Boreal forests store 30% of the world's terrestrial carbon (C). Consequently, climate change mediated alterations in the boreal forest fire regime can have a significant impact on the global C budget. Here we synthesize the effects of forest fires on the stocks and recovery rates of C in boreal forests using 368 plots from 16 long?term (?100 year) fire chronosequences distributed throughout the boreal zone. Forest fires led to a decrease in total C stocks (excluding mineral soil) by an average of 60% (range from <10% to >80%), which was primarily a result of C stock declines in the living trees and soil organic layer. Total C stocks increased with time since fire largely following a sigmoidal shape Gompertz function, with an average asymptote of 8.1 kg C m?2. Total C stocks accumulated at a rate of 2–60 g m?2 yr?1 during the first 100 years. Potential evapotranspiration (PET) was identified as a significant driver of C stocks and their post?fire recovery, likely because it integrates temperature, radiation, and the length of the growing season. If the fire return interval shortens to ?100 years in the future, our findings indicate that many boreal forests will be prevented from reaching their full C storage potential. However, our results also suggest that climate warming?induced increases in PET may speed up the post?fire recovery of C stocks.

The foliage biomass–sapwood relationship (the pipe model) is critical for tree growth and is used in tree growth models for understanding the implications of this structural relationship on the allocation of resources. In this research, we compared this relationship for two commercially important and sympatric species, black spruce (Picea mariana (Mill.) B.S.P.) and white spruce (Picea glauca (Moench) Voss). At locations in eastern Canada, 57 black and 50 white spruce trees were destructively sampled to obtain foliage biomass, crown structure, and tree stem measures. Using a model-based approach, we compared foliage biomass–branch basal area and foliage biomass–sapwood relationships at the tree and disk (i.e. along the tree stem) levels (i.e. pipe-model ratios) between these two species. We found that (i) branch foliage biomass–branch basal area was greater for black spruce than white spruce and (ii) pipe-model ratios along the tree stem given tree size were greater for black spruce than for white spruce. We attributed these differences to: (i) greater shade tolerance and leaf longevity of black spruce; (ii) slower growth rates of black spruce; and (iii) differing hydraulic strategies and mechanical requirements.

Ecosystem-based management (EBM) of forests is gaining acceptance for its focus on the maintenance of the long-term integrity of ecosystem processes, but climate change challenges this view because of its impacts on these very processes. We have therefore evaluated the robustness of EBM to projected climate change, considering the role of climate on forest growth and fire regime in a boreal forest of eastern Canada. A climate sensitive growth index model was calibrated for three commercial species and used to project the evolution of merchantable volume for two climate scenarios (B1 and A2) under conventional and EBM strategies. Current burn rate and burn rates under future climate scenarios were also considered. Under the most extreme projected climate scenario, the periodic timber supply could be reduced by up to 79% through direct (growth reduction) and indirect (fire) effects. However, ecological indicators show that EBM is a more robust forest management strategy than conventional one demonstrating its adaptation potential to climate change at least in the short term.

Ecosystem productivity estimated with a model calibrated with eddy-covariance data was related to tree-ring growth of two different boreal conifers along a latitudinal gradient. The relationship between ecosystem productivity and growth changed with species and site. Greater photosynthesis in spring and summer increased annual anomalies of radial growth in both species, and the response of growth to productivity was earlier in warmer southern stands particularly for pine. Radial growth of jack pine increased in the long-term with higher productivity, whereas this relationship was more reduced in black spruce. This could express species-specific differences in carbon allocation strategies but likely it is a consequence of the limiting marginal soils where spruce is found in the south. Only tree-rings of jack pine at some sites showed certain potential as direct proxies for ecosystem productivity at the low and high-frequency responses. Introduction Climate warming and the increase in atmos-pheric-CO 2 concentrations cause changes in forest growth and ecosystem productivity. There are reports of contrasting growth responses to warming over recent decades in different types of forests. Although some boreal species show negative growth trends in response to recent climate change (Hoofgaard et al. 1999, D'Arrigo et al. 2004), net ecosystem productivity in boreal and temperate conditions is generally expected to increase with increasing temperatures (Myneni et al. 1997, Boisvenue and Running 2006). Forest growth measurements and models assume that there is a close connection between the stem

Nous avons étudié les effets de l’épaississement de la couche organique au sol (COS) sur la croissance et la distribution de l’épinette noire (Picea mariana (Mill.) Britton, Sterns, Poggenb.) et du peuplier faux-tremble (Populus tremuloides Michx.) dans la ceinture d’argile du Québec. À l’échelle du paysage, l’épinette couvrait un plus large gradient d’épaisseur de la COS (1 à 100 cm) que celui du peuplier (1 à 30 cm). Pour les arbres âgés de 60 à 100 ans, l’épaisseur de la COS n’avait pas d’effet sur l’accroissement en surface terrière (AST) de l’épinette, mais montrait une forte corrélation négative avec l’AST du peuplier. La croissance radiale de l’épinette noire était favorisée par de fortes précipitations en juin de l’année précédente, des températures élevées au début de l’hiver et au printemps, et par des températures froides en été. Bien que statistiquement significative, l’épaisseur de la COS avait des effets modérés sur la relation entre le climat et la croissance de l’épinette, en ayant apparemment un impact sur l’isolation des racines durant la période de dormance et sur la disponibilité en eau durant la période de croissance. Dans le cas du peuplier, la température en juin de l’année courante était le plus important facteur corrélé positivement à la croissance. L’épaisseur de la COS influençait la relation entre la croissance du peuplier et (i) la température de janvier et (ii) l’indice de sécheresse mensuel de juin à août. Nous prévoyons que la réaction de l’épinette noire aux changements climatiques devrait être assez uniforme dans la région étudiée, alors que celle du peuplier sera probablement fortement influencée par l’épaisseur de la COS.

Immediate phenotypic variation and the lagged effect of evolutionary adaptation to climate change appear to be two key processes in tree responses to climate warming. This study examines these components in two types of growth models for predicting the 2010–2099 diameter growth change of four major boreal species Betula papyrifera, Pinus banksiana, Picea mariana, and Populus tremuloides along a broad latitudinal gradient in eastern Canada under future climate projections. Climate-growth response models for 34 stands over nine latitudes were calibrated and cross-validated. An adaptive response model (A-model), in which the climate-growth relationship varies over time, and a fixed response model (F-model), in which the relationship is constant over time, were constructed to predict future growth. For the former, we examined how future growth of stands in northern latitudes could be forecasted using growth-climate equations derived from stands currently growing in southern latitudes assuming that current climate in southern locations provide an analogue for future conditions in the north. For the latter, we tested if future growth of stands would be maximally predicted using the growth-climate equation obtained from the given local stand assuming a lagged response to climate due to genetic constraints. Both models predicted a large growth increase in northern stands due to more benign temperatures, whereas there was a minimal growth change in southern stands due to potentially warm-temperature induced drought-stress. The A-model demonstrates a changing environment whereas the F-model highlights a constant growth response to future warming. As time elapses we can predict a gradual transition between a response to climate associated with the current conditions (F-model) to a more adapted response to future climate (A-model). Our modeling approach provides a template to predict tree growth response to climate warming at mid-high latitudes of the Northern Hemisphere.

• To understand how the future climate will affect the boreal forest, we studied growth responses to climate variability in black spruce (Picea mariana [Mill.] B.S.P.) and trembling aspen (Populus tremuloides Michx.), two major co-occurring boreal tree species of the eastern Canadian boreal forest.
• We analysed climate–growth interaction during (i) periods of non-anomalous growth and (ii) in years with strong growth anomalies. We utilized paired tree-level data for both growth and soil variables, which helped ensure that the studied growth variability was a function of species-specific biology, and not of within stand variation in soil conditions.
• Redundancy analysis conducted on spruce and aspen tree ring chronologies showed that their growth was affected differently by climate. During non-anomalous years, growth of spruce was favoured by cooler temperatures and wetter conditions, while aspen growth was favoured by higher temperatures and drier conditions.
• Black spruce and trembling aspen also showed an inverse pattern in respect to expression of growth anomalies (pointer years). A negative growth anomaly in spruce tended to be associated with positive ones in aspen and vice versa. This suggested that spruce and aspen had largely contrasting species-specific responses to both ‘average’ weather conditions and extreme weather events.
• Synthesis. Species-specific responses to environmental variability imply that tree responses to future climate will likely be not synchronized among species, which may translate into changes in structure and composition of future forest communities. In particular, we speculate that outcome of climate change in respect to relative abundance of black spruce and trembling aspen at the regional levels will be highly dependent on the balance between increasing temperatures and precipitation. Further, species-specific responses of trees to annual climate variability may enhance the resilience of mixed forests by constraining variability in their annual biomass accumulation, as compared with pure stands, under periods with high frequency of climatically extreme conditions.

• Premise of the study: In a warming climate, boreal trees may have adjusted their growth strategy (e.g., onset and coordination of growth among different organs such as stem, shoot, and foliage, within and among species) to cope with the extended growing seasons. A detailed investigation into growth of different organs during a growing season may help us assess the potential effects of climate change on tree growth in the boreal forest.

• Methods: The intra-annual growth of stem xylem, shoot tips, and foliage area of Pinus banksiana, Populus tremuloides, and Betula papyrifera was monitored in a boreal forest in Quebec, Canada during the growing season of 2007. Xylem formation was measured at weekly intervals, and shoot elongation and foliage expansion were measured three times per week from May to September. Growth indices for stem, shoot, and foliage were calculated and used to identify any climate–growth dependence.

• Key results: The time periods required for stem growth, branch extension, and foliage expansion differed among species. Of the three species, P. banksiana had the earliest budburst (20 May) yet the latest completion date of the foliage growth (2 August); P. tremuloides had the latest budburst (27 May) yet the earliest completion date of the foliage growth (10 July). Air temperature positively affected shoot extension growth of all three species. Precipitation positively influenced stem growth of the two broadleaf species, whereas growing season temperature positively impacted stem growth of P. banksiana.

• Conclusion: The results show that both the timing of growth processes and environmental dependences differ among co-occurring species, thereby leading to different adaptive capability of these boreal tree species to climate change.

A decrease in the average diameter of commercially harvested tree species in the Eastern boreal forest of Canada has led to a decrease in availability of quality wood for the forest industry. Commercial thinning has been proposed as a means to increase stem diameter growth. However, little is known about physiological responses underlying species responses to thinning. We assessed the effect of canopy opening on the photosynthetic response of mature jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana (Mill.) BSP) trees. Two years after thinning and for each species, light response curves and the diurnal course of photosynthesis were characterized from measurements taken in a completely randomized block experiment on current-year and one-year-old needles of 12 trees from stands subjected to different levels of canopy opening. Soil water content, air and soil temperatures, and needle N concentration were not affected by thinning for either species. However, light availability increased with basal area removed and could explain the significantly positive relationship between thinning intensity and diurnal course of photosynthesis for one-year-old needles of jack pine. Black spruce photosynthesis did not respond to increases in light. Light-saturated rate of net photosynthesis (A_max), photosynthetic efficiency (α), light compensation point (LCP), and diurnal respiration (R_d) did not vary with thinning for either of the species. Jack pine and black spruce responses to thinning should be interpreted in light of species autecology.

Future climate will alter the soil cover of mosses and snow depths in the boreal forests of eastern Canada. In field manipulation experiments, we assessed the effects of varying moss and snow depths on the physiology of black spruce (Picea ­mariana (Mill.) B.S.P.) and trembling aspen (Populus tremuloides Michx.) in the boreal black spruce forest of western Québec. For 1 year, naturally regenerated 10-year-old spruce and aspen were grown with one of the following treatments: additional N fertilization, addition of sphagnum moss cover, removal of mosses, delayed soil thawing through snow and hay addition, or accelerated soil thawing through springtime snow removal. Treatments that involved the addition of insulating moss or snow in the spring caused lower soil temperature, while removing moss and snow in the spring caused elevated soil temperature and thus had a warming effect. Soil warming treatments were associated with greater temperature variability. Additional soil cover, whether moss or snow, increased the rate of photosynthetic recovery in the spring. Moss and snow removal, on the other hand, had the opposite effect and lowered photosynthetic activity, especially in spruce. Maximal electron transport rate (ETRmax) was, for spruce, 39.5% lower after moss removal than with moss addition, and 16.3% lower with accelerated thawing than with delayed thawing. Impaired photosynthetic recovery in the absence of insulating moss or snow covers was associated with lower foliar N concentrations. Both species were affected in that way, but trembling aspen generally reacted less strongly to all treatments. Our results indicate that a clear negative response of black spruce to changes in root-zone temperature should be anticipated in a future climate. Reduced moss cover and snow depth could adversely affect the photosynthetic capacities of black spruce, while having only minor effects on trembling aspen.