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.

Mixed?wood boreal forests are characterized by a heterogeneous landscape dominated by coniferous or deciduous species depending on stand moisture and fire activity. Our study highlights the long?term drivers of these differences between landscapes across mixed?wood boreal forests to improve simulated vegetation dynamics under predicted climate changes. We investigate the effects of main climate trends and wildfire activities on the vegetation dynamics of two areas characterized by different stand moisture regimes during the last 9000 years. We performed paleofire and pollen analyses in the mixed?wood boreal forest of north?western Ontario, derived from lacustrine sediment deposits, to reconstruct historical vegetation dynamics, which encompassed both the Holocene climatic optimum (ca. 8000–4000 a bp) and the Neoglacial period (ca. 4000 a bp). The past warm and dry period (Holocene climatic optimum) promoted higher fire activity that resulted in an increase in coniferous species abundance in the xeric area. The predicted warmer climate and an increase in drought events should lead to a coniferization of the xeric areas affected by high fire activity while the mesic areas may retain a higher broadleaf abundance, as these areas are not prone to an increase in fire activity.

Plant species mixtures are often seen as being able to achieve higher productivity and carbon (C) sequestration than their single-species counterparts, but it is unclear whether this is true in natural forests. Here, we investigated whether naturally-regenerated mixtures of common North American boreal tree species were more productive and stored more C than single-species stands. We also examined how closely the different C pools and fluxes were interrelated and whether these relationships varied with species composition. Single- and mixed-species stands of trembling aspen, black spruce and jack pine on mesic sites were selected in two regions of the Canadian boreal forest to assess aboveground and belowground productivity and C storage. Although previous studies conducted in these stands found synergistic effects of tree species mixtures on specific C pools and fluxes, such as higher organic layer C stocks and higher fine root productivity in some mixtures, no effects were detected on combined C pools or fluxes at the ecosystem level in the current study. Aspen abundance was linked with higher aboveground tree productivity, higher aboveground living biomass and higher soil heterotrophic respiration, indicating that aspen acts as a key driver of ecosystem C storage and fluxes in these natural forest ecosystems, more so than species richness. However, our results do not rule out the possibility of increased productivity and C storage in mixed stands under environmental conditions or stand developmental stages that are different from the ones studied here. Furthermore, when the entire forest ecosystem is considered (not only tree parts), synergistic effects of tree species mixtures may be more difficult to observe because the beneficial effect of species mixing on one specific C pool may be counterbalanced by a negative effect on another pool.

Understanding fire regime dynamics is central to predicting forest structure and the compositional dynamics of boreal forests. Spatial and temporal variations in fire frequency in central Canadian boreal forests over the last 10 000 years were examined to evaluate the influence of bottom-up controls on the regional fire regime. We analysed macroscopic charcoal larger than 160??m from sediment cores from six lakes to reconstruct fire history and performed GIS analysis of regional landscape features to investigate how fire frequency has changed temporally and how non-climatic factors may have affected long-term fire frequency. Our generalized linear mixed model revealed that temporal changes in fire return intervals (FRIs) were highly dependent on landscape connectivity as inferred through the abundance of natural firebreaks in the form of open water lakes and wetlands. FRIs did not change significantly among highly connected landscapes throughout the Holocene; in contrast, FRIs were significantly longer among poorly connected landscapes in the early Holocene (10–5 cal ka BP), suggesting that the abundant regional firebreaks limited fire spread. All sites had similar FRIs in the late Holocene. The diminishing influence of firebreaks suggests that the regional climate during the late Holocene has overshadowed the influences of the bottom-up controls on fire activities.

The southern portions of the boreal region across Canada are dominated by boreal mixedwoods forests, which are characterized by varying canopy dominance of boreal broadleaf and conifer trees. This forest region encompasses a large east-to-west gradient of climate and disturbance regimes. Although the same major boreal tree species occur in all parts of the boreal mixedwood region, they vary greatly in relative abundance. This is a reflection of the interactions among the different abiotic and biotic components. As a result, there is considerable variation in post-disturbance stand development, producing a wide variety of mixedwood forest conditions existing as a mosaic in time and space. Post-disturbance dominance by broadleaf species followed by a transition to conifers is the “classic” pathway in all regions. However, there is wide variation in the transition rate and the species sequence across the gradient depending on factors such as moisture, abundance of each species, fire cycle, climate and secondary disturbances (mainly insect outbreaks). Future changes in climate and disturbance regime could influence the nature of stand dynamics of boreal mixedwoods and the prominence of different pathways among regions. Focussing on the commonality of processes in mixedwood stand development across the boreal is a promising way to address the management of this important forest ecosystem.

Trembling aspen (Populus tremuloides) is widely distributed in North American forests. Increased stand productivity with resource availability has been reported, but the relationship between soil microbial community structure and stand productivity remains unclear. To examine soil microbial composition of 4 aspen stand productivity classes, we assessed soil properties, microbial biomass and respiration, and bacterial and ectomycorrhizal diversity. Most variables showed no significant differences between productivity classes. However, mean values for basal respiration (0.05 to 27.99 µg CO2-C·g-1 soil·h-1), bacterial biomass, and metabolic quotient (0.08 to 5.22 CO2-C·mg-1 Cmic·h-1) were lowest in low productivity (Class 1) sites. Bacteria to fungi ratios were significantly lower (P = 0.05) in Class 1 compared to other classes. Microbial biomass ranged from 1.39 to 8.11 mg Cmic·g-1soil. Thirty-seven distinct aspen ectomycorrhizas (ECM) were characterized, 21 were considered rare (from ≤3 trees). ECM richness did not differ significantly between classes, although relative abundance for some types did. Canonical correspondence analysis showed productivity class explained most microbial community variation, e.g., ECM fungi (80% explained) and soil bacteria (46%). Despite some differences, we could not identify statistically significant bacterial or ECM assemblages linked to stand productivity. Results may reflect a strong association between microbial processes and the dominant host, aspen. Aspen associated with widely distributed fungi common to all classes, possibly facilitating its survival and growth, including on sites exhibiting low pH and low soil fertility.

Changes in forest composition as a result of forest management, natural disturbances, and climate change may affect the accumulation of soil organic carbon (SOC). We examined the influence of common boreal tree species (trembling aspen, black spruce, and jack pine), either in pure stands or in conifer-broadleaf mixtures, on the amount, distribution, and quality of SOC in two regions of the Canadian boreal biome. Long-term laboratory incubations were used to assess SOC quality by quantifying proportions of fast carbon (C) (that is, proportion of total C released during the first 100 days of incubation) and active C (that is, modeled proportion of total C that can be potentially released). Total amounts of SOC did not differ between stand types, but the effects of stand type on SOC stocks and quality differed with soil depth. Among stand types, aspen stands had the greatest relative proportion of total SOC in deeper mineral layers and the lowest amount of active C in the organic layer. For these reasons, the SOC stock that developed under aspen was more stable than in the other stand types. Although black spruce stands allowed a greater accumulation of SOC in surface layers, these stocks, however, might become more vulnerable to extra losses if environmental conditions are to become more favorable to decomposition in the future. Our work highlights that boreal forest composition influences the stability of SOC stocks and how climate change could alter this large C pool.

Les préoccupations environnementales croissantes en foresterie ont provoqué un intérêt pour l’aménagement mixte comme une stratégie possible dans un contexte d’aménagement forestier durable. Cette revue de littérature se concentre sur les effets des peuplements mixtes sur la biodiversité, incluant les plantes de sous-bois, la faune aviaire, la faune du sol, et les ectomycorrhizes (ECM). Elle examine la diversité et la composition spécifiques à l’échelle des peuplements mais focalise particulièrement sur l’échelle du paysage (diversité gamma) en recherchant la présence d’espèces indicatrices des peuplements mixtes. Les principales conclusions sont les suivantes : (i) L’existence de différentes espèces d’arbres dans la canopée est associée avec une plus grande diversité de microhabitats, permettant l’addition des espèces de plantes de sous-bois associées à chacune des espèces de la canopée. Il n’y a toutefois que peu d’indications quant à l’existence d’espèces de plantes de sous-bois associées exclusivement aux peuplements mixtes. (ii) Certaines espèces d’oiseaux exigent ou préfèrent la présence de différentes espèces d’arbres dans un paysage ou un peuplement. Les peuplements ou paysages forestiers mixtes sont donc critiques pour la conservation de telles espèces. (iii) Quelques études ont montré un effet positif des peuplements ou litières mélangés sur certains groupes d’organismes du sol, mais la forte variabilité de ces résultats rend toute conclusion hasardeuse. (iv) Certains taxa d’ECM sont associés à des hôtes multiples, et pourraient donc bénéficier de la disponibilité de plusieurs hôtes dans les peuplements mixtes. Plusieurs études ont confirmé la plus grande abondance de ces taxa à hôtes multiples dans les peuplements mixtes.

Question: The effect of overstorey composition on above?ground dynamics of understorey vegetation is poorly understood. This study examines the understorey biomass, production and turnover rates of vascular and non?vascular plants along a conifer–broadleaf gradient of resource availability and heterogeneity.

Location: Canadian boreal forests of northwest Quebec and Ontario.

Methods: We sampled mature stands containing various proportions of black spruce (Picea mariana (Mill.) BSP), trembling aspen (Populus tremuloides Michx.) and jack pine (Pinus banksiana Lamb.). Above?ground biomass of the understorey vegetation was assessed through harvesting; annual growth rates were calculated as the differences between biomass in 2007 and 2008, as estimated by allometric relationships, and turnover rates were estimated as net primary production divided by the biomass in 2007.

Results: Higher aspen presence, linked to greater nutrient availability in the forest floor, was generally associated with higher vascular biomass and production in the understorey. This effect was less pronounced in sites of high intrinsic fertility. In contrast, bryophyte biomass was positively associated with conifer abundance, particularly in wet sites of the Quebec study area. Non?linear responses resulted in total understorey biomass being lower under mixed canopies than under pure aspen or pure conifer canopies. Turnover rates did not differ with overstorey composition.

Conclusions: While resource availability is a main driver of understorey productivity, resources as drivers appear to differ with differences in understorey strata components, i.e. vascular versus non?vascular plants. Resource heterogeneity induced by a mixed canopy had overall negative effects on understorey above?ground productivity, as this productivity seemed to rely on species adapted to the specific conditions induced by a pure canopy.

Processes governing tree interspecific interactions, such as facilitation and competition, may vary in strength over time. This study tried to unveil them by performing dendrometrical analyses on black spruce Picea mariana, trembling aspen Populus tremuloides and jack pine Pinus banksiana trees from pure and mixed mature boreal forest stands in the Clay Belt of northwestern Quebec and on the tills of northwestern Ontario. We cored 1430 trees and cut 120 for stem analysis across all stand composition types, tree species and study regions. Aspen annual growth rate was initially higher when mixed with conifers, but then progressively decreased over time compared to pure aspen stands, while jack pine growth rate did not differ with black spruce presence throughout all stages of stand development. When mixed with aspen, black spruce showed a contrary response to aspen, i.e. an initial loss in growth but a positive gain later. On the richer clay soil of the Quebec Clay Belt region, however, both aspen and spruce responses in mixed stands reversed between 37 and 54 years. Overall, our results demonstrate that interspecific interactions were present and tended to change with stand development and among species. Our results also suggest that the nature of interspecific interactions may differ with soil nutrient availability. © 2011 The Authors.

Determination of the direct causal factors controlling wildfires is key to understanding wildfire–vegetation–climate dynamics in a changing climate and for developing sustainable management strategies for biodiversity conservation and maintenance of long-term forest productivity. In this study, we sought to understand how the fire frequency of a large mixedwood forest in the central boreal shield varies as a result of temporal and spatial factors. We reconstructed the fire history of an 11,600-km2 area located in the northwestern boreal forest of Ontario, using archival data of large fires occurring since 1921 and dendrochronological dating for fires prior to 1921. The fire cycle decreased from 295 years for the period of 1820–1920 to approximately 100 years for the period of 1921–2008. Spatially, fire frequency increased with latitude, attributable to higher human activities that have increased fragmentation and fire suppression in the southern portion of the study area. Fire frequency also increased with distance to waterbodies, and was higher on Podzols that were strongly correlated with moderate drainage and coniferous vegetation. The temporal increase of fire frequency in the central region, unlike western and eastern boreal forests where fire frequency has decreased, may be a result of increased warm and dry conditions associated with climate change in central North America, suggesting that the response of wildfire to global climate change may be regionally individualistic. The significant spatial factors we found in this study are in agreement with other wildfire studies, indicating the commonality of the influences by physiographic features and human activities on regional fire regimes across the boreal forest. Overall, wildfire in the central boreal shield is more frequent than that in the wetter eastern boreal region and less frequent than that in the drier western boreal region, confirming a climatic top-down control on the fire activities of the entire North American boreal forest.

Summary

1. The diversity–productivity debate has so far been focused above-ground, despite that below-ground production can account for approximately half of total annual net primary production, mostly from fine roots.

2. Here, we investigate the fine root productivity of mature, fire-origin stands of Populus tremuloides–Picea spp.–Abies balsamea (mixed-species stands) and relatively pure P. tremuloides (single-species stands) in two regions of North American boreal forest to better understand the link between plant diversity and below-ground productivity in forest ecosystems. We hypothesized that: (i) mixed-species stands have higher fine root productivity compared with single-species stands and (ii) this difference may be the result of greater soil space filling by the fine roots due to the contrasting rooting traits of the component species in the mixed-species stands.

3. We found that fine root productivity, measured by annual production and total biomass, was higher in mixed- than single-species stands. We also found that mixed-species stands had lower and higher horizontal and vertical fine root biomass heterogeneity, respectively, indicating that soil space is more fully occupied by fine roots in the mixed- than single-species stands.

4. In all, our study supports that below-ground niche differentiation may be a key driver of higher fine root productivity in mixed stands of species with contrasting rooting traits than single-species stands by facilitating greater soil space filling of fine roots and soil resource exploitation.

Abstract: This study investigates the potential of mixed forest stands as better aboveground carbon sinks than pure stands. According to the facilitation and niche complementarity hypotheses, we predict higher carbon sequestration in mature boreal mixedwoods. Aboveground carbon contents of black spruce (Picea mariana (Mill.) Britton, Sterns, Poggenb.) and trembling aspen (Populus tremuloides Michx.) mixtures were investigated in the eastern boreal forest, whereas jack pine (Pinus banksiana Lamb.) and trembling aspen were used in the central boreal forest. No carbon gain was found in species mixtures; nearly pure trembling aspen stands contained the greatest amount of aboveground carbon, black spruce stands had the least, and mixtures were intermediate with amounts that could generally be predicted by linear interpolation with stem proportions. These results suggest that for aspen, the potentially detrimental effect of spruce on soils observed in other studies may be offset by greater light availability in mixtures. On the other hand, for black spruce, the potentially beneficial effects of aspen on soils could be offset by greater competition by aspen for nutrients and light. The mixture of jack pine and trembling aspen did not benefit any of these species while inducing a loss in trembling aspen carbon at the stand level.

Résumé : Cette étude vise à déterminer si les peuplements forestiers mixtes peuvent être des puits de carbone aériens plus efficaces que les peuplements purs. En accord avec les hypothèses de facilitation et de séparation des niches écologiques, nous prédisons une plus grande séquestration du carbone dans les peuplements mixtes. Nous avons donc déterminé les stocks de carbone aériens dans des mélanges d’épinettes noires (Picea mariana (Mill.) Britton, Sterns, Poggenb.) et de peupliers faux-tremble (Populus tremuloides Michx.) situés dans la forêt boréale de l’est, tandis que des mélanges de pin gris (Pinus banksiana Lamb.) et de peupliers faux-tremble furent plutôt utilisés dans la forêt boréale centrale. Les mélanges d’espèces ne présentaient aucun gain en carbone. Les peuplements dominés par le tremble contenaient la plus grande quantité de carbone aérien, les pessières la plus petite quantité, et les mélanges en contenaient des quantités intermédiaires qui pouvaient généralement être prédites par interpolation linéaire en fonction de la proportion de tiges de chaque espèce. Ces résultats suggèrent que dans le cas du tremble, l’effet potentiellement néfaste de l’épinette sur les sols peut être compensé par une meilleure disponibilité de la lumière dans les peuplements mixtes. Parallèlement, dans le cas de l’épinette noire, l’effet potentiellement bénéfique du peuplier faux-tremble sur les sols pourrait être contrebalancé par une compétition accrue pour la lumière et les nutriments en sa présence. Le mélange de pin gris et de peuplier faux-tremble n’a apporté aucun bénéfice pour aucune des deux espèces tout en induisant une perte en carbone chez le tremble à l’échelle du peuplement.

Plant root systems are highly dynamic over various temporal and spatial scales, and are responsive to changes in environmental conditions. The objective of this review is to describe the dynamics of root structure and function in boreal and northern temperate forests and examine how edaphic and climatic characteristics and intra- and interspecific root competition impact root dynamics. Fine roots exhibit distinct seasonal trends of production and mortality. Over the extent of stand development, coarse root biomass increases until maturity, while the response of fine roots remains unclear. Roots are predominantly restricted to the upper soil layers, and spatial variability of roots in the horizontal direction decreases with decreasing root size. Root/shoot ratio decreases gradually through stand development. On nutrient-rich sites, roots are more concentrated around respective stems and root systems are more branched than on nutrient-poor sites. Plants generally root deeper under low soil moisture conditions, while roots tend to grow horizontally into rich rather than poor patches of soil. Plants adapt their biomass allocation strategies to edaphic and climatic variation according to the functional equilibrium hypothesis. Although root production is projected to increase, providing nutrients are not limiting, following elevated carbon dioxide concentrations and temperatures, how root turnover and above- and below-ground carbon allocation may change remains uncertain. Stands composed of species with different rooting characteristics may attain greater root production compared to single-species stands or mixtures of similar species from reduced exploitative competition. Alternatively, plants can produce greater root biomass with a competing plant than growing alone as a result of self root discrimination. Future research is needed to address how elevated carbon dioxide concentrations and temperatures will feedback upon soil resource availability to influence plant responses from the organism- to the ecosystem-level.