Eastern Canadian Boreal Forest Research Articles

Many places called home: the adaptive value of seasonal adjustments in range fidelity.

The vast majority of animal species display range fidelity, a space-use behaviour enhancing familiarity with local habitat features. While the fitness benefits of this behaviour have been demonstrated in a variety of taxa, some species or populations rather display infidelity, displacing their home range over time. Others, such as many ungulate species, show seasonal adjustments in their range fidelity to accommodate changes in the dominance of limiting factors or in the distribution of resources. Few empirical studies have explored the adaptive value of seasonal adjustments in range fidelity. Using boreal populations of woodland caribou (Rangifer tarandus caribou) as a biological model, we evaluated how range fidelity impacted individual performance during two seasons where juvenile and adult survival are limited by different predation pressures. Between 2004 and 2013, we monitored the survival, reproductive success, habitat selection and range fidelity of female caribou in the boreal forest of eastern Canada. Using resource selection functions, we assessed how seasonal range fidelity was linked to two fitness correlates: calf survival in summer and adult female survival in winter. Females displayed season-specific space use tactics: they selected previously used areas during calving and summer, but tended to shift their winter range from 1year to the next. During calving and summer, range fidelity yielded relatively high fitness benefits, as females that did not lose their calf displayed stronger fidelity than females that did. In winter, however, adult survival was negatively linked to range fidelity, as females that survived selected areas further away from their seasonal range of the previous year than females that died. We provide one of the first evidences that making seasonal adjustments in range fidelity can be an adaptive behaviour influencing the spatial distribution of a threatened species. Assessing the seasonal nature of range fidelity tactics may improve our predictions of space use and associated fitness implications for species displaying this behaviour.

Understanding tree growth responses after partial cuttings: A new approach.

Forest ecosystem management heads towards the use of partial cuttings. However, the wide variation in growth response of residual trees remains unexplained, preventing a suitable prediction of forest productivity. The aim of the study was to assess individual growth and identify the driving factors involved in the responses of residual trees. Six study blocks in even-aged black spruce [Picea mariana (Mill.) B.S.P.] stands of the eastern Canadian boreal forest were submitted to experimental shelterwood and seed-tree treatments. Individual-tree models were applied to 1039 trees to analyze their patterns of radial growth during the 10 years after partial cutting by using the nonlinear Schnute function on tree-ring series. The trees exhibited different growth patterns. A sigmoid growth was detected in 32% of trees, mainly in control plots of older stands. Forty-seven percent of trees located in the interior of residual strips showed an S-shape, which was influenced by stand mortality, harvested intensity and dominant height. Individuals showing an exponential pattern produced the greatest radial growth after cutting and were edge trees of younger stands with higher dominant height. A steady growth decline was observed in 4% of trees, represented by the individuals suppressed and insensitive to the treatment. The analyses demonstrated that individual nonlinear models are able to assess the variability in growth within the stand and the factors involved in the occurrence of the different growth patterns, thus improving understanding of the tree responses to partial cutting. This new approach can sustain forest management strategies by defining the best conditions to optimize the growth yield of residual trees.

The relationships of soil total nitrogen concentrations, pools and C:N ratios with climate, vegetation types and nitrate deposition in temperate and boreal forests of eastern Canada

Forest soils contain large organic N pools which might react to global warming and eventually impact C sequestration in these ecosystems. Quantifying these N pools and understanding the factors controlling their size is therefore crucial. Here, we report on C:N ratio, N concentrations and pools in the forest floor (FF) and the mineral soil (MS) horizons of 21 forests of eastern Canada and on their relationships with 13 biophysical variables. The C:N ratio decline with soil depth at most sites. Total soil C:N ratio ranged from 9.8 to 24.9 and increased with mean annual precipitation (MAP) and decreased with the percentage of hardwoods (Phwd). Total soil N pools to the C-horizon ranged from 475 to 1261gNm−2, and on average, 82% of soil N was located in the MS. Nitrogen storage in the FF increased with MAP, the percentage of conifers (Pc) and altitude, whereas it decreased with mean annual air temperature (MAAT) and FF pH. These relationships mainly resulted from the impact of these variables on FF thickness. Nitrogen pool in the MS was strongly positively correlated with N-NO3 deposition. Together with mean annual soil temperature (MAST), N-NO3 deposition was also positively correlated with total soil N storage. The depth at which half of the mineral soil N pool was reached (D50) averaged 20cm and increased with altitude, Pc and MAP:PET while it decreased with MAAT. These results show that while MAP and N-NO3 deposition are major drivers of N pools in the FF and the MS respectively, the distribution of N within the MS was rather explained by MAAT and the vegetation type. These data also suggest that increasing temperature might increase N sequestration in eastern Canadian forest soils in the future.

Assessing the potential of the differenced Normalized Burn Ratio (dNBR) for estimating burn severity in eastern Canadian boreal forests

There is considerable variation in the degree of burn severity in boreal fires. One approach that has been used to capture this variation from field and remote sensing perspectives for western Canadian boreal forests is the Composite Burn Index (CBI) and differenced Normalized Burn Ratio (dNBR). Of interest was how well these methods may perform for fires in eastern Canada. This study investigated the CBI-dNBR relationship for selected fires in the eastern boreal forests of Canada, with a view towards contributing to the generalisation of a Canada-wide model. Results for the sampled region showed no difference in the CBI-dNBR relationship between black spruce- and jack pine-dominated stands, whereas this relationship was best described by a Generalised Additive Model (GAM). The dNBR-derived maps would also be useful in support of research and post-fire management in burns outside the studied territory and time frame covered by the existing burn severity mapping system already used in this region. The Saturated growth model proposed for the western boreal region also performed well for our eastern boreal region, thus further supporting the development of a national model.

Fire is a stronger driver of forest composition than logging in the boreal forest of eastern Canada

AbstractAimsOur study aimed to: (1) document the preindustrial (1925) forest composition prior to extensive logging; (2) document the magnitude of changes from 1925 to 2005; and (3) identify the relative influence of logging and natural disturbances as drivers of the present‐day forest composition.LocationBoreal forest in central Quebec, eastern Canada.MethodsWe used a dense network of georeferenced historical (~1925) forest plots (n = 30 033) to document preindustrial forest composition. We evaluated the magnitude of changes with the present‐day using modern plots (1980s to 2000s). We reconstructed a long‐term, spatially explicit history of logging, spruce budworm outbreaks (Choristoneura fumiferana [Clem.], SBO), and fire using historical maps and field surveys.ResultsIn the preindustrial period, late successional coniferous taxa (Abies balsamea and Picea spp.) dominated the landscape, whereas early successional deciduous taxa (Betula spp. and Populus spp.) were confined to recently burned areas. In the present‐day landscape, large areas dominated by late successional coniferous taxa have been replaced by early successional deciduous taxa. Forest communities dominated by early successional deciduous taxa increased sharply throughout the study area. Logging has been a minor driver of these changes compared to fire and SBOs.ConclusionsThis study demonstrates the importance of documenting the long‐term history of both anthropogenic and natural disturbances in order to assess their relative contributions to the development of the present‐day forest ecosystems. Natural disturbances have remained the main drivers of forest composition during the 20th century, whereas logging played a less important role. In the current context of global change, long‐term experimental research is required to help forecast impacts of natural disturbances and forest management on boreal forest composition.

Sprucing up the mixedwoods: growth response of white spruce (Picea glauca) to partial cutting in the eastern Canadian boreal forest

Mixed-species stands present a number of opportunities for and challenges to forest managers. Boreal mixedwood stands in eastern Canada are often characterized by a dominant canopy of shade-intolerant aspen (Populus tremuloides Michx.) with more shade-tolerant conifers in the mid- to sub-canopy layers. Because the aspen and conifer components often attain optimal merchantable sizes at different moments in stand development, there is an interest in developing silvicultural practices that allow partial or total removal of aspen and favour accelerated growth of residual conifers. We tested four partial harvesting treatments in mixed aspen – white spruce (Picea glauca (Moench.) Voss) stands in which different proportions of aspen (0%, 50%, 65%, and 100% basal area) were removed. Ten years after treatments, 72 spruce stems representing dominant, co-dominant, and suppressed social classes were destructively sampled for stem analysis. Using linear mixed effect models, we analyzed growth as a function of treatment intensity, time since treatment, social status, pretreatment growth rate, and neighbourhood competition. Relative to control stands, radial and volume growth responses were detected only in the extreme treatment of 100% aspen removal. In relative terms, suppressed trees showed the greatest magnitude of cumulative growth increase. Compared with control trees, average annual radial and volume increments were, respectively, 23.5% and 7.1% higher for dominant trees, 67.7% and 24.1% higher for co-dominant trees, and 115.8% and 65.6% higher for suppressed trees over the 10 years after treatment. Growth response was proportional to pretreatment growth rate, and among neighbouring trees, only coniferous neighbours had a negative effect on white spruce growth. Our results suggest that in similar mixed-stand conditions, relatively heavy removal of overstory aspen accompanied by thinning of crowded conifers would result in greatest growth response of residual spruce stems.

Cumulative patterns of logging and fire (1940–2009): consequences on the structure of the eastern Canadian boreal forest

Although logging has affected circumboreal forest dynamics for nearly a century, very few studies have reconstructed its influence on landscape structure at the subcontinental scale. This study aims to document spatiotemporal patterns of logging and fire since the introduction of logging in the early twentieth-century, and to evaluate the effects of these disturbances on landscape structure. We used historical (1940–2009) logging and fire maps to document disturbance patterns across a 195,000-km2 boreal forest landscape of eastern Canada. We produced multitemporal (1970s–2010s) mosaics providing land cover status using Landsat imagery. Logging significantly increased the rate of disturbance (+74 %) in the study area. The area affected by logging increased linearly with time resulting in a significant rejuvenation of the landscape along the harvesting pattern (south–north progression). From 1940 to 2009, fire was the dominant disturbance and showed a more random spatial distribution than logging. The recent increase of fire influence and the expansion of the proportion of area classified as unproductive terrestrial land suggest that regeneration failures occurred. This study reveals how logging has modified the disturbances dynamics, following the progression of the logging frontier. Future management practices should aim for a dispersed spatial distribution of harvests to generate landscape structures that are closer to natural conditions, in line with ecosystem-based management. The challenges of defining sustainable practices will remain complex with the predicted increase in fire frequency, since this factor, in combination with logging, can alter both the structure and potentially the resilience of boreal forest.

High conservation value forests for burn‐associated saproxylic beetles: an approach for developing sustainable post‐fire salvage logging in boreal forest

Abstract Fire‐killed timber is considered as a loss of potential revenues and is thus increasingly salvaged, though not without concerns for biodiversity conservation. Indeed, a large diversity of burn‐associated saproxylic beetles use recently burned trees. This study intends to reduce potential impacts of salvage logging on biodiversity by identifying high conservation value forests (HCVFs) for burn‐associated beetles, which are considered the most at risk. In five burns ignited naturally in 2010 in the eastern Canadian boreal forest, we selected 31 and 29 stands of black spruce and jack pine respectively. Three 50‐cm bole segments were retrieved from each stand and placed in emergence cages to measure tree utilisation by saproxylic beetles. This yielded 7235 beetles from 103 taxa, of which 67 were considered rare (<5% occurrence in logs) and 36 as common taxa (>5% occurrence in logs). Among the common taxa, we identified six groups of ecologically related species using co‐occurrence‐based hierarchical clustering, among which three were mainly formed by opportunistic species that are currently of little concern in a post‐fire logging context. The three other groups were formed by burn‐associated species that could be affected by salvage logging. HCVFs include jack pine stands and large trees of either tree species of low‐ to mid‐range burn severity. We also recommend retaining the periphery of burned stands, as ecotones are rich habitats used by several burn‐associated species that are found in low numbers in green forests but they benefit from burned habitats by increasing their populations significantly.

Rating a Wildfire Mitigation Strategy with an Insurance Premium: A Boreal Forest Case Study

Risk analysis entails the systematic use of historical information to determine the frequency, magnitude and effects of unexpected events. Wildfire in boreal North America is a key driver of forest dynamics and may cause very significant economic losses. An actuarial approach to risk analysis based on cumulative probability distributions was developed to reduce the adverse effects of wildfire. To this effect, we developed spatially explicit landscape models to simulate the interactions between harvest, fire and forest succession over time in a boreal forest of eastern Canada. We estimated the amount of reduction of timber harvest necessary to build a buffer stock of sufficient size to cover fire losses and compared it to an insurance premium estimated in units of timber volume from the probability of occurrence and the amount of damage. Overall, the timber harvest reduction we applied was much more costly than the insurance premium even with a zero interest rate. This is due to the fact that the insurance premium is directly related to risk while the timber harvest reduction is not and, as a consequence, is much less efficient. These results, especially the comparison with a standard indicator such as an insurance premium, have useful implications at the time of choosing a mitigation strategy to protect timber supplies against risk without overly diminishing the provision of services from the forest. They are also promoting the use of insurance against disastrous events in forest management planning.

Regional Instability in the Abundance of Open Stands in the Boreal Forest of Eastern Canada

Fires are a key disturbance of boreal forests. In fact, they are the main source of renewal and evolution for forest stands. The variability of fire through space and time results in a diversified forest mosaic, altering their species composition, structure and productivity. A resilient forest is assumed to be in a state of dynamic equilibrium with the fire regime, so that the composition, age structure and succession stages of forests should be consistent with the fire regime. Dense spruce-moss stands tend, however, to diminish in favour of more open stands similar to spruce-lichen stands when subjected to more frequent and recurring disturbances. This study therefore focused on the effects of spatial and temporal variations in burn rates on the proportion of open stands over a large geographic area (175,000 km2) covered by black spruce (Picea mariana (Mill.) Britton, Sterns, Poggenb.). The study area was divided into 10 different zones according to burn rates, as measured using fire-related data collected between 1940 and 2006. To test if the abundance of open stands was unstable over time and not in equilibrium with the current fire regime, forest succession was simulated using a landscape dynamics model that showed that the abundance of open stands should increase progressively over time in zones where the average burn rate is high. The proportion of open stands generated during a specific historical period is correlated with the burn rate observed during the same period. Rising annual burn rates over the past two decades have thereby resulted in an immediate increase in the proportion of open stands. There is therefore a difference between the current proportion of open stands and the one expected if vegetation was in equilibrium with the disturbance regime, reflecting an instability that may significantly impact the way forest resources are managed. It is apparent from this study that forestry planning should consider the risks associated with the temporal variability of fire regimes on the forest ecosystem, as the resulting changes can have a significant impact on biodiversity and allowable cut estimates.

Previous growing season climate controls the occurrence of black spruce growth anomalies in boreal forests of Eastern Canada

To better understand climatic origins of annual tree-growth anomalies in boreal forests, we analysed 895 black spruce (Picea mariana (Mill.) B.S.P.) tree-growth series from 46 xeric sites situated along three latitudinal transects in Eastern Canada. We identified interannual (based on comparison with previous year growth) and multidecadal (based on the entire tree-ring width distribution) growth anomalies between 1901 and 2001 at site and transect levels. Growth anomalies occurred mainly at site level and seldom at larger spatial scales. Both positive interannual and multidecadal growth anomalies were strongly associated with below-average temperatures and above-average precipitation during the previous growing season (Junet–1 – Augustt–1). The climatic signature of negative interannual and multidecadal growth anomalies was more complex and mainly associated with current-year climatic anomalies. Between the early and late 20th century, only negative multidecadal anomalies became more frequent. Our results highlight the role of previous growing season climate in controlling tree growth processes and suggest a positive association between climate warming and increases in the frequency of negative multidecadal growth anomalies. Projected climate change may further favour the occurrence of tree-growth anomalies and enhance the role of site conditions as modifiers of tree response to regional climate change.

Vertical distribution of three longhorned beetle species (Coleoptera: Cerambycidae) in burned trees of the boreal forest

This study aimed to characterize the vertical distribution of longhorned beetle larvae in burned trees of the eastern Canadian boreal forest. Black spruce (Picea mariana (Mill.) Britton, Sterns & Poggenb.) and jack pine (Pinus banksiana Lamb.) trees burned at three severity levels were cut, and 30 cm boles were collected from the ground up to a height of 9.45 m. Boles were debarked and dissected to collect insect larvae. Results show that the three most abundant longhorned beetle species were vertically segregated among burned jack pine and black spruce trees, but the section having the highest timber value was heavily infested by woodborer larvae. Larval density distribution of Monochamus scutellatus scutellatus (Say) and of Acmaeops proteus proteus (Kirby) could be linked with bark thickness, which also depends on fire severity. Lightly burned stands of black spruce were the most heavily infested and should be salvaged only if they are easily accessible and can thus be rapidly harvested and processed at the mill. More severely burned stands should be salvaged later as they will be less affected by woodborers, as should jack pine, which is lightly infested compared with black spruce. The ecological role of stumps should be further investigated because they could still have an ecological value after salvage logging as Arhopalus foveicollis (Haldeman) uses them specifically.

Evaluating the impacts of climate variability and cutting and insect defoliation on the historical carbon dynamics of a boreal black spruce forest landscape in eastern Canada

In this study, the Carbon and Nitrogen coupled Canadian Land Surface Scheme (CN-CLASS) was used to investigate the impact of climate variability, seasonal weather effects, disturbance, and CO2 fertilization effects on the historical carbon (C) dynamics of an eastern Canadian boreal forest landscape (6275ha) from 1928 to 2008. The model was parameterized with ecological, soil texture, forest inventory and historical disturbance data and driven by hourly meteorological data constructed from the historical climate records. Before performing the landscape-level simulation, model results were evaluated against site-level eddy covariance (EC) measurements. Landscape-level simulated C fluxes showed that the forest ecosystem was a small C sink in all of the years prior to cutting and insect defoliation in 1963, which resulted in the removal of 23849Mg C from the forest landscape. As a consequence, the study area was a large C source in 1963 (net biome productivity, NBP=−537gCm−2yr−1). After that, the forest landscape was mainly a net annual C sink, with total ecosystem C stocks increasing from 14.8 to 16.0kgCm−2 by 2008, during which total biomass increased from 3.1 to 4.2kgCm−2. Analysis of landscape-level, age-detrended, simulated C fluxes for the undisturbed forest landscape from 1928 to 2002 indicated that summer temperature was the dominant control on C fluxes with higher temperature causing a much faster increase in landscape-level annual Re than that of GPP (i.e. 12.3 vs. 1.3gCm−2yr−1°C−1, respectively). Scenario analysis suggested that forest disturbances had a less profound impact on landscape-level C fluxes and stocks compared to inter-annual climate variability in this landscape. Climate sensitivity analysis revealed that landscape-level simulated C fluxes and stocks were sensitive to the change of air temperature, while only dead organic matter (DOM) and soil organic matter (SOM) were sensitive to the change of precipitation. This study will help to explore the impact of future climate change scenarios and forest management on boreal forest landscapes.

Negative impacts of high temperatures on growth of black spruce forests intensify with the anticipated climate warming.

An increasing number of studies conclude that water limitations and heat stress may hinder the capacity of black spruce (Picea mariana (Mill.) B.S.P.) trees, a dominant species of Canada's boreal forests, to grow and assimilate atmospheric carbon. However, there is currently no scientific consensus on the future of these forests over the next century in the context of widespread climate warming. The large spatial extent of black spruce forests across the Canadian boreal forest and associated variability in climate, demography, and site conditions pose challenges for projecting future climate change responses. Here we provide an evaluation of the impacts of climate warming and drying, as well as increasing [CO2 ], on the aboveground productivity of black spruce forests across Canada south of 60°N for the period 1971 to 2100. We use a new extensive network of tree-ring data obtained from Canada's National Forest Inventory, spatially explicit simulations of net primary productivity (NPP) and its drivers, and multivariate statistical modeling. We found that soil water availability is a significant driver of black spruce interannual variability in productivity across broad areas of the western to eastern Canadian boreal forest. Interannual variability in productivity was also found to be driven by autotrophic respiration in the warmest regions. In most regions, the impacts of soil water availability and respiration on interannual variability in productivity occurred during the phase of carbohydrate accumulation the year preceding tree-ring formation. Results from projections suggest an increase in the importance of soil water availability and respiration as limiting factors on NPP over the next century due to warming, but this response may vary to the extent that other factors such as carbon dioxide fertilization, and respiration acclimation to high temperature, contribute to dampening these limitations.

Cover density recovery after fire disturbance controls landscape aboveground biomass carbon in the boreal forest of eastern Canada

In existing carbon budget models, carbon stocks are not explicitly related to forest successional dynamics and environmental factors. Yet time-since-last-fire (TSLF) is an important variable for explaining successional changes and subsequent carbon storage. The objective of this study was to predict the spatial variability of aboveground biomass carbon (ABC) as a function of TSLF and other environmental factors across the landscape at regional scales. ABC was predicted using random forest models, both at the sample-plot level and at the scale of 2-km2 cells. This cell size was chosen to match the observed minimum fire size of the Canadian large fire database. The percentage variance explained by the empirical sample-plot level model of ABC was 50%. At that scale, TSLF was not significantly related to ABC. At the 2-km2 scale, ABC was influenced mainly by the proportions of cover density classes, which explained 83% of the variance. Changes in cover density were related to TSLF at the same 2-km2 scale, indicating that the increase in cover density following fire disturbance is a dominant mechanism through which TSLF acts upon ABC at the scale of landscapes.

Bryophyte species assemblages in fire and clear-cut origin boreal forests

Natural and anthropogenic disturbances could have different impacts on understory plant communities. Investigating these differences could help improve silvicultural and management practices in order to better achieve biodiversity protection objectives. Using post-fire (20–90years) and post-clearcutting (20–70years) forest chronosequences placed on similar sites, we examined which environmental factors are the main drivers of bryophyte community assembly in eastern Canadian boreal forests, using information on bryophyte life-history strategies (colonist: high reproductive effort but a short potential life span; perennial: low reproductive effort and a long potential life span) to interpret the resulting patterns. The fire origin stands were affected by high-severity fires followed by natural regeneration, whereas the clear-cut stands were regenerated through the advance regeneration present in the understory of the harvested stands. Our results indicate that by killing the existing mosses and baring the mineral soil, fire tends to decrease the cover of perennial species (such as Pleurozium scherberii) and increase the presence of colonist species compared with clear-cut. Overall species richness does not increase much in older stands, but some species that have been identified by previous studies as being more sensitive to management activities, such as liverworts, tend to be strongly associated with balsam fir basal area, which is higher in mature clear-cut origin stands. This tree species tends to be heavily affected by partial mortality events after >50years (insect outbreaks, windthrow), which could accelerate the creation of heterogeneous canopy structure and generate a greater diversity of microhabitats suitable for sensitive bryophyte species. More research needs to be conducted to better understand the underlying functional relationships between overstory tree composition and bryophyte communities.

Salvage logging following fires can minimize boreal caribou habitat loss while maintaining forest quotas: An example of compensatory cumulative effects

Protected area networks are the dominant conservation approach that is used worldwide for protecting biodiversity. Conservation planning in managed forests, however, presents challenges when endangered species use old-growth forests targeted by the forest industry for timber supply. In many ecosystems, this challenge is further complicated by the occurrence of natural disturbance events that disrupt forest attributes at multiple scales. Using spatially explicit landscape simulation experiments, we gather insights into how these large scale, multifaceted processes (fire risk, timber harvesting and the amount of protected area) influenced both the persistence of the threatened boreal caribou and the level of timber supply in the boreal forest of eastern Canada. Our result showed that failure to account explicitly and a priori for fire risk in the calculation of timber supply led to an overestimation of timber harvest volume, which in turn led to rates of cumulative disturbances that threatened both the long-term persistence of boreal caribou and the sustainability of the timber supply itself. Salvage logging, however, allowed some compensatory cumulative effects. It minimised the reductions of timber supply within a range of ∼10% while reducing the negative impact of cumulative disturbances caused by fire and logging on caribou. With the global increase of the human footprint on forest ecosystems, our approach and results provide useful tools and insights for managers to resolve what often appear as lose–lose situation between the persistence of species at risk and timber harvest in other forest ecosystems. These tools contribute to bridge the gap between conservation and forest management, two disciplines that remain too often disconnected in practice.

The Interaction Between Insect Outbreaks and Debris Slides in a Glacial Valley of the Eastern Canadian Shield

Disturbance interactions, including compound and linked disturbances, are increasingly recognized to have important effects on landscape structure and function. Most research on linked disturbances has focused on the interaction between insect outbreaks and fire, and between fire or forest harvesting and other physical processes such as soil erosion and landslides, but to our knowledge, none have explored the possible interaction between insect outbreaks and landslides. We tested the hypothesis that severe outbreaks of the spruce budworm, an insect defoliator, could increase the probability of debris slide occurrence in the southern boreal forest of eastern Canada. We mapped 28 debris slides that occurred between 1996 and 2002 along a deep U-shaped glacial valley (Jacques-Cartier River valley, Quebec) and compared their characteristics with those of 50 random sites unaffected by debris slides using maps of the abiotic (topography, surficial deposits, and so on) and biotic (forest cover and cumulative spruce budworm defoliation from aerial surveys) factors that could influence the probability of debris slide occurrence. Logistic regression showed that probability of debris slide occurrence increased with mean slope, outbreak severity, and abundance of balsam fir, and that it was associated with thin surficial deposits and northeast aspects. Spruce budworm outbreaks probably increase susceptibility to debris slides primarily by reducing the biomass of soil-stabilizing root systems, and possibly also by reducing forest transpiration, which in turn increases soil water content. Future research should focus on the mechanisms underlying insect outbreak-landslide interactions.

Effects of 80 years of forest management on landscape structure and pattern in the eastern Canadian boreal forest

Forest management alters patterns generated by natural disturbances, particularly in ecosystems with infrequent fires. Management effects can differ according to spatial scale and affect ecological processes. To assess the effect of 80 years of forest management at both the landscape and burn/harvest scales on forest age, composition, density, spatial pattern and heterogeneity. Forest inventory maps and satellite images were used to compare two contiguous landscapes, respectively managed and unmanaged, of the eastern boreal forest of Canada, in a region with infrequent fires. Burns and harvests occurring from 1920–1950 were also compared. In addition to reducing the proportion of old-growth stands in the landscape, forest management changed forest composition at both scales, favouring the late-successional species balsam fir. Landscape metrics indicated that old-growth forests and spruce-dominated ones were more fragmented, less connected, and confined to smaller patches in the managed landscape than in the unmanaged one. Forest management increased heterogeneity at the landscape scale, but decreased it at the burn/harvest scale. Logging had a homogenizing effect at the burn/harvest scale by attenuating the effect of the physical environment on forest density. This study provides knowledge to help reduce effects of forest management at both scales. In this forest region with low fire recurrence, the goal should be to manage for greater forest heterogeneity at the burn/harvest scale whereas, at the landscape scale, restoration strategies should aim to create large contiguous patches of coniferous forests to increase spatial continuity as these were reduced by past management activities.

Frost hollows of the boreal forest: a spatiotemporal perspective

Summary Frost hollows (FH) are small, confined lichen‐dominated ecosystems surrounded by lichen woodlands. FH are impacted by repetitive frosts occurring during the growing season. We evaluated the dynamics of FH across the boreal forest in eastern Canada and assessed the influence of latitude as the main geoclimatic factor controlling their development. The distribution and abundance of FH along 39 longitudinal transects were based on aerial photographs and field validation. Black spruce (Picea mariana) in FH and adjacent woodlands were sampled for age determination, year of tree establishment, height growth rate of spruce, and intensity and frequency of frost rings. Most FH are located in the southern part of the boreal forest, in depressions bordered by dry, open, lichen woodlands. The inception and development of FH are not dependent on latitude but rather on the presence of lichen woodlands. Frost‐induced thúfurs and frost rings were more frequent and abundant in the southernmost FH, whereas the northernmost FH were devoid of thúfurs. Black spruce established synchronously in FH across the boreal forest, an indication of a climatic control associated with cloudier, humid conditions reducing radiative frost activity. Most FH spruce established after 1933 and peaked in the 1980s. Fewer frost events were recorded in the 1980s than in the 1990s. In the latter decade, frost events became more frequent, intense and synchronous across the boreal forest. Synthesis. The distribution of frost hollows (FH) is related to the presence of dry lichen woodlands that exacerbate frost activity during the growing season. Synchronous frost‐ring frequency across the biome also suggests that shared climatic conditions, in particular freezing temperature, humidity and cloudiness, are at the origin of the dynamics of FH. A longer growing season creates propitious conditions for a higher frequency of frost events and more intense soil cryoturbation. FH are climate‐sensitive ecosystems which can be used as proxies of the impact of global change on the boreal biome.