Impact of global warming on the tree species composition of boreal forests in Finland and effects on emissions of isoprenoids

This study aims to identify how climate change may influence total emissions of monoterpene and isoprene from boreal forest canopies. The whole of Finland is assumed to experience an annual mean temperature (T) increase of 4 °C and a precipitation increase of 10% by the year 2100. This will increase forest resources throughout the country. At the same time, the proportions of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) in southern Finland (60°≤ latitude < 65°N) will be reduced from the current 40–50% to less than 10–20%, with increased dominance of birches (Betula pendula and Betula pubescens). In northern Finland (65°≤ latitude < 70°N), the proportions of Norway spruce and Scots pine will be balanced at a level of about 40% as the result of an increase in Norway spruce from the current 21% to 37% and a concurrent reduction in Scots pine from 63% to 40%. The proportion of birches is predicted to increase from the current 17% to 23%, but these will become the dominant species only on the most fertile sites. Total mean emissions of monoterpene by Scots pine will be reduced by 80% in southern Finland, but will increase by 62% in the north. Emissions from Norway spruce canopies will increase by 4% in the south but by 428% in the north, while those from birch canopies will increase by about 300% and 113%, respectively. Overall emissions of monoterpene over the whole country amount to about 950 kg km^?2 y^?1 under current temperature conditions and will increase by 17% to 1100 kg km^?2 y^?1 with elevated temperature and precipitation, mainly because of an increase at northern latitudes. Under current conditions, emissions of isoprene follow the spatial distribution of spruce canopies (the only isoprene‐emitting tree species that forms forests in Finland) with four times higher emissions in the south than in the north. The elevated temperature and the changes in the areal distribution of Norway spruce will result in increases in isoprene emissions of about 37% in southern Finland and 435% in northern Finland. Annual mean isoprene emissions from Norway spruce canopies over the whole country will increase by about 60% up to the year 2100.     

大兴安岭北部樟子松树木生长与气候因子的关系

 在大兴安岭北部漠河(MH I、MH II 2个样点)、塔河蒙克山(MKS)、满归(MG)地区共采集樟子松(Pinus sylvestris var. mongolica)年轮样芯139个, 成功地建立了MH I、MH II、MKS和MG 4个样点的樟子松差值年表, 最长达377年(1631–2007年, 有效年表为1743–2007年)。樟子松年轮指数与气候因子的响应函数分析表明, 气温是这4个样点樟子松径向生长的主要限制因子。但4个样点限制其生长的月份有所差异, 漠河的2个样点樟子松年轮指数与6月气温负相关, 满归和塔河蒙克山樟子松年轮指数与前一年10月气温正相关。樟子松年表与区域气候的冗余分析(redundancy analysis, RDA)基本与响应函数分析的结果一致, 进一步验证了气温对大兴安岭北部樟子松生长的限制作用。该研究为全球变暖下大兴安岭北部樟子松林的经营管理及区域气候重建提供了基础数据。     

Contrasting climate-growth relationship between Larix gmelinii and Pinus sylvestris var. mongolica along a latitudinal gradient in Daxing’an Mountains,China

The Daxing’an Mountains is one of the areas with the most serious climate warming in northern China. Dahurian larch (Larix gmelinii) and Mongolian Scots pine (Pinus sylvestris var. mongolica) are two major coniferous species in boreal forests of the region. Their growth-climate relationship is crucial for understanding the effects of climate change on boreal forest ecosystems. To examine and compare the changes of climate-growth relationship between larch and pine, a total of 418 tree-ring cores of the two species were collected at six sites in the Daxing’an Mountains, and the tree-ring chronologies were developed. The results showed that water availability (Palmer Drought Severity Index, PDSI) played a key role in the stable growth of larch and pine. The temperature and precipitation in January, June-August are important factors affecting the radial growth of the two coniferous species along the latitude gradient. The correlation coefficients of growth and the seasonal temperature and precipitation of larch and pine showed a completely opposite trend with the increase of latitude. In summer and autumn, the correlation coefficients between larch growth and seasonal mean temperature decreased first and then increased with the increase of latitude, while that of pine, on the contrary, increased first and then decreased. In winter, spring and autumn, the correlation coefficients between larch growth and seasonal total precipitation decreased first and then increased with the increase of latitude, while that of pine was opposite. However, the correlation coefficients between larch and pine growth and PDSI showed the same trend with the increase of latitude, decreasing at first and then increasing. Before and after rapid warming (around 1980), the correlation coefficients between larch and pine growth and PDSI showed a completely opposite change. Our findings emphasize that the growth-climate relationships of Dahurian larch and Mongolian Scotts pine shows an opposite trend with latitude, which means that the two species may exhibit a completely opposite response with climate change along the latitude gradient.     

Sensitivity of French temperate coniferous forests to climate variability and extreme events (Abies alba,Picea abies and Pinus sylvestris)

Questions: (1) How do extreme climatic events and climate variability influence radial growth of conifers (silver fir, Norway spruce, Scots pine)? (2) How do elevation and soil water capacity (SWC) modulate sensitivity to climate? Location: The sampled conifer stands are in France, in western lowland and mountain forests, at elevations from 400 to 1700 m, and an SWC from 50 to 190 mm. Methods: We established stand chronologies for total ring width, earlywood and latewood width for the 33 studied stands (985 trees in total). Responses to climate were analysed using pointer years and bootstrapped response functions. Principal component analysis was applied to pointer years and response function coefficients in order to elucidate the ecological structure of the studied stands. Results: Extreme winter frosts are responsible for greater growth reductions in silver fir than in Norway spruce, especially at the upper elevation, while Scots pine was the least sensitive species. Exceptional spring droughts caused a notable growth decrease, especially when local conditions were dry (altitude<1000 m and SWC<100 mm for silver fir, western lowlands for Scots pine). Earlywood of silver fir depended on previous September and November and current‐year February temperature, after which current June and July water supply influenced latewood. Earlywood of Norway spruce was influenced by previous September temperature, after which current spring and summer droughts influenced both ring components. In Scots pine, earlywood and latewood depended on the current summer water balance. Local conditions mainly modulated latewood formation. Conclusions: If the climate becomes drier, low‐elevation dry stands or trees growing in western lowlands may face problems, as their growth is highly dependent on soil moisture availability.     

Geographical adaptation prevails over species‐specific determinism in trees’ vulnerability to climate change at Mediterranean rear‐edge forests

Climate change may reduce forest growth and increase forest mortality, which is connected to high carbon costs through reductions in gross primary production and net ecosystem exchange. Yet, the spatiotemporal patterns of vulnerability to both short‐term extreme events and gradual environmental changes are quite uncertain across the species’ limits of tolerance to dryness. Such information is fundamental for defining ecologically relevant upper limits of species tolerance to drought and, hence, to predict the risk of increased forest mortality and shifts in species composition. We investigate here to what extent the impact of short‐ and long‐term environmental changes determines vulnerability to climate change of three evergreen conifers (Scots pine, silver fir, Norway spruce) and two deciduous hardwoods (European beech, sessile oak) tree species at their southernmost limits of distribution in the Mediterranean Basin. Finally, we simulated future forest growth under RCP 2.6 and 8.5 emission scenarios using a multispecies generalized linear mixed model. Our analysis provides four key insights into the patterns of species’ vulnerability to climate change. First, site climatic marginality was significantly linked to the growth trends: increasing growth was related to less climatically limited sites. Second, estimated species‐specific vulnerability did not match their a priori rank in drought tolerance: Scots pine and beech seem to be the most vulnerable species among those studied despite their contrasting physiologies. Third, adaptation to site conditions prevails over species‐specific determinism in forest response to climate change. And fourth, regional differences in forests vulnerability to climate change across the Mediterranean Basin are linked to the influence of summer atmospheric circulation patterns, which are not correctly represented in global climate models. Thus, projections of forest performance should reconsider the traditional classification of tree species in functional types and critically evaluate the fine‐scale limitations of the climate data generated by global climate models.     

Simulation of tree species composition and organic matter accumulation in Finnish boreal forests under changing climatic conditions

A model simulating the regeneration, growth and death of trees and the consequent carbon and nitrogen dynamics of the forest ecosystem was applied to determine the effect of expected temperature rise on tree species composition and the accumulation of organic matter in the boreal forest ecosystem in Finland (between latitudes 60°–70° N). In the southern and middle boreal zones a temperature rise of 2–3° C (temperature for 2 x CO₂) over a period of one hundred years increased the competitive capacity of Scots pine (Pinus sylvestris) and birch species (Betula pendula and B. pubescens), and slowed down the invasion by Norway spruce (Picea abies). In the northern boreal zone a corresponding rise in temperature promoted the invasion of sites by Norway spruce. The accumulation of organic matter was promoted only slightly compared to that taking place in the current climatic conditions.A further doubling of temperature (temperature for 4 x CO₂) over an additional period of two hundred years led to the replacement of coniferous stands with deciduous onesin the southern and middle boreal zones. In the northern boreal zone an admixture of coniferous and deciduous species replaced pure coniferous stands with the latter taking over sites formerly classified as tundra woodland. In the southern and middle boreal zones the replacement of coniferous species induced a substantial decrease in the amount of organic matter; this returned to its former level following the establishment of deciduous species. In the northern boreal zone there was no major change in the amount of organic matter such as occurred in the case of the tundra woodland where the amount of organic matter accumulated was nearly as high as in the northern boreal zone.     

Size-dependent responses to summer drought in Scots pine, Norway spruce and common oak

In this study, we provide a detailed analysis of tree growth and water status in relation to climate of three major species of forest trees in lower regions of Bavaria, Southern Germany: Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and common oak (Quercus robur). Tree-ring chronologies and latewood δ¹³C were used to derive measures for drought reaction across trees of different dimensions: growth reduction associated with drought years, long-term growth/climate relations and stomatal control on photosynthesis. For Scots pine, growth/climate relations indicated a stronger limitation of radial growth by high summer temperatures and low summer precipitation in smaller trees in contrast to larger trees. This is corroborated by a stronger stomatal control on photosynthesis for smaller pine trees under average conditions. In dry years, however, larger pine trees exhibited stronger growth reductions. For Norway spruce, a significantly stronger correlation of tree-ring width with summer temperatures and summer precipitation was found for larger trees. Additionally, for Norway spruce there is evidence for a change in competition mode from size-asymmetric competition under conditions with sufficient soil water supply to a more size-symmetric competition under dry conditions. Smaller oak trees showed a weaker stomatal control on photosynthesis under both dry and average conditions, which is also reflected by a significantly faster recovery of tree-ring growth after extreme drought events in smaller oak trees. The observed patterns are discussed in the context of the limitation-caused matter partitioning hypothesis and possible species-specific ontogenetic modifications.     

Climate change induces multiple risks to boreal forests and forestry in Finland: A literature review

Climate change induces multiple abiotic and biotic risks to forests and forestry. Risks in different spatial and temporal scales must be considered to ensure preconditions for sustainable multifunctional management of forests for different ecosystem services. For this purpose, the present review article summarizes the most recent findings on major abiotic and biotic risks to boreal forests in Finland under the current and changing climate, with the focus on windstorms, heavy snow loading, drought and forest fires and major insect pests and pathogens of trees. In general, the forest growth is projected to increase mainly in northern Finland. In the south, the growing conditions may become suboptimal, particularly for Norway spruce. Although the wind climate does not change remarkably, wind damage risk will increase especially in the south, because of the shortening of the soil frost period. The risk of snow damage is anticipated to increase in the north and decrease in the south. Increasing drought in summer will boost the risk of large‐scale forest fires. Also, the warmer climate increases the risk of bark beetle outbreaks and the wood decay by Heterobasidion root rot in coniferous forests. The probability of detrimental cascading events, such as those caused by a large‐scale wind damage followed by a widespread bark beetle outbreak, will increase remarkably in the future. Therefore, the simultaneous consideration of the biotic and abiotic risks is essential.     

Regeneration of planted conifers across climatic moisture gradients on the Canadian prairies: implications for distribution and climate change

The influence of dry climates on white spruce ( Picea glauca (Moench) Voss)) regeneration was examined by conducting surveys of seedlings and small trees that had regenerated naturally at 100 farm shelterbelts and plantations in southern Saskatchewan, Canada. The sites surveyed were located along a climate moisture gradient extending from the relatively moist boreal forest, across the aspen parkland, to the semi-arid prairie grasslands. Natural regeneration was greatest at sites in the boreal forest and northern aspen parkland, decreased in the southern aspen parkland, and was negligible in the grassland zone. Furthermore, the few seedlings found in the drier zones were usually in poor condition. Similar results were obtained for the introduced Colorado spruce ( Picea pungens Engelm.) and Scots pine ( Pinus sylvestris L.). It is concluded that the present climate of the southern parkland and grassland is too dry to permit natural regeneration of white spruce and other conifers. If increases in atmospheric CO₂ levels lead to a drier future climate in the southern boreal forest of western Canada, the ability of conifers to regenerate naturally may be significantly reduced.     

Spatiotemporal variation in the relationship between boreal forest productivity proxies and climate data

The impacts of climate change on high-latitude forest ecosystems are still uncertain. Divergent forest productivity trends have recently been reported both at the local and regional level challenging the projections of boreal tree growth dynamics. The present study investigated (i) the responses of different forest productivity proxies to monthly climate (temperature and precipitation) through space and time; and (ii) the local coherency between these proxies through time at four high-latitude boreal Scots pine sites (coastal and inland) in Norway. Forest productivity proxies consisted of two proxies representing stem growth dynamics (radial and height growth) and one proxy representing canopy dynamics (cumulative May-to-September Normalized Difference Vegetation Index (NDVI)). Between-proxy and climate-proxy correlations were computed over the 1982–2011 period and over two 15-yr sub-periods. Over the entire period, radial growth significantly correlated with current year July temperature, and height growth and cumulative NDVI significantly correlated with previous and current growing season temperatures. Significant climate responses were quite similar across sites, despite some higher sensitivity to non-growing season climate at inland sites. Significant climate-proxy correlations identified over the entire period were temporarily unstable. Local coherency between proxies was generally insignificant. The spatiotemporal instability in climate-proxy correlations observed for all proxies underlines evolving responses to climate and challenges the modelling of forest productivity. The general lack of local coherency between proxies at our four study sites suggests that forest productivity estimations based on a single proxy should be considered with great caution. The combined use of different forest growth metrics may help circumvent uncertainties in capturing responses of forest productivity to climate variability and improve estimations of carbon sequestration by forest ecosystems.     

Sub-cortical Coleoptera in dead pines and spruces: is primeval species composition maintained in managed forests?

The sub-cortical beetle fauna of dead Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) trunks was compared in primeval forests and managed forests in central Finland. The numbers of both individuals and species were higher in the managed forest in spite of the smaller trunk surface area examined. The proportion of rare species was higher in the primeval forest. Although most species occurred both in primeval and managed forests, there were striking differences in the abundance relations: there was only one species (Pytho depressus) in common among the ten most abundant species. The proportion of bark beetles (Scolytidae) was more than 50% in the managed forests, and less than 5% in the primeval forests. The number of species per site was associated with observation date, occurrence of snails and trunk position (standing or lying) in the primeval forest, and with trunk diameter in the managed forests. The occurrence of rare beetle species in dead conifer trunks was related to man's effects on the forest. Although many sub-cortical species can live in managed forests, the fauna differs drastically from that of primeval forests.     

Understanding 3D structural complexity of individual Scots pine trees with different management history

Tree functional traits together with processes such as forest regeneration, growth, and mortality affect forest and tree structure. Forest management inherently impacts these processes. Moreover, forest structure, biodiversity, resilience, and carbon uptake can be sustained and enhanced with forest management activities. To assess structural complexity of individual trees, comprehensive and quantitative measures are needed, and they are often lacking for current forest management practices. Here, we utilized 3D information from individual Scots pine (Pinus sylvestris L.) trees obtained with terrestrial laser scanning to, first, assess effects of forest management on structural complexity of individual trees and, second, understand relationship between several tree attributes and structural complexity. We studied structural complexity of individual trees represented by a single scale‐independent metric called “box dimension.” This study aimed at identifying drivers affecting structural complexity of individual Scots pine trees in boreal forest conditions. The results showed that thinning increased structural complexity of individual Scots pine trees. Furthermore, we found a relationship between structural complexity and stem and crown size and shape as well as tree growth. Thus, it can be concluded that forest management affected structural complexity of individual Scots pine trees in managed boreal forests, and stem, crown, and growth attributes were identified as drivers of it.     

Stand characteristics and biodiversity indicators along the productivity gradient in boreal forests: Defining a critical set of indicators for the monitoring of habitat nature quality

Abstract

We quantified the effects of anthropogenic disturbances on the structure and biodiversity of boreal forests on acidic soils and created a statistically supported rational set of indicators to monitor the stand “naturalness”. For that, we surveyed various traits of tree layer, understory, herb layer, forest floor and several widely accepted biodiversity epiphytic indicators in 252 old‐aged boreal stands in Estonia, mostly dominated by Scots pine or Norway spruce. Multifactorial general linear model analyses showed that many forest characteristics and potential indicators were confounded by the gradient of soil productivity (reflected by the forest site type), local biogeographic gradients and also by stand age. Considering confounding effects, boreal forests in a near‐natural state have more large‐diameter trees (diameter at breast height >40 cm) and larger variety of diameter classes, higher proportion of spruce or deciduous trees, a larger amount of coarse woody debris in various stages, a more closed tree canopy and denser understory than managed mature forests. By increasing light availability above the field layer, forest management indirectly increases the coverage of herbs and lichens on the forest floor but reduces the alpha‐ and beta‐diversity of herbs and the proportion of graminoids. Human disturbances reduce the relative incidence of many commonly accepted biodiversity indicators such as indicator lichens, woodpeckers, wood‐dwelling insects or fungi on trees. The test for the predictive power of characteristics reacting on disturbance revealed that only a fraction of them appeared to be included in a diagnostic easy‐to‐apply set of indicators to assess the nature quality of boreal forest: the amount of dead wood, the proportion of deciduous trees, the presence of specially shaped trees and woodpeckers and, as an indicator of disturbances, the forest herb Melampyrum pratensis. Many of these indicators have already been implemented in practice.     

Persistence of vascular plants in a Norwegian boreal coniferous forest

Persistence, the tendency of a species to remain in its original position and not to colonize new sites, is studied for the most abundant forest vascular plants (25 species in spruce forest and 7 in pine forest) in Solhomfjell, Gjerstad, S Norway Data sets included presence/absence in 199 meso plots (1 m²) and 3184 subplots (1/16 m²), analyzed over a 5 yr interval, and a subset of 50 meso plots and 800 subplots, analyzed for six consecutive years Relationships between species variables (seedling frequency and mobility rate compiled from the literature, and cover and abundance means in the study area) are studied, and related to species optima along ecologically interpreted DCA ordination axes Vascular plant mobility may increase towards nutrient-poor sites Dominance in the boreal forest floor is mostly by clonal species Persistence was calculated for different temporal (1-5 yr) and spatial (1/16 and I m²) scales Persistence patterns in the spruce and pine forests were similar, but persistence decreased towards the xeric pine forests One main component of variation in persistence was demonstrated by PCA analyses the absolute level of persistence, which is related to seedling recruitment vs clonal growth, and within clonal species to ramet longevity, abundance, mobility, growth pattern and mode of surviving the infavourable season Minor components of variation in persistence were related to spatial scale and temporal scale Persistence characteristics were species-specific and varied little between years Numerous species characteristics were relevant to interpretation of variation in persistence, indicating a continuous, multidimensional variation in life history characteristics     

Fine root foraging strategies in Norway spruce forests across a European climate gradient

Fine root acclimation to different environmental conditions is crucial for growth and sustainability of forest trees. Relatively small changes in fine root standing biomass (FRB), morphology or mycorrhizal symbiosis may result in a large change in forest carbon, nutrient and water cycles. We elucidated the changes in fine root traits and associated ectomycorrhizal (EcM) fungi in 12 Norway spruce stands across a climatic and N deposition gradient from subarctic‐boreal to temperate regions in Europe (68°N–48°N). We analysed the standing FRB and the ectomycorrhizal root tip biomass (EcMB, g m^?2) simultaneously with measurements of the EcM root morphological traits (e.g. mean root length, root tissue density (RTD), N% in EcM roots) and frequency of dominating EcM fungi in different stands in relation to climate, soil and site characteristics. Latitude and N deposition explained the greatest proportion of variation in fine root traits. EcMB per stand basal area (BA) increased exponentially with latitude: by about 12.7 kg m^?2 with an increase of 10° latitude from southern Germany to Estonia and southern Finland and by about 44.7 kg m^?2 with next latitudinal 10° from southern to northern Finland. Boreal Norway spruce forests had 4.5 to 11 times more EcM root tips per stand BA, and the tips were 2.1 times longer, with 1.5 times higher RTD and about 1/3 lower N concentration. There was 19% higher proportion of root tips colonized by long‐distance exploration type forming EcM fungi in the southern forests indicating importance of EcM symbiont foraging strategy in fine root nutrient acquisition. In the boreal zone, we predict ca. 50% decrease in EcMB per stand BA with an increase of 2 °C annual mean temperature. Different fine root foraging strategies in boreal and temperate forests highlight the importance of complex studies on respective regulatory mechanisms in changing climate.     

Climate‐diameter growth relationships of black spruce and jack pine trees in boreal Ontario,Canada

To predict the long‐term effects of climate change – global warming and changes in precipitation – on the diameter (radial) growth of jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana [Mill.] B.S.P.) trees in boreal Ontario, we modified an existing diameter growth model to include climate variables. Diameter chronologies of 927 jack pine and 1173 black spruce trees, growing in the area from 47°N to 50°N and 80°W to 92°W, were used to develop diameter growth models in a nonlinear mixed‐effects approach. Our results showed that the variables long‐term average of mean growing season temperature, precipitation during wettest quarter, and total precipitation during growing season were significant (alpha = 0.05) in explaining variation in diameter growth of the sample trees. Model results indicated that higher temperatures during the growing season would increase the diameter growth of jack pine trees, but decrease that of black spruce trees. More precipitation during the wettest quarter would favor the diameter growth of both species. On the other hand, a wetter growing season, which may decrease radiation inputs, increase nutrient leaching, and reduce the decomposition rate, would reduce the diameter growth of both species. Moreover, our results indicated that future (2041–2070) diameter growth rate may differ from current (1971–2000) growth rates for both species, with conditions being more favorable for jack pine than black spruce trees. Expected future changes in the growth rate of boreal trees need to be considered in forest management decisions. We recommend that knowledge of climate–growth relationships, as represented by models, be combined with learning from adaptive management to reduce the risks and uncertainties associated with forest management decisions.     

Effects of nutrition and soil warming on stemwood production in a boreal Norway spruce stand

The boreal forest is expected to experience the greatest warming of all forest biomes. The extent of the boreal forest, the large amount of carbon contained in the soil, and the expected climate warming, make the boreal forest a key biome to understand and represent correctly in global carbon models. It has been suggested that an increase in temperature could stimulate the release of CO₂ caused by an increased decomposition rate, more than biomass production, which could convert current carbon sinks into carbon sources. Most boreal forests are currently carbon sinks, but it is unclear for how long in the future the carbon sink capacity of the boreal forest is likely to be maintained. The impact of soil warming on stem volume growth was studied during 6 years, in irrigated (I) and irrigated‐fertilized (IL) stands of 40‐year‐old Norway spruce in Northern Sweden. From May to October heating cables were used to maintain the soil temperature on heated‐irrigated plots (Ih and ILh) 5 °C above that on unheated control plots (Ic and ILc). After six seasons' warming, stem volume production (m³ ha^?1 a^?1) was 115% higher on Ih than on unheated (Ic) plots, and on heated and irrigated‐fertilized plots (ILh) it was 57% higher than on unheated plots (ILc). The results indicate that in a future warmer climate, an increased availability of nitrogen, combined with a longer growing season, may increase biomass production substantially, on both low‐ and high‐fertility sites. It is, however, too early to decide whether the observed responses are transitory or long lasting. It is therefore crucial to gain a better understanding of the responses of boreal forest ecosystems to climate change, and to provide data to test and validate models used in predicting the impact of climate change.     

Simulating boreal forest dynamics from perspectives of ecophysiology,resource availability,and climate change

Boreal forests are under strong influences from climate change, and alterations in forest dynamics will have significant impacts on global climate-biosphere feedback as well as local to regional conservation and resource management. To understand the mechanisms of forest dynamics and to assess the fate of boreal forests, simulation studies should be based on plant ecophysiological responses onto environmental conditions. In central Canadian boreal forests, local geomorphology created by past glacial activities often generates a mosaic of very distinctive forest types. On sandy hilltop of a glacial till, due to limitations in moisture availability and short fire return intervals, drought-tolerant and fire-adapted jack pine usually becomes the dominant species. On mesic and nutrient-rich slopes, fast-growing and resource-demanding trembling aspen forms mixed forests with coniferous species. In bottomland, black spruce, slowly growing but tolerant species, is often the only species that can survive to the adult stage. These three very distinctive forest types often occur within a scale of 10 m. Simulation models of boreal forests should be able to reproduce this heterogeneity in forest structure and composition as an emergent property of plant ecophysiological responses to varying environmental properties. In this study, a process-based forest dynamics model, ecosystem demography model version 1.0, is used to mechanically reproduce the landscape heterogeneity due to edaphic variations. First, boreal tree species of northern Manitoba, Canada, are parameterized according to field observations, and, to explicitly capture interactions among tree saplings, allometric equations based on diameter at height of 0.15 m, instead of the conventional breast height of 1.37 m, is parameterized. Then, soil moisture regime and nutrient concentrations are statistically incorporated from a dataset. The resultant simulation successfully reproduces the distinctive forest dynamics influenced by the edaphic heterogeneity. The sequences of succession and the trajectories of forest development are generally consistent with the field observations. The differences in resource availability are the essential control on equilibrium values of total forest leaf area index. Next, to show the effect of anthropogenic atmospheric changes, changes in temperature and CO₂ concentrations are studied by a set of factorial experiments. The magnitude of CO₂ fertilization is largely affected by soil fertility. The temperature rise will increase the length of growing season, but can have a negative impact on forest growth by increasing aridity and autotrophic respiration. Overall, the boreal forest responses to climate change are complex due to the inherent edaphic variations and ecophysiological responses.     

Assessing forest vulnerability to climate warming using a process‐based model of tree growth: bad prospects for rear‐edges

Growth models can be used to assess forest vulnerability to climate warming. If global warming amplifies water deficit in drought‐prone areas, tree populations located at the driest and southernmost distribution limits (rear‐edges) should be particularly threatened. Here, we address these statements by analyzing and projecting growth responses to climate of three major tree species (silver fir, Abies alba; Scots pine, Pinus sylvestris; and mountain pine, Pinus uncinata) in mountainous areas of NE Spain. This region is subjected to Mediterranean continental conditions, it encompasses wide climatic, topographic and environmental gradients, and, more importantly, it includes rear‐edges of the continuous distributions of these tree species. We used tree‐ring width data from a network of 110 forests in combination with the process‐based Vaganov–Shashkin‐Lite growth model and climate–growth analyses to forecast changes in tree growth during the 21st century. Climatic projections were based on four ensembles CO₂ emission scenarios. Warm and dry conditions during the growing season constrain silver fir and Scots pine growth, particularly at the species rear‐edge. By contrast, growth of high‐elevation mountain pine forests is enhanced by climate warming. The emission scenario (RCP 8.5) corresponding to the most pronounced warming (+1.4 to 4.8 °C) forecasted mean growth reductions of ?10.7% and ?16.4% in silver fir and Scots pine, respectively, after 2050. This indicates that rising temperatures could amplify drought stress and thus constrain the growth of silver fir and Scots pine rear‐edge populations growing at xeric sites. Contrastingly, mountain pine growth is expected to increase by +12.5% due to a longer and warmer growing season. The projections of growth reduction in silver fir and Scots pine portend dieback and a contraction of their species distribution areas through potential local extinctions of the most vulnerable driest rear‐edge stands. Our modeling approach provides accessible tools to evaluate forest vulnerability to warmer conditions.