativa más importantes de la Patagonia. Se halla en bosques puros en el límite superior arbóreo a ambos lados de la Cordillera de los Andes. El objetivo de este estudio fue evaluar la variación geográfica y genética de poblaciones naturales de la especie en rasgos cuantitativos potencialmente adaptativos. Se muestrearon poblaciones naturales de la Provincia de Chubut representando los tres gradientes ambientales más relevantes de su área de distribución: latitudinal, altitudinal y de precipitación. Se analizó variación natural en caracteres seminales y se instalaron ensayos de ambiente común en los que se evaluaron variables correspondientes a plántulas en invernadero y plantines en campo. Con los datos obtenidos se estudió variación en caracteres cuantitativos entre y dentro de poblaciones. Los resultados principales muestran evidencias de variación clinal y ecotípica en los gradientes latitudinal y altitudinal, respectivamente, con indicios de adaptación local en el caso del gradiente altitudinal. También se verificó variación genética entre morfotipos. En algunas de las variables consideradas se registró una importante diversidad genética, tanto entre como dentro de poblaciones, mientras que en otras la diversidad fue muy baja.
En la discusión se da cuenta del complejo patrón de variación geográfica y genética de la especie, presumiblemente determinado por factores históricos (probablemente relacionado con la última glaciación) y la presión de selección que opera actualmente. Las diferencias fueron evidentes en los gradientes evaluados y entre los diferentes tipos de caracteres, operando procesos de adaptación local y plasticidad fenotípica en un equilibrio variable.
Palabras claves: adaptación, conservación, plasticidad fenotípica, genética cuantitativa, Nothofagus pumilio.
... Tree lines occur at the point where carbon gained during the summer is equal to, or only just greater than, that lost due to respiration or tissue mortality through lack of hardening to withstand winter temperatures (Wardle 1965 ). Because summer temperatures and the length of the growing season strongly influence carbon production and shoot hardening, higher temperatures should increase tree productivity and survival at the tree line (Brubaker 1986). Tree line responses to warming over the last century have been detected in many parts of the Northern Hemisphere (see Kullman 1990; Rochefort et al . ...
... Disturbances could also modify the response of tree line populations to climate change. Light-demanding tree species that rely on disturbances for recruitment may be unable to respond to climate warming unless it coincides with canopy opening (Brubaker 1986), whereas the response of tree line forests to warming may be enhanced during periods of disturbance (Overpeck et al . 1990). ...
1. We used forest stand history reconstruction to infer the relative roles of disturbance and climate warming on the population dynamics of Nothofagus menziesii (silver beech) dominated tree lines in north Westland, South Island, New Zealand. 2. Stem recruitment in tree line forests over the last 300 years has been episodic, has tended to occur in small, scattered parches, and has been dominated by the production of new stems from existing trees. Pulses of stem recruitment also coincide with episodes of abrupt decline in the radial growth of established trees. These patterns suggest that infrequent natural disturbances form localized canopy openings, damage trees that survive the event, and initiate the establishment of new trees and the production of new stems by surviving trees that fill these openings. 3. Climate warming in New Zealand since 1950 has had little effect on the recruitment of Nothofagus close to the tree line. There is a large seedling pool within the tree line forests, but recruitment will probably require a disturbance-related canopy opening. 4. Natural disturbances drive the population dynamics of Nothofagus tree lines and may modify their response to climate warming. Unlike many Northern Hemisphere tree lines, there has been no recent upward movement of the tree line or increase in seedling establishment. This difference could reflect the greater importance of natural disturbance for recruitment in the abrupt closed-canopy tree lines formed by light-demanding Nothofagus species in the Southern Hemisphere. However, given the ubiquity of disturbance effects in low-altitude forests in both hemispheres, future studies need to consider their role when investigating any tree line response to climate warming.
... This has been central to the approach we develop here with red spruce. Habitat fragmentation and anthropogenic disturbance have been found to contribute to genetic erosion, i.e., the loss of GD due to inbreeding and genetic drift (Woodruff, 2001), and reduce the adaptive responses of populations experiencing environmental stress (Brubaker, 1986;Frankham et al., 1999;Bijlsma and Loeschcke, 2012;González et al., 2020). In that context, GD is recognized as an important element of conservation planning and policy implementation (Hoban et al., 2021). ...
Premise
Global anthropogenic change threatens the health and productivity of forest ecosystems. Assisted migration and reforestation are tools to help mitigate these impacts. However, questions remain about how to approach sourcing seeds to ensure high establishment and future adaptability.
Methods
Using exome‐capture sequencing, we demonstrate a computational approach to finding the best n‐sets from a candidate list of seed sources that collectively achieve high genetic diversity (GD) and minimal genetic load (GL), while also increasing evolvability in quantitative traits. The benefits of this three‐part strategy (diversity‐load‐evolvability) are to increase near‐term establishment success while also boosting evolutionary potential to respond to future stressors. Members of The Nature Conservancy and the Central Appalachian Spruce Restoration Initiative planted 58,000 seedlings across 255 acres. A subset of seedlings was monitored for establishment success and variation in growth.
Results
The results show gains in GD relative to GL and increases in quantitative genetic variation in seedling growth for pooled vs. single‐source restoration. No single “super source” was observed across planting sites; rather, monitoring results demonstrate that pooling of multiple sources helps achieve higher GD:GL and evolvability.
Discussion
Our study shows the potential for integrating genomics into local‐scale restoration and the importance of building partnerships between academic researchers and applied conservation managers.
... Tree growth plasticity enables trees to grow slowly during periods with poor growing conditions; however, when the conditions improve, increased tree growth resumes [56]. Plants exhibit resistance to external stress through morphological and physiological changes that improve their adaptability to environmental stress [57,58]. ...
Tree growth strongly responds to climate change, especially in semiarid mountainous areas. In recent decades, China has experienced dramatic climate warming; however, after 2000 the warming trend substantially slowed (indicative of a warming hiatus) in the semiarid areas of China. The responses of tree growth in respect to elevation during this warming hiatus are poorly understood. Here, we present the responses of Qinghai spruce (Picea crassifolia Kom.) growth to warming using a stand-total sampling strategy along an elevational gradient spanning seven plots in the Qilian Mountains. The results indicate that tree growth experienced a decreasing trend from 1980 to 2000 at all elevations, and the decreasing trend slowed with increasing elevation (i.e., a downward trend from −10.73 mm 2 year −1 of the basal area increment (BAI) at 2800 m to −3.48 mm 2 year −1 of BAI at 3300 m), with an overall standard deviation (STD) of 2.48 mm 2 year −1. However, this trend reversed to an increasing trend after 2000, and the increasing trends at the low (2550-2900 m, 0.27-5.07 mm 2 year −1 of BAI, p > 0.23) and middle (3000-3180 m, 2.08-2.46 mm 2 year −1 of BAI, p > 0.2) elevations were much weaker than at high elevations (3300 m, 23.56 mm 2 year −1 of BAI, p < 0.01). From 2000-2013, the difference in tree growth with elevation was much greater than in other sub-periods, with an overall STD of 7.69 mm 2 year −1. The stronger drought conditions caused by dramatic climate warming dominated the decreased tree growth during 1980-2000, and the water deficit in the 2550-3180 m range was stronger than at 3300 m, which explained the serious negative trend in tree growth at low and middle elevations. After 2000, the warming hiatus was accompanied by increases in precipitation, which formed a wetting-warming climate. Although moisture availability was still a dominant limiting factor of tree growth, the relieved drought pressure might be the main reason for the recent recovery in the tree growth at middle and low elevations. Moreover, the increasing temperature significantly promoted tree growth at 3300 m, with a correlation coefficient between the temperature and BAI of 0.77 (p < 0.01). Our results implied that climate change drove different growth patterns at different elevations, which sheds light into forest management under the estimated future climate warming: those trees in low and middle elevations should be paid more attention with respect to maintaining tree growth, while high elevations could be a more suitable habitat for this species.
... It was also decided to check for significant trends in the nitrogen (N) concentrations in current's year needles of fir trees in the period 1997-2019 as Abies species, especially silver fir (Abies alba Mill.), Greek fir (Abies cephalonica Loudon) and Bulgarian fir (Abies borisii-regis Mattf), are sensitive to environmental changes (Potocic et al., 2005). Tree populations found at the limit of the species geographical distribution may be responding more dramatically to climate change than those at the core of the range (Brubaker, 1986). In higher altitudes, the increase in temperature can enhance the mineralization of organic matter and consequently organic N. The forest stand under consideration was surveyed for the N content in needles in the past (Michopoulos et al., 2015) because of the high N content in soils. ...
In the present work meteorological data, concerning temperature and precipitation in a mountainous Bulgarian fir (Abies borisii-regis Mattf.) forest, was analyzed in order to find trends for the period 1973-2019. The results showed that temperature values on annual and seasonal base, especially in summers, showed a significant increasing trend. In addition, the number of days with particularly high temperatures increased together with the daily temperature range. With regard to the rain height an increasing trend was found significant at the 0.1 probability level. The nitrogen concentration in current year needles of the Bulgarian fir measured in the period 1997-2019 showed an increasing trend. This fact could be a sign of increased mineralization rates of organic matter as a response to the temperature increase.
... Competition indirectly affects plant distribution by altering the original living environment and resource conditions [42]. The dynamics of age structure and quantitative seedling characteristics of long-lived plant populations can be used as indicators of change in environmental and resource conditions caused by competition [43,44]. The age structure of a plant population refers to the age-matching status of individuals in the population, and the number of individuals at different age stages reflects dynamics changes and development trends of the population to a certain extent [45]. ...
The Chinese pine (Pinus tabuliformis) community on the ridge is one of the most important zonal forest communities on the southern slope of the mid-Qinling Mountains. This study aimed to investigate the driving factors of Chinese pine population distribution in the ridge habitats and its adaptability characteristics. Population age structure and the relationship between regeneration dynamics and environmental factors were investigated in 32 plots in the Huoditang Forest region. The results showed that the niche of Chinese pine was wide but overlapped greatly with that of Quercus aliena var. acutiserrat, an oak species. The population in the ridge habitats exhibited an expansion trend, while that inhabiting slope habitats was declining. Seedling density in ridge habitats was much higher than that of the understory in the slope habitats. Still, the seedling growth rate in both ridge and understory habitats was much lower than that characteristic of gap habitats. Seedling density positively correlated with understory solar conditions, while growth positively correlated with soil fertility, indicating that environmental factors significantly influence the regeneration process. Thus, light conditions and intrinsic biological traits of Pinus tabuliformis influence its distribution. In ridge habitats, sufficient light conditions promote Pinus tabuliformis regeneration and recruitment of larger classes, but poor soil conditions also limit its growth.
... More specifically, it is generally approved that the history and dynamics of the alpine and arctic treeline ecotones, at population and landscape levels, reflect recent and past climate change and variability in interaction with local drivers and circumstances, eg. topoclimate, herbivory and land use (Payette et al. 1985;Brubaker 1986;Kullman 1998a;Holtmeier 2009;Kullman & Öberg 2009). Preferably, repeated present-day insitu records, combined with megafossil historical inferences, can provide this perspective in a proper way. ...
... Studies of climatic influence on trees are also complicated by the fact that intensity of climatic reaction is modulated by a whole spectrum of external and internal factors from local conditions to ontogenetic or phenotypic characteristics [6,[41][42][43][44]. Moreover, the impact of these factors can also be ambiguous. ...
Siberian stone pine (Pinus sibirica Du Tour) is one of the keystone conifers in Siberian taiga, but its radial growth is complacent and thus rarely investigated. We studied its growth in subalpine stands near the upper timberline along theWestern Sayan Mountains, Southern Siberia, because climatic responses of trees growing on the boundaries of species distribution help us better understand their performance and prospects under climate change. We performed dendroclimatic analysis for six tree-ring width chronologies with significant between-site correlations at distances up to 270 km (r = 0.57–0.84, p < 0.05). We used ERA-20C (European Reanalysis of the Twentieth Century) daily climatic series to reveal weak but spatially coherent responses of tree growth to temperature and precipitation. Temperature stably stimulated growth during the period from the previous July–August to current August, except for an adverse effect in April. Precipitation suppressed growth during periods from the previous July–September to December (with reaction gradually strengthening) and from the current April to August (weakening), while the snowfall impact in January–March was neutral or positive. Weather extremes probably caused formation of wide tree rings in 1968 and 2002, but narrow rings in 1938, 1947, 1967, 1988, and 1997. A subtle increase in the climatic sensitivity of mature trees was observed for all significant seasonal climatic variables except for the temperature in the previous October–January. The current winter warming trend is supposedly advantageous for young pine trees based on their climatic response and observed elevational advance.
... Moreover, ecotones are also likely to be areas of new colonization, such as at upper and lower treelines, forest-grassland ecotones, and more generally at the climatic limits of species distributions (Allen and Breshears, 1998;Brubaker, 1986;Thuiller et al., 2008;Williams et al., 2010). For instance, warmer temperatures and longer growing seasons have led to increased tree growth and productivity, and new colonization into areas that were not previously occupied by trees at highelevation sites (Peterson et al., 2002;Peterson, 2001, 1994;Zald et al., 2012). ...
Rising greenhouse gases are changing the Earth’s climate and adversely affecting ecosystems that currently provide a suite of invaluable benefits, from cleaning water to sequestering carbon. Some of the world’s most productive forests grow in the Pacific Northwest region of North America, but our understanding of climate change effects on these forests and their carbon is still emerging. Here, we synthesize the current state of research (including empirical, paleo, and modeling studies), discuss the implications on forest growth and carbon storage in Pacific Northwest forests, and identify key knowledge gaps and future research opportunities based on a combination of published studies and expert opinion. Two case studies are presented that illustrate the expected effects of climate change on moist and dry forest ecology and carbon storage. In response to these impacts, we highlight a number of appropriate regional forest restoration and management adaptation strategies. Filling in knowledge gaps will improve the accuracy of forest carbon accounting, a crucial part of the strategy to meet climate mitigation targets and prevent the most severe impacts of climate change.
... The focus of our research was on juvenile quantitative trait variation. Natural selection, and therefore adaptation, intervenes along tree life, but seedling establishment is one of the most sensitive stages (Brubaker 1986;Green 2005). In addition, regeneration offers the highest levels of diploid genetic variation (with the exception of the embryo stage), and over this variation selection acts early. ...
The Patagonian cypress is the native conifer with the largest distribution area in Argentina: its binational distribution and ecological range are presented. Its wood is quite appreciated and used in house building of North Andean-Patagonia. Isozyme and microsatellite markers were used to investigate the geographic patterns of neutral genetic variation, advancing towards structure analyses and the definition of genetic zones. The study of variation in key quantitative juvenile traits, through nursery and field common garden trials, allowed gaining knowledge on adaptive variation: seedling growth and seasonal rhythms, architecture, physiology, drought stress tolerance, and survival traits were considered. Starting on a regional scale analysis, first results led to compare humid and xeric populations and finally to concentrate on marginal populations from the steppe. Based on results from markers and quantitative traits, provenance regions were delineated. The low-intensity breeding proposal for the species lies on the identification of seed production areas and the installation of progeny seed orchards for each of the provenance regions separately. A provenance and progeny tests network was installed.
... Forest ecosystems in elevated regions are under dynamic influences of climate, geomorphic conditions and human impact (Brubaker, 1986;Jonášová and Prach, 2004;Pauchard and Alaback, 2004). National parks established to protect the last remnants of natural forests are unique and suitable spots to study these three main agents shaping the tree growth pattern and the spatial extent of vegetation (Whittaker, 1960;Evangelista et al., 2016;Gazda et al., 2019). ...
During changing climates, tree species distribution and productivity are subject of dynamic changes. However, two other factors can play a role in forest development: human impact and terrain properties. Furthermore, terrain properties can frequently modify climatic limitation of tree species growth both positively and negatively. Here, we chose to study five national parks in southern Poland. We tested the impact of climate, soil type and geomorphic indices on the occurrence and biomass of tree species using random forest models. We assumed that despite the presence of human impact, fundamental relationships between landscape properties and climate were still detectable in the selected parks. Elevation and valley depth were the most important individual predictors of tree species distribution; site-specificity was an additional important factor. In addition to the strong age-dependency of aboveground biomass (mainly for Norway spruce Picea abies (L.) Karst and European beech Fagus sylvatica L.), elevation negatively impacted the productivity of all tree species. Additionally, the topographic wetness index negatively affected the biomass of F. sylvatica, while the slope positively influenced its biomass. Using partial dependence plots, we described how geomorphic variables modify climate-dependent elevational patterns of species studied distributions and biomass. Due to protection and preservation of these sites as provided by the national park system, we could separate the effects of particular variables on tree species studied. Our results broaden understanding of the influence of geomorphological variability on species distributions under similar climatic and soil conditions. This allows for predicting sites with a higher probability of species persistence under changing climates. Therefore, our results might be used to identify sites less vulnerable to climate change, therefore important for conservation prioritization.
... Tree growth plasticity enables trees to grow slowly during periods with poor growing conditions; however, when the conditions improve, increased tree growth resumes [56]. Plants exhibit resistance to external stress through morphological and physiological changes that improve their adaptability to environmental stress [57,58]. ...
Tree growth strongly responds to climate change, especially in semiarid mountainous areas. In recent decades, China has experienced dramatic climate warming; however, after 2000 the warming trend substantially slowed (indicative of a warming hiatus) in the semiarid areas of China. The responses of tree growth in respect to elevation during this warming hiatus are poorly understood. Here, we present the responses of Qinghai spruce (Picea crassifolia Kom.) growth to warming using a stand-total sampling strategy along an elevational gradient spanning seven plots in the Qilian Mountains. The results indicate that tree growth experienced a decreasing trend from 1980 to 2000 at all elevations, and the decreasing trend slowed with increasing elevation (i.e., a downward trend from −10.73 mm2 year−1 of the basal area increment (BAI) at 2800 m to −3.48 mm2 year−1 of BAI at 3300 m), with an overall standard deviation (STD) of 2.48 mm2 year−1. However, this trend reversed to an increasing trend after 2000, and the increasing trends at the low (2550–2900 m, 0.27–5.07 mm2 year−1 of BAI, p > 0.23) and middle (3000–3180 m, 2.08–2.46 mm2 year−1 of BAI, p > 0.2) elevations were much weaker than at high elevations (3300 m, 23.56 mm2 year−1 of BAI, p < 0.01). From 2000–2013, the difference in tree growth with elevation was much greater than in other sub-periods, with an overall STD of 7.69 mm2 year−1. The stronger drought conditions caused by dramatic climate warming dominated the decreased tree growth during 1980–2000, and the water deficit in the 2550–3180 m range was stronger than at 3300 m, which explained the serious negative trend in tree growth at low and middle elevations. After 2000, the warming hiatus was accompanied by increases in precipitation, which formed a wetting–warming climate. Although moisture availability was still a dominant limiting factor of tree growth, the relieved drought pressure might be the main reason for the recent recovery in the tree growth at middle and low elevations. Moreover, the increasing temperature significantly promoted tree growth at 3300 m, with a correlation coefficient between the temperature and BAI of 0.77 (p < 0.01). Our results implied that climate change drove different growth patterns at different elevations, which sheds light into forest management under the estimated future climate warming: those trees in low and middle elevations should be paid more attention with respect to maintaining tree growth, while high elevations could be a more suitable habitat for this species.
... That vegetation could lag behind climatic changes is particularly relevant at the species level. Species demography, seed dispersibility, phenotypic plasticity, interspecific competition, gene flow, and mutation rates may all affect how well individual species track climate (Brubaker, 1986;Corlett & Westcott, 2013;Davis, Shaw, & Etterson, 2005;Sittaro, Paquette, Messier, & Nock, 2017), potentially leading to asynchronous migrations that produce novel species assemblages (Alexander, Diez, & Levine, 2015). As such, the combinations of species assimilating in the future are uncertain and may diverge from the vegetation communities that occupy today's climate space. ...
Climate change poses a serious threat to biodiversity and unprecedented challenges to the preservation and protection of natural landscapes. We evaluated how climate change might affect vegetation in 22 of the largest and most iconic protected area (PA) complexes across North America. We use a climate analog model to estimate how dominant vegetation types might shift under mid‐ (2041–2070) and late‐century (2071–2100) climate according to the RCP 8.5 scenario. Maps depicting vegetation for each PA and time period are provided. Our analysis suggests that half (11 of 22) of the PAs may have substantially different vegetation by late‐21st century compared with reference period conditions. The overall trend is toward vegetation associated with warmer or drier climates (or both), with near complete losses of alpine communities at the highest elevations and high latitudes. At low elevation and latitudes, vegetation communities associated with novel climate conditions may assemble in PAs. These potential shifts, contractions, and expansions in vegetation portray the possible trends across landscapes that are of great concern for conservation, as such changes imply cascading ecological responses for associated flora and fauna. Overall, our findings highlight the challenges managers may face to maintain and preserve biodiversity in key PAs across North America.
... Kierunek i naturalny charakter stwierdzanych w przeszłości przemian składu gatunkowego lasów, nasuwa przypuszczenie, że mogą być one efektem długofalowych zmian klimatycznych (Brubaker 1986;Kowalski 1994;Kullman 1995). Trwające od połowy XIV do XIX wieku ochłodzenie klimatu Europy, nazywane małą epoką lodową lub małym glacjałem (Trepińska 1994), charakteryzowało się surowymi, długimi zimami z dużą ilością opadów średnia roczna temperatura powietrza była wówczas o 0,4 o C do 0,5 o C niższa niż obecnie (Obrębska-Starklowa 1997), a chłodny i wilgotny klimat wyraźnie sprzyjał rozwojowi i rozprzestrzenianiu się świerka. ...
... Dada la longevidad de los árboles, la variación adaptativa puede contribuir a la aptitud reproductiva (fitness en la terminología genética internacional) a lo largo de las diferentes fases de sus vidas. Sin embargo, el establecimiento representa una etapa particularmente sensible a la selección, debido a una mayor variación microclimática cerca del suelo, a altas presiones físicas y biológicas, y a una menor resistencia al estrés durante el estadio de plantín(Brubaker 1986, Rehfeldt et al. 1999, Green 2005. Además, la regeneración ofrece los mayores niveles potenciales de variación genética en la fase diploide (a excepción de la etapa embrionaria) y sobre dicha variación actúan tempranamente las fuerzas selectivas. ...
Forests are the most diverse and threatened ecosystems. In the context of climate
change, we need to know the genetic variation of key adaptive traits. The Patagonian
cypress Austrocedrus chilensis (D. Don) Pic. Ser. et Bizzarri (Cupressaceae) has high
ecological and economic importance in northern Argentine Patagonia. This conifer
inhabits from wet forest understory to arid steppe environments. This allows us to study
the genetic variation of a species with a seemingly wide plasticity. We address the
adaptation during the seedling stage on three levels of genetic organization: 1) initial
adaptation on an ecologically wide scale (at the provenance level), evaluated through
seedling survival and growth under natural conditions, 2) the variation in seedling annual
growth rhythm traits, at the cypress xeric border, previously characterized with molecular
markers as the zone of highest genetic diversity of the species’ distribution in Argentina
and 3) the variation of tolerance to extreme climatic events, along the species’ xeric
margin, assessed through seedling survival under summer water deficit and extreme
seasonal cold.
The hypothesis is that the cypress has evolved a general adaptive strategy for the
seedling stage; homogenizing selection would be the main process modeling the variation
in annual growth rhythm traits. Since cypress’ arid limit may be increasingly subject to
climatic extremes, its persistence may depend on its adaptability to changes in
environmental variance. If mechanisms preventing local adaptation prevail during
seedling recruitment, we would expect to find intra-population heritable variation, rather
than inter-population variance (evidence of divergent selection) for survival to drought
and extreme cold.
This thesis is organized into six chapters. Chapter 1 is a general introduction,
providing the conceptual framework that underpins the whole thesis, and presenting the
hypotheses and objectives of the work; Chapter 2 discusses the general methods used for
the development of the work. In the regional scale study (Chapter 3), we assayed
material from 14 populations, five from the arid margin, four mesic and five from the wet
range of cypress distribution in Argentina. We analyzed the variance using generalized
linear mixed models; the contribution of the geographical and genetic sources in the
variation of survival and growth was low overall. However, in a mesic experimental
location, the marginal populations showed higher levels of variation than the mesic and
humid, and a wide capacity of responding to a more favorable environment than that of
their home areas; the average growth was negatively correlated with the population
mean annual rainfall. In Chapter 4, we studied the variation of the second year, annual
growth rhythm traits, in seedlings from 10 marginal populations, testing an average of 15
families per population. In addition, we analyzed the variation of the first year, sylleptic
branching degree, and its correlation with the growth rhythm traits. The branching
degree had high genetic variances and was heritable in most populations. The amongpopulation
differentiation (QST= 0.08) was equal to the degree of neutral differentiation;
nevertheless, there was an ecotypic trend: the branching degree might decrease with
altitude. The correlations of the branching degree with the growth traits were very low
(except the genetic correlation with growth initiation). For the growth rhythm traits,
family and population effects represented on average 4.03% and 2.74% of the variation
(the residual variance was 84.57%). Genetic differentiation was high for the maximum
rate (QST= 0.29) and growth initiation (QST≈ 1); their mean population phenotypes were
negatively associated with altitude, suggesting an ecotypic pattern. The additive variance
and heritability of the growth traits varied between populations. Although there were no
correlations between additive variances and altitude and/or latitude, the northern part of
the marginal distribution showed higher variation for the pool of traits; this is consistent
with the patterns of neutral variation. Although the ecological range here studied was
quite narrow, the additive variance of growth cessation was positively associated with
population mean precipitation, suggesting a threshold at which the intra-population
environmental heterogeneity (cause of additive variance retention) would lose relevance
to macro-environmental shaping the genetic variance. In Chapter 5, we analyzed the
heritability of survival to summer water deficit and an exceptional winter cold event,
using two trials planted in mesic sites, with 140 and 163 families from 10 marginal
populations. The first trial suffered lower than the average environmental pressures,
which were sufficient to reveal additive effects on seedling survival. The second trial
suffered a more severe water stress and an extreme cold event. In that environment, the
heritability of survival to water deficit was high in all populations (h2 = 0.84 on average);
for cold survival the heritabilities were lower (h2= 0.28), even zero in several populations.
The regional scale analysis indicates the relevance of the marginal limit of cypress as a
reserve of genetic variation. Its behavior, at least equal to that of other populations,
suggests ample capacity to adapt to varied environments during seedling recruitment.
The hypothesis of genetic differentiation between areas of contrasting rainfall deserves
further research on adaptation towards the wet front of cypress. Marginal populations
could be limited in their micro-evolutionary capabilities, especially if environmental
changes imply a shift of the optimum for growth initiation. If their persistence was more
dependent on growth cessation (associated with drought and/or episodic frost
tolerance), several populations may adapt, particularly the northernmost. There was no
evidence of differentiation among marginal populations in their abilities to tolerate
drought and extreme cold. The results suggest that even when climate changes cause the
extinction of the most vulnerable populations, high levels of heritable variation for traits
underlying drought and cold tolerances would allow the marginal cypress to persist,
without losing overall genetic diversity. There was general agreement with the patterns
of neutral variation. The marginal zone had at least the same levels of variation than the
rest of the distribution, as well as ability to outperform in more favorable environments.
Northern marginal populations (approximately to 41 °S) had higher additive variance and
heritability for several traits. This, together with the background on neutral variation,
indicates the relevance of the northern cypress populations for conservation of the
species’ genetic diversity. The marginal populations should be considered key adaptive
elements. The “Native Forests” (26,431) and the “Planted Forests Promotion” (26,432)
laws provide a framework that encourages the use of cypress genetic resources for
various purposes, such as in situ conservation of marginal populations, and their use in
restoration, enrichment and commercial afforestation.
... However, these opportunities may be less frequent in forests with infrequent disturbance regimes where disturbance intervals tend to be measured in centuries rather than decades (e.g., maritime and subalpine temperate conifer forests [6][7][8][9]), and large patches of standreplacement fire (10 3 −10 5 ha) are relatively common and within normal system behavior. Such forests can be described as having greater 'landscape inertia' because of longer disturbance-free periods, fewer broad-scale regeneration opportunities, and long-lived tree species that, in mature form, can tolerate suboptimal conditions for long time periods [10]. ...
Future vegetation shifts under changing climate are uncertain for forests with infrequent stand-replacing disturbance regimes. These high-inertia forests may have long persistence even with climate change because disturbance-free periods can span centuries, broad-scale regeneration opportunities are fewer relative to frequent-fire systems, and mature tree species are long-lived with relatively high tolerance for sub-optimal growing conditions. Here, we used a combination of empirical and process-based modeling approaches to examine vegetation projections across high-inertia forests of Washington State, USA, under different climate and wildfire futures. We ran our models without forest management (to assess inherent system behavior/potential) and also with wildfire suppression. Projections suggested relatively stable mid-elevation forests through the end of the century despite anticipated increases in wildfire. The largest changes were projected at the lowest and uppermost forest boundaries, with upward expansion of the driest low-elevation forests and contraction of cold, high-elevation subalpine parklands. While forests were overall relatively stable in simulations, increases in early-seral conditions and decreases in late-seral conditions occurred as wildfire became more frequent. With partial fire suppression, projected changes were dampened or delayed, suggesting a potential tool to forestall change in some (but not all) high-inertia forests, especially since extending fire-free periods does little to alter overall fire regimes in these systems. Model projections also illustrated the importance of fire regime context and projection limitations; the time horizon over which disturbances will eventually allow the system to shift are so long that the prevailing climatic conditions under which many of those shifts will occur are beyond what most climate models can predict with any certainty. This will present a fundamental challenge to setting expectations and managing for long-term change in these systems.
... Mature individuals of long-lived tree species can tolerate unfavorable climate conditions for several centuries (Brubaker 1986, Noss 2001. In the absence of stand-replacing fire, climateinduced shifts in composition and distribution of existing forests will likely be muted or substantially lagged (Franklin et al. 1992). ...
Building resilience to natural disturbances is a key to managing forests for adaptation to climate change. To date, most climate adaptation guidance has focused on recommendations for frequent-fire forests, leaving few published guidelines for forests that naturally experience infrequent, stand-replacing wildfires. Because most such forests are inherently resilient to stand-replacing disturbances, and burn severity mosaics are largely indifferent to manipulations of stand structure (i.e., weather-driven, rather than fuel-driven fire regimes), we posit that pre-fire climate adaptation options are generally fewer in these regimes relative to others. Outside of areas of high human value, stand-scale fuel treatments commonly emphasized for other forest types would undermine many of the functions, ecosystem services, and other values for which these forests are known. For stand-replacing disturbance regimes, we propose that (1) managed wildfire use (e.g., allowing natural fires to burn under moderate conditions) can be a useful strategy as in other forest types, but likely confers fewer benefits to long-term forest resilience and climate adaptation, while carrying greater socio-ecological risks; (2) reasoned fire exclusion (i.e., the suppression component of a managed wildfire program) can be an appropriate strategy to maintain certain ecosystem conditions and services in the face of change, being more ecologically justifiable in long-interval fire regimes and producing fewer of the negative consequences than in frequent-fire regimes; (3) low-risk pre-disturbance adaptation options are few, but the most promising approaches emphasize fundamental conservation biology principles to create a safe operating space for the system to respond to change (e.g., maintaining heterogeneity across scales and minimizing stressors); and (4) post-disturbance conditions are the primary opportunity to implement adaptation strategies (such as protecting live tree legacies and testing new regeneration methods), providing crucial learning opportunities. This approach will provide greater context and understanding of these systems for ecologists and resource managers, stimulate future development of adaptation strategies, and illustrate why public expectations for climate adaptation in these forests will differ from those for frequent-fire forests.
... The catalyzing role of fire at times of high climate velocity arises because fire accelerates population turnover when climatic conditions are unfavorable for seedling establishment of dominant species (Brubaker 1986). Given long-lived tree species, mature forests without disturbance can exist for decades to centuries after climate has become inhospitable for regeneration, resulting in considerable inertia in response to climate change (i.e., "the storage effect," Chesson and Warner 1981). ...
Disturbance can catalyze rapid ecological change by causing widespread mortality and initiating successional pathways, and during times of climate change, disturbance may contribute to ecosystem state changes by initiating a new successional pathway. In the Pacific Northwest of North America (PNW), disturbance by wildfires strongly shapes the composition and structure of lowland forests, but understanding the role of fire over periods of climate change is challenging, because fire-return intervals are long (e.g., millennia) and the coniferous trees dominating these forests can live for many centuries. We developed stand-scale paleorecords of vegetation and fire that span nearly the past 14,000 years to study how fire was associated with state changes and rapid dynamics in forest vegetation at the stand scale (1-3 ha). We studied forest history with sediment cores from small hollow sites in the Marckworth State Forest, located ~1 km apart in the Tsuga heterophylla Zone in the Puget Lowland ecoregion of western Washington, USA. The median rate of change in pollen/spore assemblages was similar between sites (0.12 and 0.14% per year), but at both sites, rates of change increased significantly following fire events (ranging up to 1% per year, with a median of 0.28 and 0.38%, p < 0.003). During times of low climate velocity, forest composition was resilient to fires, which initiated successional pathways leading back to the dominant vegetation type. In contrast, during times of high climate variability and velocity (e.g., the early Holocene) forests were not resilient to fires, which triggered large-scale state changes. These records provide clear evidence that disturbance, in the form of an individual fire event, can be an important catalyst for rapid state changes, accelerating vegetation shifts in response to large-scale climate change. This article is protected by copyright. All rights reserved.
... Increased fire in these ecosystems could hasten climate-driven changes by removing cold-adapted and alpine species at the margins of their ranges (Lesica and McCune 2004;Gottfried et al. 2012), and by creating growing space that allows lowerelevation species to become established and spread. Conversely, increased fire could counteract ongoing responses to climate change, including upward movement of the treeline (Brubaker 1986;Harsch et al. 2009) and tree invasion of subalpine meadows (Franklin et al. 1971;Taylor 1995;Rochefort and Peterson 1996;Miller and Halpern 1998) by reducing tree cover and increasing the prevalence of non-forested vegetation. Fire may also interact with other stressors and disturbances to maintain existing or create new non-forested areas. ...
The direct effects of climate change on alpine treeline ecotones – the transition zones between subalpine forest and non-forested alpine vegetation – have been studied extensively, but climate-induced changes in disturbance regimes have received less attention. To determine if recent increases in area burned extend to these higher-elevation landscapes, we analysed wildfires from 1984–2012 in eight mountainous ecoregions of the Pacific Northwest and Northern Rocky 5 Mountains. We considered two components of the alpine treeline ecotone: subalpine parkland, which extends upward from subalpine forest and includes a fine-scale mosaic of forest and non-forest vegetation; and non-forest alpine vegetation. We expected vegetation types to burn proportionally less than the entire ecoregion, reflecting higher fuel moisture and longer historical fire rotations. In four of eight ecoregions, the proportion of area burned in subalpine parkland (3%–8%) was greater than the proportion of area burned in the entire ecoregion (2%–7%). In contrast, in all but one ecoregion, a small proportion (4%) of the alpine vegetation burned. Area burned regionally was a significant predictor of area burned in subalpine parkland and alpine, suggesting that similar climatic drivers operate at higher and lower
elevations or that fire spreads from neighbouring vegetation into the alpine treeline ecotone.
... Climatic variation can be a critically important influence on tree recruitment, growth, and mortality patterns (Brubaker 1986, Auclair 1993, Woodward et al. 1995, Savage et al. 1996, Villalba and Veblen 1998, Pedersen 1998, Breshears et al. 2005) as well as on disturbances such as fire and insect outbreaks (Swetnam and Lynch 1989. Consequently, we briefly review climatic variation, especially over the past c. ...
... The effectiveness of local population adaptation depends on several factors, including genetic variability, fecundity, dispersal, and generation time (Aitken et al. 2008, Brubaker 1986, Jump and Peñuelas 2005). Populations with high genetic variability have a higher potential for adaptation if the genetic variability includes traits that affect performance and reproduction responses to climate (Shaw 2001, Davis et al. 2005). ...
The purpose of this study was to review scientific knowledge and model projections on vegetation vulnerability to climatic and other environmental changes in the Pacific Northwest, with emphasis on five major biome types: subalpine forests and alpine meadows, maritime coniferous forests, dry coniferous forests, savannas and woodlands (oak and juniper), and interior shrub-steppe. We started by reviewing and synthesizing the scientific literature on past and projected changes in atmospheric carbon dioxide concentrations and climate for the Pacific Northwest (and globally), and how these changes are likely to influence snowpack dynamics, soil water availability, and selected disturbance regimes. We also reviewed and synthesized the scientific literature on plant growth, reproduction, and mortality in response to changing climate and disturbance regimes, and on the ability of plants to adapt to these changes through phenotypic plasticity, local adaptation, and migration. We then reviewed the strengths and weaknesses of several types of simulation models commonly used to project vegetation responses to climate change and discussed recent model projections of vegetation responses to future climate change scenarios in the Pacific Northwest, as well as how these projections might best be used in developing management plans for forests and rangelands. We next reviewed the existing scientific literature on plant sensitivity and adaptation to changing climate and disturbance regimes for five major vegetation biomes in the Pacific Northwest. We concluded with a discussion of current approaches and resources for developing climate change adaptation strategies, including restoring historical vegetation structure and composition, promoting resistance to change, promoting resilience to change, and facilitating anticipated responses to change.
... The plant population structure is the common result of the survivability of individuals and the impact of environmental conditions (Khan & Shaukat, 1997;Fuchsa et al., 2000;Svensson, 2001;Manuel & Molles, 2002), and may also provide important information on the past and present regeneration of species (Agren & Zackarisson, 1990;Chen, 1999). So, the population structure of plant species, especially long-lived ones could be considered as indicators of vegetation succession as well as climate changes along the tree lined ecotone (Begon & Mortimer, 1981;Brubaker, 1986;Johnson, 1989;Camarero & Gutiérrez, 2004). Describing the population structure is a well-known method for analyzing population dynamics. ...
Populus euphratica Oliv. is a common tree species found along the Tarim River and is important for the development of the local economy. We did our study in the upper and lower reaches of the Tarim River during 2.5 months, from September to November. We used the static life table and survivor curve for our investigations, and population dynamics were predicted by time sequence model. According to our results, young individuals were dominant in the upper reaches of the P.euphratica population, with the survivor curve belonging to the Deevey C type category. The time sequence model demonstrated that the number of mid-aged individuals in the upper reaches of the river will increase in population over the next 20, 30, to 40 years, when there will be continuous development. In contrast, in the lower reaches, young individuals were rare, and mid-aged individuals comprised the largest proportion of the population. The time sequence model for this population demonstrated that the number of old individuals will increase and young individuals will decrease during the next 20, 30, to 40 years. The main cause of this difference between the upper and lower reaches of the Tarim River is connected mostly to the level of ground water, which played an important role in the determination of age structure. Therefore, the crucial factors for the natural regeneration and restoration of P.euphratica were the rising ground water level and improvement in their habitat.
... Based on our simulations, we conclude that warmer and more variable climatic conditions are diminishing the conditions for extant and adjacent tree species regeneration in Alberta, Canada. Some studies indicate that forest regeneration conditions should be improving at higher elevations and latitudes (Brubaker, 1986;Lenoir et al., 2009) and declining in lower elevation forests (Bertrand et al., 2011b;Loarie et al., 2009), while others provide mixed results potentially related to changes in human activity (Boisvert-Marsh et al., 2014). Our findings support a relative improvement in regeneration conditions in low-elevation northern forests (Crimmins et al., 2011;Dobrowski et al., 2013;Zhu et al., 2014Zhu et al., , 2012. ...
The regeneration niche of trees greatly narrows the fundamental niche and is sensitive to climatic change. Development from seed and phenology are regulated by biological and environmental controls, shaping forest successional pathways. We hypothesized that recent climate change is reducing regeneration suitability in northern forests. We used a process-based ecophysiological model to examine changes in forest regeneration conditions across an elevational and latitudinal gradient in Alberta, Canada from 1923 to 2012. We compared these results to a recent empirical study in the region to infer the recent drivers of regeneration change in northern forests. Our results suggest that these forests are experiencing climatically driven declines in conditions suitable for regeneration. Contrary to previous findings indicating poorer current conditions in low elevation forests, we found more stable regeneration potential there, attributable to a relative abundance of soil moisture. Rocky soils resulted in modeled losses of soil moisture at higher elevations, potentially preventing upslope migrations of species despite warming. We identify potential mechanisms driving unexpected tree regeneration patterns described in previous studies. Our simulations suggest a delayed response of forest regeneration to warming throughout the past 90 years.
... In my opinion, this sudden fall resists the release phase of understorey beech after a long suppression phase. This phenomenon is a likely consequence of high plasticity of trees; by growing slowly they can overcome the environmental stress to survive periods of poor growing conditions; normal growth usually resumes when favourable conditions return (Brubaker, 1986). ...
European beech (Fagus sylvatica L.) is the most important broad-leaved tree species in Central and Western Europe. Beech juveniles have been found to be particularly sensitive to soil drought. The growth and the survival of beech juveniles are influenced by water availability. This thesis aimed to find the effect of soil drought on tree vitality and growth for beech juvenile and understorey. Semi-natural oak stand surrounded with beech stands in Schlossberg near Freiburg in southwestern Germany was selected for the study.
Crown dieback or branch mortality, expressed as the percentage of dead above ground biomass (stems, branches and leaves), was used as a measure of tree vitality. The pattern of branch mortality distribution was recorded in different vertical parts of the crown. Biomass was calculated from the regression models, prepared from the harvested samples. Both height and lateral increment were measured for calculating growth. Tree-ring analysis was performed to calculate lateral increment, expressed by basal area increment (BAI), with harvested tree discs. Soil drought was quantified by calculating available soil water storage capacity (ASWSC).
A significant negative strong correlation between soil drought and tree vitality was found. Branch mortality threshold was found 40%. In the stand having 67.38 mm mean ASWSC. Highest branch mortality (42%) was noticed in the lower crown. Significant difference was found between height and length of the trees proving stunted growth. ‘2003 summer drought’ had significantly high adverse effect on basal area increment.
At the drought limit of beech, drought causes partial up to complete crown dieback of beech individuals. Particularly extreme dry year is resisting the growth and survival of beech. Soil drought together with extreme summer drought impedes the establishment of beech trees in semi-natural oak forests.
We incorporated climate adaptation into a forest management plan for Michigan Technological University’s Ford Forest, a 2,000-ha property in Michigan’s Upper Peninsula used for education, research, and timber revenues. Our process was an opportunity to test the existing climate adaptation literature, pulling from multiple sources to meet the diverse needs of the institution at the time the plan was created. We present outcomes as well as lessons learned summarized in a series of broad takeaway messages. (1) Climate adaptation is a means to an end. The end must be defined and is made up of critical values and an adapted state. (2) Given the instability inherent in climate change, achieving adaptation means adjusting forest attributes in response to change but also preparing for unforeseen outcomes via adaptive management. (3) Decisions surrounding goals affect the entire process. Considering climate change–driven constraints when setting goals will improve outcomes. (4) Flawed decision-making is a risk associated with certain organizational contexts and affects the identification of goals, vulnerabilities, and adaptation options. (5) Climate adaptation actions cannot be evaluated in the short term for efficacy because climate change is ongoing. However, the appropriateness of a plan in responding to anticipated change can be evaluated.
Study Implications: This work informs strategic planning for climate change adaptation in forests. We created a climate-informed forest management plan using adaptation frameworks for a university-owned forest in Upper Michigan at the confluence of the northern hardwoods and boreal forest ecotypes. We offer assessment of our plan outcomes and insights into how our decision-making context affected them. This case study expands on the ongoing scientific conversation, incorporating concepts from management science, on how best to adapt natural systems to climate change to protect human values derived from ecosystem services.
Evidence of climate change and global warming is becoming more visible; it is an ongoing process that is likely to become increasingly influential in the near future, not only at the global level but also at the local and regional levels. The fact that climate change affects the development of all forest communities and forest tree species, accordingly, has resulted in the increasing awareness in society towards this phenomenon. Having this in mind, the main aim of this paper is to evaluate the relationship between climate change and coniferous forests in the Balkan Peninsula, as well as to review the management strategies that may contribute to forest adaptation to climate change, with a special emphasis on the conservation of forest genetic resources. Hence, we have analyzed 202 papers regarding climate change and its effects on coniferous forests in the Balkan region, as well as papers dealing with adaptive forest management and forest genetic resources conservation. We concluded that climate change will likely represent one of the major challenges for coniferous forests on the Balkan peninsula in the future, imposing a need for the application of different management strategies to address these challenges and to facilitate adaptation of forests to the altered environmental conditions.
Silver fir is one of the most threatened conifer species in Croatia, especially at the western edge of its spread in Croatian Dinarides, where the decline in fir trees has resulted in significant ecological and economic issues. The aim of this study was to determine, over an 18-year monitoring period, the relationships of silver fir crown defoliation with climatic factors and structural attributes. We further analyzed the tree retention time in a given defoliation class and transition dynamics between defoliation classes, as well as the survival/mortality of trees. Data on silver fir defoliation were analyzed in two different forest types: in pure silver fir and in mixed silver fir and common beech stands. The climatic factors, primarily vegetation period air temperature, potential evapotranspiration, and dry season water deficit, were correlated with crown defoliation. Regarding the structural attributes, in the mixed stand with predominantly smaller trees, crown defoliation increased with reduced diameter at breast height, crown diameter, social class, and crown illumination. In the pure fir stand, crown defoliation increased with reduced crown diameter, greater crown asymmetry, greater crown illumination, and on trees with a stork’s nest crown. The retention time in defoliation classes differed for research sites. Transition dynamics were different only for trees in the highest defoliation class (dead trees). At the end of the study period, silver fir mortality was higher in the pure fir stand. Increased silver fir defoliation and mortality can be expected in the future, particularly in overmature stands under prolonged drought stress. Permanent forest monitoring could ensure the high-quality data needed for adaptive management of fir stands that could positively influence the structure of these stands and, thus, improve their health status.
Rapid climate change is a significant threat to the long-term persistence of native tree populations. Concern has been expressed that tree populations might fail to adapt due to rate of change, insufficient adaptive variation in tree populations and limits to dispersal. In contrast, others have contended that most tree species have high phenotypic plasticity, maintain high levels of within-population genetic variation and exhibit effective gene dispersal capability, all characteristics which should enable an adaptive response. To assess the potential adaptability of tree populations we need to understand their genetic diversity and phenotypic plasticity to build on the currently limited evidence base and guide decisions about seed sourcing for establishment of new woodlands desired to meet ambitious planting targets. Currently the seed sourcing system divides the island in four regions of similar size although it is not based on any genetic or ecological information. We discuss the suitability of this system with the insight of the data collected from native tree populations growing in experimental trials. In this thesis we study genetic diversity and phenotypic plasticity patterns in over 30 native tree populations across all Great Britain for three broadleaved species: ash (Fraxinus excelsior), rowan (Sorbus aucuparia), and silver birch (Betula pendula). To obtain these data we assessed the variation in multiple traits in several common garden experiments for each species, which were grown in contrasting environments. There is a tendency in provenance experiments to consider height as a proxy for fitness. We demonstrate that tree height is not enough to understand tree fitness and its adaptability capacity. We assessed our tree populations for growth (survival, tree height, DBH), stem form (number of forks), leaf phenology (leaf flushing and senescence) and leaf anatomical traits (leaf area, stomatal density and stomatal size).Great Britain has very distinct and heterogeneous environments likely to have given rise to adaptive differentiation. Knowing the geographical pattern of the genetic differences we can see the direction selective pressures have had on each of the traits studied, and we compare differences in patterns across the traits and species. Comparing populations growing in different environments we assessed the variation in phenotypic plasticity by trait and the direction of these plasticity. We found that tree populations across Great Britain are highly genetically variable and show genetic differences which have a geographical pattern, and that the patterns and size of the differences vary by species. Phenotypic plasticity varies across traits and interactions between genotype and environment make plasticity in some traits more unpredictable than others. We conclude that tree populations of ash, rowan and birch are well adapted to the diverse and oceanic climate of Great Britain, and that levels of genetic diversity and phenotypic plasticity provide a high capacity to respond to environmental change.
Izvod: Evropska bukva (Fagus sylvatica L.) je jedna od najvažnijih šumskih vrsta drveća u Bosni i Hercegovini kako sa ekonomskog tako i sa ekološkog aspekta. Trenutno stanje bukovih šuma u Republici Srpskoj je zadovoljavajuće, bukva nije značajnije ugrožena devitalizacijom i sušenjem. S obzirom da se prema različitim klimatskim scenarijima predviđa povećanje temperatura i smanjenje količine padavina, postoji verovatnoća da će doći do pojave sušenja bukovih šuma i smanjenja debljinskog prirasta. Iz tog razloga potrebno je sprovesti detaljna istraživanja koja će imati za cilj da istraže dinamiku debljinskog prirasta stabala bukve i uticaj ekstremno sušnih godina na širinu godova na području Republike Srpske. U radu su korišćene dendrohronološke metode za analizu prirasta bukovih stabala u tri sastojine koje se nalaze na donjoj granici, u optimumu i gornjoj granici vertikalnog rasprostranjenja ovih šuma u zapadnom delu Republike Srpske. U istraživanim sastojinama utvrđene su različite reakcije stabala na ekstremno sušne godine i različita dinamika debljinskog prirasta stabala bukve.
Beech (Fagus sylvatica L.) is one of the most important broadleaved tree species in Europe. In addition to its economic role through timber production, it plays an important role as an ecological organism— a‘building stone’ of the European biodiversity. Many species depend exclusively or primarily on a co-existence with beech, and many ecosystems would not exist in their current form if beech was not present. Consequently, optimal conditions for beech forest dynamics are vital for maintaining ecosystem services. Climate change will undoubtedly have an impact on the distribution on F. sylvatica and on the associated biodiversity. This review conducted on European beech forests with regard to climate change is based on 109 references and clearly describe the extent of possible forest changes both in compositions, patterns and processes. The extent of this impact is very difficult to predict because mostly studies didn’t considered beech forests as a whole, with an holistic approach. Already, we propose to implement certain measures to maintain beech forest biodiversity for the future at its highest possible level.
Subfossil pine (Pinus sylvestris L.) occurs in various deposits and in great frequency well above the current altitudinal pine limit in the Scandes Mountains. These tree remnants constitute a fairly continuous record of range-limit dynamics throughout the Holocene and offer excellent potential for palaeoecological as well as palaeoclimatological research. General aspects of this record are discussed, including the fundamental issue of the circumstances of preservation. It is proposed that throughout the Holocene conditions for preservation have existed at the fluctuating tree-limit. Hypothetically, temporal clusters and gaps in the frequency distribution of radiocarbon dates can fairly consistently reflect climatic variability. Tree-limit recession in particular may be rapid, in response to episodes of severe winter cold. Such events may appear disproportionately protracted backwards in time, due to frost-heaving and rain-induced erosion of deposits with subfossils. Additional aspects of subfossil pine, which are briefly discussed, include relative altitude, ecological setting, population structure, stress and disturbance, dendrochronology and recent processes.
Baobab trees are long-lived trees for which reproduction and biology have led various research questions. This thesis aims understanding the biological processes involved in seed dispersal of two species of Malagasy baobabs (A. rubrostipa and A. grandidieri) and the effect on population spatial pattern. Experiments were conducted to clarify the role of vertebrate dispersers on seed dissemination and seed germination. Analysis of the spatial distribution was discussed at the population level. The spatial referencing of adult baobabs has been achieved on the basis of a census and crown delimitation using imagery with very high spatial resolution. This work demonstrated the mutualistic relationships within ecosystems for seed dispersals and the baobab recruitment. In particular, our results showed that (i) the disappeared Malagasy megafauna as giant tortoises can provide dispersal of intact baobabs seeds, (ii) the large livestock can replace the role of the Malagasy megafauna ; pulp removal and dormancy breaking are facilitated by the passage of seeds in the digestive tract of these animals (iii) the big size of the fruits is compatible with the involvement of current frugivores such as lemurs and (iv) dispersal by floating seeds can prove to be an equally effective mechanism in the absence of main animals dispersers. This study also demonstrated that the capacity of seed dispersal and the access to the nearest water point due to arid conditions of their habitats are main determinants of spatial patterns of baobabs. These conditions can play the role of environmental filter selecting individuals that are far from water point.
The Late Archaic culture period has been characterized as a time of rapid and widespread changes in settlement-subsistence patterns, social organization, and technologies in the Middle Atlantic region (Catlin et al. 1983; Custer 1978, 1984a, b, 1986a,b; Gardner 1978, 1980; Mouer 1986; Mouer et al. 1981; Witthoft 1953; but see Cavallo and Joyce 1985). Recent explanatory models using a cultural-ecological theoretical perspective have focused on environmental change as the major cause of these shifts (Catlin et al. 1983; Custer 1978, 1984a,b, 1986a,b; Custer and Stewart 1983; Custer and Watson 1986). One of the major environmental factors viewed as being significant in generating Late Archaic cultural change is the mid-postglacial xerothermic. This climatic interval has been characterized as a period of extreme climatic oscillations, including maximum Holocene warm/dry conditions (Carbone 1976:77; Catlin et al. 1983:125; Custer 1978:2, 1984a:90, 1984b:33; 1986a,b; Custer and Stewart 1983:4; Custer and Wallace 1982:147; Custer and Watson 1986). It has been proposed that those climatic conditions caused changes in the abundance and distribution of resources exploited by human groups, resulting in an adaptive response by those populations that involved an increasing focus on estuarine and riverine sources and the emergence of long-range exchange networks (Catlin et al. 1983; Custer 1978, 1984a,b, 1986a,b; Custer and Stewart 1983).
This chapter examines the effects of temperature stress and elevated CO2 on ecosystem processes, including primary production, nutrient cycling, and landscape water and energy balance. Environmental stress varies depending on whether it is applied to a cultivated production system or to an ecosystem. In its broadest sense, environmental stress is any factor that lowers primary production below its optimum. In this context, any nonoptimum temperature can be considered stressful. However, a more narrow view of environmental stress would require an environmental condition (for example, freezing/chilling temperatures, heat stress) to cause damage to plants. Severe damage to plants, causing either mortality or an irreversible loss in productivity, will have clear impacts on ecosystem processes. Also of interest from an ecosystem perspective is recurring environmental stress, which can result in either physiological hardening in plants (which may be irreversible within a given growing season) or genetic responses of populations over time, such that the ecosystem equilibrates to nonoptimum conditions through a loss of productivity. One important aspect of the global change debate is the potential rapidity with which global climate is predicted to change and its effects on ecological systems.
Mount Fuji is a relatively young mountain, which erupted intermittently until 1707. The tree limit on Mount Fuji is composed of larches (Larix leptolepis). The limit ranges from 1,400 to 2,900 meters in altitude, depending on the slope. Around Oniwa on the northwestern slope of Mount Fuji, a larch scrub community is scattered in patches, forming an island, where the tree line is 2,650 meters. All of the larches are severely deformed toward the northeast due to strong winds. Factors maintaining the larch scrub are examined referring to tree size and tree age. Tree size decreases rapidly above an altitude of 2,390 meters on the northwestern slope, where the forest limit is located. On this slope, we can observe both a group of trees showing a stronger tendency toward growth in terms of height and another group growth in terms of diameter. These growth patterns change depending on the altitude; that is, the higher the altitude, the greater the diameter, and the lower the altitude, the greater the height. On the other hand, the relationships between tree size and tree age show a tendency at lower altitudes of older trees having greater heights, and at higher altitudes of older trees having greater diameter, but not heights. These facts suggest that a larch scrub community forms by controlling exposure to the severe environment. This is also in accord with the observation of older trees having greater deformation. It is considered that embolism is a plausible cause controlling tree size, especially tree height, because frost action with severe transpiration frequently occurs on this slope. As a result, a scrub formation would be fixed. This explanation of the growth mechanism of the landscape around Oniwa on the northwestern part of the Ochu-do trail running along the side of Mount Fuji will assist eco- and geo-tourism development on the Ochu-do.
Climate on earth has always been changing. Despite decades of investigation, our limited knowledge of the ecological and evolutionary effects of climate changes often translates into uncertain predictions about the impact of future climates on biodiversity. Integrative biogeographical approaches using palaeobotanical, phylogenetic and niche-based species distribution models, when permitted by data availability, may provide valuable insights to address these key questions. Here we combine palaeobotanical and phylogeographical information with hindcast modelling of species distribution changes to reconstruct past range dynamics and diff erentiation in the bay laurel (Laurus spp., Lauraceae), an emblematic relict tree from the subtropical laurel forests that thrived in Tethyan realms during most of the Tertiary period. We provide plausible examples of climate-driven migration, extinction and persistence of populations and taxa, and discuss the factors that infl uence niche conservatism or adaptation to changing environments. Finally, we discuss the likely impacts of the predicted climate change on laurophyllous taxa in the Mediterranean and Macaronesia.
Climatic change is expected to cause dramatic shifts in low-elevation treeline in mountainous environments. Ponderosa pine (Pinus ponderosa) were sampled across an elevation gradient adjacent to the Methow Valley of the Okanogan National Forest in the eastern Cascades, Washington to examine the potential response of ponderosa pine to climatic change. Response function analyses were used to compare climate-growth relationships among 12 sites, four elevations on three different mountains. Response function analysis attributes 42-55% of the inter-annual variation in growth to climate. Growth is positively correlated with November precipitation prior to the growing season on all 12 sites, suggesting that November precipitation is critical for increased root growth, increased nutrient availability through decomposition. building snowpack, or non-growing season photosynthesis and carbon storage. Growth is positively correlated with previous October, January, June, and July precipitation at more than one site. Temperature is not correlated with growth on any sites. Climate models predict that the Pacific Northwest will experience warmer and wetter winters and drier summers in the future. Growth-climate correlations suggest that the short-term growth response of ponderosa pine is most sensitive to non-growing season precipitation. Therefore, predicting ponderosa pine's response to projected climatic change is problematic. with wetter falls increasing growth and drier summers decreasing growth. Our results indicate that ponderosa pine is much more sensitive to precipitation than temperature and that any predictions of this arid species' response to climatic change are difficult, due to uncertainty in predicting future precipitation patterns.
Plant genetic resources (PGR) are the basic raw materials required to cater current and future needs of crop improvement. Climate change is expected to result in increased frequency of abiotic stresses like drought, heat stress, submergence, increased soil salinity etc. The negative impacts of climate change are visible in the form of declining crop productivity, shifting in crop suitability areas, species migration and extinction, emergence of new pests and weeds and altered phenology. Already, the existing genetic base of our crops and varieties has shrunken, and in future we may find it difficult to cope with new climatic challenges with the existing information on genetic resources. Consequently, food and sustainable livelihood security of larger section of populations is jeopardized. Substantial knowledge and insight is, therefore, needed to gauge what types of diversity now exist in the gene banks, and what will be needed in the future. There is a need to assemble and screen germplasm strategically and discover new sources of variations which will enable us to address the very pertinent issue of climate change. Strategies like genetic enhancement/ pre-breeding using crops wild relatives, developing core sets, focused identification of germplasm, mapping and cloning gene and gene constructs, allele mining, bioprospecting for novel biomolecules, and promoting on farm conservation in order to allow genes to evolve and respond to new environments would be of great help to mitigate the climate change impacts. There is also need to mobilize national and international opinion to make food security and poverty alleviation central in climate negotiations
Long-term, landscape-scale vegetation studies are relatively new to ecology. Conventional sampling strategies of randomly selecting a few study plots in a relatively homogeneous study unit may not be adequate when assessing large-scale processes and subtle long-term vegetation change. The technology (eg geographic information systems, global positioning devices) is advancing more rapidly than field methodologies. Regardless of the initial intent of the long-term study or the size of the study area, initial decisions on study Goals, Objectives, Scale, Sampling design, Intensity of sampling, and Pattern of sampling (GOSSIP) affect the long-term value of datasets for time series and other analyses. Because of a backlog in research needs, taking an experimental approach and using the GOSSIP framework for decision making will help plan landscape-scale, long-term vegetation studies. Long-term landscape-scale studies must be combined with large-scale experiments and ecological modeling. The only way to validate interpretations of short-term experiments and ecological models is by careful long-term monitoring of many, widely dispersed permanent plots. -from Author
Aimed at taking the mystery out of soil science, Soils: Principles, Properties and Management is a text for undergraduate/graduate students who study soil as a natural resource. Written in a reader-friendly style, with a host of examples, figures and tables, the book leads the reader from the basics of soil science through to complex situations, covering such topics as: the origin, development and classification of soil physical, chemical and biological properties of soil water and nutrient management management of problem soils, wetland soils and forest soils soil degradation Further, the ecological and agrological functions of soil are emphasized in the context of food security, biodiversity and climate change. The interactions between the environment and soil management are highlighted. Soil is viewed as an ecosystem itself and as a part of larger terrestrial ecosystems. © Springer Science+Business Media Dordrecht 2013. All rights are reserved.
Dendrochronology and tree-ring densitometry are used to reconstruct summer temperatures and treeline dynamics in the Columbia Icefield area from 1600 AD to the present. Detailed studies at three sites, less than 10 km. apart, show different responses to regional climate over this interval. At a north-facing site, Abies lasiocarpa have maintained a population by vegetative regeneration with little change in the treeline ecotone over the last 400 years. In contrast, at a warmer, south-facing site, the Picea engelmannii dominated treeline shows catastrophic dieback during the late 1600s. Extensive upslope migration of treeline by seedling establishment occurred during the 20th Century. At an adjacent valley-floor site, tree clumps established during the 18th–19th centuries but exhibit no subsequent population expansion beyond their borders. These results suggest that the response of treeline ecotones to climate change varies with both local site conditions and the response of individual species. These data can provide important inputs to simulation models and to resource managers who wish to understand the effect of climate change on ecosystem dynamics.
Assumptions that correlations of climate and natural vegetation are also causal underlie the use of general circulation models for predicting vegetation change at a national or global scale, yet direct links between macroclimate and plant distribution are difficult to demonstrate. Studies of vegetation change in the pre-Pleistocene past and invasions of pine trees in the present indicate that the effects of climate are seldom direct. Vegetation change is most often caused by changes in disturbance regime or in competitive interactions, especially at the seedling stage. Population-level studies, using predictions of climate effects on productivity of different growth forms and the (limited) knowledge of climate effects on reproductive biology, offer an important alternative to predicting and coping with climate change. -Authors
A dendrochronological investigation of 347 seedlings and saplings from a stand of trees invading a subalpine meadow in the Wind River Mountains of western Wyoming provided the basis for this study. Growth rates of these trees were found to increase as their distance from the forest edge increased; this pattern was shown to continue with meadow trees throughout their lives. The evidence suggests that the factors which maintain the meadows in this area primarily limit tree establishment rather than growth. Invasion began about 1890, and was most intense between 1940 and 1963. Little establishment occurred after 1963. Examination of cattle grazing records suggests that changes in grazing pressures may be responsible for this expansion of the forest. Moderate levels of cattle utilization of the meadows appear to have resulted in abundant tree establishment by reducing competition with meadow vegetation. The lack of establishment after 1963 coincides with the cessation of grazing which occurred at about the same time.
Fuel weight by size classes for five understory coniferous tree species in southern Oregon was estimated using regression equations. Species included Abiesconcolor (Gord. and Glend) Lindl., Abiesmagnified Murr. var. shastensis Lenin., Pinusponderosa Dougl., Pinus contorta Dougl. var. murryana Grev. and Balf., and Tsugamertensiana (Bong.) Carr. Trees ranged from 0.1 to 3 m tall and were selected from "open" and "dense" overstory classes. Live and dead fuels from each tree were segregated into five diameter size classes. Regressions for foliage and total fuel weight are presented using as independent variables groundline diameter, total height, diameter2 × height, and basal area. Coefficients of determination (r2) were high, exceeding 95% for total weight in every case, but tended to decrease as diameter size class increased. Relative shade tolerance of the five species is estimated by comparing open- and dense-grown foliage weight ratios for each species.
Biomass distribution and above- and below-ground net primary production were determined for 23- and 180-year-old Abiesamabilis (Dougl.) Forbes ecosystems growing at 1200-m elevation in the western Washington Cascade Range. Total organic matter accumulations were 427.0 t•ha−1 in the young stand, and 1247.1 t•ha−1 in the mature stand. Aboveground tree and detritus biomass were 49.0 t•ha−1 and 130.2 t•ha−1, respectively, in the young stand compared with 445.5 t•ha−1 and 389.4 t•ha−1 in the mature stand. Net primary production (NPP) was 18.3 t•ha−1 in the young stand and 16.8 t•ha−1 in the mature stand. Belowground dry matter production was 65% of total net production in the young stand and 73% of total net production in the mature stand. Conifer fine root production was 35.9% of NPP in the young and 66.4% of NPP in the mature stand. This apparent shift in fine root production as a proportion of NPP may be related to detritus accumulation.
Theoreticians have suggested that genetic changes in long-lived perennial species may show a lag in response to selective pressures. If this suggestion is correct, then these populations should contain subpopulations adapted to dissimilar environments. The present study, employing data on age distributions and gel electrophoresis, provides some evidence for the existence of these genetically differentiated subpopulations among seedlings of Acer saccharum.
Although, as W.D. Billings notes in his chapter in this book. the development of physiological ecology can be traced back to the very beginnings of the study of ecology it is clear that the modern development of this field in North America is due in the large part to the efforts of Billings alone. The foundation that Billings laid in the late 1950s came from his own studies on deserts and subsequently arctic and alpine plants, and also from his enormous success in instilling enthusiasm for the field in the numerous students attracted to the plant ecology program at Duke University. Billings' own studies provided the model for subsequent work in this field. Physiological techniques. normally confined to the laboratory. were brought into the field to examine processes under natural environmental conditions. These field studies were accompanied by experiments under controlled conditions where the relative impact of various factors could be assessed and further where genetic as opposed to environmental influences could be separated. This blending of field and laboratory approaches promoted the design of experiments which were of direct relevance to understanding the distribution and abundance of plants in nature. Physiological mechanisms were studied and assessed in the context of the functioning of plants under natural conditions rather than as an end in itself.
measuring the amount of genetic variation in species populations has been a concern of evolutionary biologists since the time of Darwin. A variety of techniques have been used, ranging from the common garden approach of Turresson (1922, 1925) and others (e.g., Clausen, Keck, and Hiesey, 1940, 1948) to the DNA sequencing studies of modern workers. These studies, whether morphometric, physiological, developmental, or biochemical, have demonstrated that most species populations contain relatively large amounts of genetic variation. Good examples of these kinds of studies can be found in the forest genetics literature. Because of the commercial importance of forest trees, genetic variation within and between tree populations has been unusually well documented. The majority of forest tree species have considerable amounts of genetic variation within, as well as between, populations (Libby et al., 1969; Stern and Roche, 1974; J. W. Wright, 1976). A notable exception is red pine (Pinus resinosa), in which little genetic variation has been demonstrated (Fowler, 1964, 1965; Fowler and Lester, 1970; Fowler and Morris, 1977). This is remarkable, since red pine ranges from Wisconsin to Nova Scotia and from northern Pennsylvania to central Ontario. Other forest tree species occurring in essentially the same environments as those inhabited by red pine have been shown to have considerable genetic variation.
Describes techniques and results of tree ring research in the United States where old trees are relatively common. The method has been used successfully in all regions, and can be used in dating and also in the analysis of climatic change. -K.Clayton
A 0.36—ha area in the Harvard Forest, Petersham, Massachusetts, was intensively analyzed to determine its history. Natural and man—caused disturbances of varying magnitudes occurred periodically in the central New England mixed—species stand. Evidence of two hurricanes and a fire prior to 1803 were found. Between 1803 and 1952, 14 natural or man—caused disturbances of various magnitudes occurred in the area. Large disturbances created new age classes, but small disturbances did not. Species arising together after large disturbances formed a distinct vertical stratification, with northern red oak (Quercus rubra L.) arising after several decades to the dominant canopy. Smaller disturbances to the overstory allowed understory trees such as black birch (Betula lenta L.), red maple (Acer rebrum L.), and eastern hemlock (Tsuga canadensis L. Carr.) to emerge to the dominant canopy. The composition of this forest was more the result of allogenic influences rather than autogenic development. See full-text article ...
The self-thinning rule describes plant mortality because of competition in crowded even-aged stands. The rule is best understood with respect to a graph of log biomass (log B) per unit area vs. log density (log N) of survivors, known as the B–N diagram. The rule has three notable features—(1) mortality is a function only of biomass accumulation, (2) because mortality is driven by the rate of accumulation of biomass, mortality is slower when conditions for growth are worse, and (3) the thinning line has a slope of about –½ for most studied species under most conditions. Two main effects operate in developing distributions in even-aged stands. First, large plants suppress small plants. The result is a “hierarchy of dominance and suppression” in which the smaller plants are at an accumulating disadvantage and finally die. Second, the mortality of smaller individuals truncates the distribution from the left.
Data for numbers of living seedlings by two-year age-classes under old-growth forests permit examination of life-table analysis as a tool for following seedling population relationships. The population depletion curves for white pine, balsam fir and red maple are compared. The negative exponential depletion model is demonstrated as a potentially sensitive method for quantitative comparison of silvical response characteristics.
Picea mariana predominates in even-aged stands of fire origin at the forest-tundra ecotone near Inuvik, Northwest Territories, Canada. Growth rates of P. mariana decrease with increasing latitude and density although growth rates do not change significantly along the ecotone. Field measurements of water relations, when compared with laboratory determinations of leaf water potentials vs. net photosynthesis, suggest no water stress related photosynthetic reduction for mature trees during 1976. Maximum and minimum leaf water potentials for mature trees were -0.6 and -2.1 MPa on 4 August and 1 July, respectively. Winter measurements of water relations also suggest no water stress. In contrast, 1st-yr seedlings demonstrated sensitivity to water stress with high rates of mortality at low levels of water loss. A macro-mesoclimatic gradient of 4@?C was found across the tree line (135-km study transect). Picea mariana seed production and germination are limited by this environmental gradient to @?40 km south of modern forest line. Germination and survivorship are also affected by microenvironments within a well-developed soil hummock terrain. The lower cardinal germination temperature of 15@?C was the determinant of germination timing and success. Seed production estimates suggested little effect on stand reproduction with burn intervals of 100-200 yr. Shorter or longer burn intervals are believed to reduce stand reproduction. Reduced seed longevity, complete stand destruction by wild-fire, and apparent destruction of seed in the soil restrict establishment to 1-8 yr after fire. This may cause local extermination of the species or out-of-equilibrium tree line positions under conditions of deteriorating environment.
Rates of subalpine tree establishment were measured on the 1978 Hoh burn (3 yr old), the 1924 Mount Wilder burn (55 yr old), and the 1891 High Divide burn (88 yr old) in Olympic National Park, Washington, USA. All three sites were Abies lasiocarpa/Tsuga mertensiana forest at the time of burning: Vaccinium spp. were dominant after the fire. Tree establishment rates were higher on burned sites during periods with generally above-average to average spring/summer precipitation than during below-average periods. Highest rate of tree establishment occurred close to fire edges. Current species composition on the older burns closely reflects the composition of residual trees. Drought resistance of the residual tree species may interact with climate to affect establishment rates. Tsuga mertensiana establishes best during wet periods; Abies lasiocarpa, Pseudotsuga menziesii, and Pinus monticola establish well during normal periods. These patterns are quite different from tree invasion into heather (Phyllodoce/Cassiope) meadows, which occurred during a fairly discrete 1920-1940 regional drought when extended snow-free periods apparently existed in these meadows.
Gifford woods is dominated by Acer saccharum with smaller amounts of Fagus grandifolia, Fraxinus americana, Betula lutea, Ulmus americana, Tsuga canadensis and Tilia americana. The stand, a remnant of old-age hemlock-northern hardwoods forest, is located near the upper altitudinal limit of this forest type in central Vermont. Soil underlying the forest is mostly a coarse mull with thin duff mull on steep slopes and well-drained knolls. The herbaceous flora is comparatively rich with a total of 84 species. Nineteen of these are ferns and fern allies which also contribute about 3 times as much cover as all other herbaceous plants put together. The reproductive mechanisms of sugar maple (Acer saccharum) are considered.
The first 40 yr of forest succession on permanent plots at the Harvard Forest in central New England followed the initial floristic composition model forest succession. After the 1938 hurricane removed the previous white pine (Pinus strobus) canopy, species regenerated within 4-6 yr by sprouts, buried seed, and wind-blown seed, with no method of regeneration uniformly contributing more to species success than another. Hemlock (Tsuga canadensis) was the only species successfully regenerating after 1948. Pin cherry (Prunus pennsylvanica) was the early dominant in size and numbers (5000 stems/ha). At year 10, pin cherry, red maple (Acer rubrum), white ash (Fraximus americana), and red oak (Quercus rubra) were dominant. Species diversity had reached a maximum. By year 40, red oak and paper birch (Betula papyrifera) showed strong canopy dominance, making up only 7.5 and 4.9%, respectively, of total density but 37.5 and 12.5%, respectively, of the size-dominant stems. Red maple and white pine were also codominant in 1978. Some evidence for an intermediate stage dominated by red maple and gray birch (B. populifolia) was found. On one previously hardwood plot, the same species were present, and similar trends in species composition and dominance were followed, but there was more surviving hemlock advance regeneration and a lower density of some shade-intolerant early dominant species. The canopy structure was loosely multilayered, and at any given point in succession, species tended to be found in characteristic layers, although these relative positions could change with time.
Seedling and sapling growth, natality and mortality were studied over a three year period in forty 5 x 5 m plots in a mature woodland in Hunterdon County, New Jersey dominated by oaks. Year to year seedling populations remained relatively constant for most canopy species with the exception of red oak (Quercus rubra) and black birch (Betula lenta). Survivorship and growth varied greatly according to species. Beech (Fagus grandifolia) was the most persistent species while black birch was the least. Oak species and red maple (Acer rubrum) were intermediate. Sapling survivorship and growth were better than seedling. Population dynamics indicate an essential steady-state in regard to numbers but some qualitative changes are indicated. Black birch does not appear to be capable of regeneration under present conditions.
Bristlecone pines (Pinus aristata Engelm.) locally attain ages over 4,000 years. In the White Mountains, California, old trees are widely distributed near the lower forest border, but are restricted to exposed, rocky areas at higher altitudes. Comparative aridity seems to be an important feature of the @'old-age@' habitat. Occurrences of older (1,5000-4,900 years) bristlecone pines in the moister mountain ranges of Nevade and Utah are analogous to those at higher altitudes in the White Mountains, suggesting that local edaphic and physiographic factors can offset the effects of a regional precipitation gradient. Attainment of an age greater that about 1,500 years apparently depends on the adoption of a strip-growth habit, which permits the aging bristlecone pine to maintain a nearly constant ratio of green to non-green tissue. Slow growth rates, wind damage, and soil erosion may be conducive to cambial area reduction. Other features of old-age stands, such as the wide spacing of the trees, the compactness of their crowns, the sparsity of litter, and the low density of the accompanying ground-cover vegetation, would also provide a measure of protection from fire and from competition, permitting the older trees to survive.
The annual demographic changes and height increments of small, isolated, tree-limit stands of Pinus sylvestris L. (considered collectively as a single population) were studied during a 10-yr period (1972 to 1981). The individual vitality of specimens forming the population was also followed in detail throughout the study period. No recruitment to the population was recorded during the study period and the population size (total number of live plants) decreased by ca. 13%. The demographic changes were basically controlled by intrinsic factors but proximately executed by damage related to the annual differences in the air temperatures recorded during the initial 2 to 3 mon of the year. Long periods of radiation-controlled high temperatures during these months apparently lower the frost-hardiness, and the young pines, in particular, suffer serious damage by the subsequent onset of low temperatures. The individuals forming such isolated and small-sized tree-limit populations appear to be especially susceptible to this type of damage. Tree vitality in general, and the reproductive phase of life-cycle in particular, may suffer due to selfing and inbreeding. Such populations of pine seem therefore to be doomed to complete extinction, or to very drastic reduction in size within a rather short time after establishment (30 to 40 yr). The Holocene tree-limit history of pine in Scandinavia is briefly discussed in the light of this finding.
The self-thinning rule predicts that for a crowded, even-aged plant population, a log-log plot of total plant mass against plant density will give a straight line of slope -3/2. Recent reevaluations have concluded that the slope is much more variable than previous authors have claimed, that straight lines are the exception rather than the rule, and that the slope varies with aspects of the biology of the plant. Much of the reported variability in slope has resulted from the inclusion of inappropriate data sets: not all populations for which biomass and density data are available are undergoing self-thinning. There was no evidence for relationships between shade tolerance or taxonomic groups and the slope, and reported relationships between the slope and various allometric growth constants, though real, were weak. Combined data for all populations are not consistent with an interspecific relationship of slope -3/2, the slope being somewhat shallower, at -0.379, possibly because only the stem mass of trees is generally measured. There is therefore no evidence at present for a -3/2 power rule of self-thinning, but final rejection of the idea that there is an ideal slope awaits experiments in which resource levels are controlled. -from Author
Tree-ring samples of Pseudotsuga menziesii (Mirb.) Franco, Pinus ponderosa Laws., and Pinus edulis Engelm. for 1860 through 1962 were obtained from sites along a vegetational gradient, ranging from forest interior sites to semiarid lower forest border sites. Samples were analyzed using a stratified nested plot design. Near the lower forest border tree rings are narrowest, the variability in the relative ring-width response from year to year is greatest, and the variance in common or the correlation between radii and between trees is highest. This variability is more highly related to climatic fluctuations than the variability in highest. This variability is more highly related to climatic fluctuations that the variability in forest interior trees and may be attributed to longer and more frequent periods during which water stress is limiting to physiological processes in the trees. Hence, at the arid forest border tree-ring chronologies contain the most consistent, but variable, growth responses and provide the best record of climatic fluctuations. The semiarid lower forest border trees exhibit a high frequency of partial growth layers. Chronologies from the forest interior exhibit very few partial growth layers. Crossdating between both types of sites and among many individuals of different species makes absolute dating not only possible but exceedingly reliable. Such tree-ring analyses may have considerable application to the evaluation of ecological forest gradients.
In the forest-tundra ecotone region of the Washington and Oregon Cascades, massive invasion into subalpine meadows by a variety of tree species, especially Abies lasiocarpa and Tsuga mertensiana, has been noted. The most intense period of establishment in most areas was during 1928 to 1937. Little invasion has been noted in the Cascade Range since about 1945. We consider fire, grazing, and forest edge effect as possible factors influencing the massive establishment of trees during this time but suggest that climatic change is the most probable causative factor. At Paradise Valley on the slopes of Mount Rainier, trees invaded meadows during the period when the mass budget of the Nisqually Glacier was generally negative; after 1945, this mass budget was positive or balanced. The snow-free period in certain subalpine meadow communities is probably the most critical factor affecting tree establishment. Invasion densities were low on tall-forb-dominated (Valeriana sitchensis) and grass (Festuca viridula) communities and were high on low-forb-dominated (Potentilla flabellifolia) and shrub (Phyllodoce empetriformis-Vaccinium deliciosum) meadow types. However, growth rates of established trees appeared greatest on the tall-forb and grass communities.
The northern limit of the natural distribution of Tilia cordata in Britain is the English Lake District (latitude 54° 30′N). Many populations contain very old trees and in all but three of 150 localities there has been no regeneration from seed for at least a century. Seeds produced are largely infertile. Trees commonly grow in rocky situations which have probably never been entirely cleared by man. Most localities are between sea-level and 160 m, sometimes on level ground but usually on slopes with aspects between 45° and 315°. Populations occur on all the main rock-types and on soils ranging from acid rankers and brown podzolic soils to rendzinas. The climate of the region is characterized by mild winters and cool summers with mean daily maxima of air temperature for July and August not exceeding 19°C.
Three characteristics of the output of a forest-stand simulation model were matched to pollen records of actual vegetation in central Tennessee. Temporal shifts of individual pollen taxon frequencies were compared to shifts of individual plant species frequencies in simulated biomass for the last 16,000 y. Individual pollen profiles (temporally ordered species frequencies) were also compared to simulated biomass profiles during that period. Modern ratios of pollen to vegetation composition (R values) were compared with those calculated from simulated biomass percentages and fossil pollen percentages. The model output was similar to the comparable characteristics of the pollen record. The model output is therefore a plausible description of vegetation characteristics at the site of pollen deposition in central Tennessee. The model produced information unavailable from other sets of prehistoric data. This information describes the invasion and growth of the yellow-poplar which produces no windborne pollen, and of palynologically indistinguishable oak and pine species. These results suggest that many paleoecological questions can be answered through appropriate simulation modeling studies.
Within some populations, mean annual growth rate was greater for trees heterozygous at a high proportion of their isozyme loci than for relatively homozygous trees. The strength of the heterozygosity-growth relationship varied from strongly positive in the oldest stands to negative in the youngest. The regression coefficient relating growth to heterozygosity was strongly correlated with age (r = .85). Because of the record left by the annual rings, tree growth at various past ages can be related to heterozygosity, and within most stands the relationship between growth rate and heterozygosity increased as trees aged. Apparently, the superiority of heterozygotes is not expressed in young stands. The delayed expression of heterozygote superiority may reflect the greater homeostasis of heterozygotes in the face of year-to-year climatic variation, or the accentuation of differences by competition between heterozygotes and homozygotes as the canopy closes. In fact, the heterozygosity-growth relationship was strongest in the least predictable environments (r = 69), as measured by the standard deviation of mean monthly maximum temperatures over years of record. But competition may also be involved because heterozygote superiority was most strongly expressed in the densest stands (r = .28). The failure of previous investigations to find correlations may result from scoring too few loci to adequately characterize heterozygosity, or to sampling populations in which competition was not strongly expressed. The relationship of growth to heterozygosity appears explicable with reference to age, stand structure, and climatic variability There was no heterosis associated with any single locus, suggesting that depression of growth in homozygotes is a result of linked deleterious recessives. Deleterious recessives are in high frequency in most conifers. Homozygosity in pitch pine probably identifies individuals carrying chromosome segments identical by descent from recent ancestors, and apparent heterozygote superiority may actually measure inbreeding depression in homozygotes. Homeostasis in forest trees has been measured by the inverse of variability in ring width or annual increment. For pitch pine there was no consistent relationship of variability to heterozygosity, perhaps because of random error and because variability in annual increment is not a cumulative characteristic like mean annual increment.
Tree stems 2 106 yr old (i.e., established before significant European influence in this area) in a 7.3-ha old-growth ponderosa pine (Pinus ponderosa) forest in northern Arizona were aged and mapped. Age structure analysis showed that successful establishment of ponderosa pine was infrequent. The periods without successful establishment could be quite long, as suggested by four consecutive decades in which only two surviving trees were established. The stems were strongly aggregated, as measured with nearest neighbor analysis, and groups were visually distinct in the field. Most of the stems occurred in groups of three or more, with group size ranging from 3 to 44 stems and area occupied by a group ranging from 0.02 to 0.29 ha. Ages of stems within groups were variable, the most homogeneous group having a range of 33 yr and the least having a range of 268 yr. The data are not consistent with the commonly held view that southwestern ponderosa pine occurs in even-aged groups and that each group became established following the demise of the group previously occupying the site. Instead, it seems more likely that seedlings became established when one or two trees within the group died, the additional fuel surrounding the dead trees causing an intensely burned spot in the otherwise low-intensity fires that were frequent in the area. The hot spot would create a potential seedbed for pine by eliminating, at least temporarily, the competing grasses on that small area. This decreased competition, in conjunction with adequate seed production and favorable moisture conditions in the spring and early summer, may well have been critical for pon- derosa pine establishment. The relative infrequency of all these events occuning in the necessary sequence could explain the erratic age structure data from this area.
The dynamies of small openings in a hemlock–hardwood stand at the Harvard Forest, Petersham, Massachusetts, were studied. Rates of lateral growth of canopy tree species into openings ranged from 6 to 14 cm/year with a maximum of 26. Red oak (Quercusrubra L.) (RO) had the highest rate of lateral expansion. In small openings (r < 0.25 × tree height), regenerating species ranked by rate of height growth were as follows: black birch (Betulatenta L.) (BB) > red maple (Acerrubrum L.) (RM) > yellow birch (B. alleghaniensis Britton) (YB) > hemlock (Tsugacanadensis (L.)Carr.) (HK) = red oak; in moderate size openings (r = 0.25 to 0.5 × tree height), the ranking was birches = RM > RO > HK; in open grown even-aged stands, the ranking was RO > BB = RM = YB > paper birch (B. papyrifera Marsh.) HK. A comparison of rates of height growth with opening closure rates indicates that tree reproduction is not successful in openings of less than about 0.5 × tree height in diameter. This is primarily because small openings close quickly by lateral growth of the surrounding canopy trees and is not simply a factor of changes in rates of height growth with opening size.