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Climate change is projected to dramatically increase boreal wildfire activity, with broad ecological and socioeconomic consequences. As global temperatures rise, periods with elevated fire weather are expected to increase in frequency and duration, which would be expected to increase the number and size of fires. Statistical forecasts or simulations of future fire activity often account for direct climatic effects only, neglecting other controls of importance, such as biotic feedbacks. This could result in overestimating the effects of climate change on fire activity, if the future distribution of vegetation or fuels were to change. We incorporated sensitivity to climate or fire weather and vegetation in a fire simulation model and represented explicitly two key biotic feedbacks linked to succession and regeneration processes. We used this model to forecast annual fire activity from 2011 to 2099 over a large region of boreal forest in Québec, Canada, dominated by balsam fir (Abies balsamea (L.) Mill) and yellow birch (Betula alleghaniensis Britt.) or paper birch (Betula papyrifera Marsh.), with and without the biotic feedbacks. Our simulations show that vegetation changes triggered by fire disturbance altered future fire activity and may even be as important a driver as climate change itself. Indeed, over the course of the century, vegetation changes were projected to offset much of the increase in fire activity that would be expected due to global warming as such. It follows that if biotic feedbacks are not included in statistical or simulation-based forecasts, the resultant projections of future fire activity could be biased upward to a very considerable degree. For the case of end-of-century mean annual burn rate, we estimated this positive bias to be as high as 400%. Accounting for biotic feedbacks in simulation models is therefore necessary for accurate projection of future wildfire activity and associated vegetation changes. Purely statistical forecasts based on current vegetation cannot be relied upon, in the presence of biotic feedbacks. Our results further suggest that vegetation management could reduce fire risk in some systems by altering the abundance and distribution of the most highly flammable fuels and thus mitigate the impact of climate change on fire activity. [ABSTRACT FROM AUTHOR]

The natural disturbance-based management (NDBM) aims to maintain specific structural and compositional attributes of natural forests in managed stands. Operationally, NDBM relies on diversifying and adapting silvicultural practices, including partial harvesting (PC), to expand the range of options beyond that of simply clearcuts (CC). Established in 1998, the Sylviculture et Aménagement Forestier Écosystémique (SAFE) project evaluates this potential in hardwood, mixedwood, and coniferous stands in northwestern Québec, Canada. Our results confirmed a part of the NDBM objectives, i.e., PC allowed the maintenance of stands with mixed structure and composition, constituting an interesting complement to CC, which reset stand regeneration. However, PC did not accelerate the stand transition to later stages with less intensive harvesting or to earlier stages with more intensive harvesting. We essentially had an initial impact, delaying or stopping the stand evolution that dissipates over time and more quickly with less intensive harvesting. Furthermore, our results did not support the ability of PC to enhance the development of old-growth attributes like deadwood. Despite the 20-year horizon of this study, further field surveys will be required in the future to better understand the impact of different silvicultural treatments on forest productivity and biodiversity preservation throughout a forest rotation. [ABSTRACT FROM AUTHOR]

• Boreal forest is resilient in the face of repeated disturbances during the Holocene. • Moderate fire frequency leads to a recurrence dynamics. • Resilience remains precarious as shown by lichen woodlands formation. • Climate change is expected to amplify the forest/woodland phenomenon. • Charcoals in mineral soils allow reconstruction of past local tree assemblages. The dynamics of the boreal forest has followed several successional trajectories during the Holocene caused by recurrent stand-scale, abiotic and biotic disturbances. Under stable environmental conditions, site disturbances should favor the regeneration of forest communities enabling the recovery process to produce forests similar to the pre-disturbance states. However, a failure in the post-disturbance recovery process can also occur to shift the forest communities to alternative states. Although fire is the main disturbance factor affecting the dynamics of the boreal forest, long-term resilience of most forest ecosystems remains poorly known because of lack of stand-scale paleoecological evidence. To evaluate the resilience ability of boreal forest ecosystems, we reconstructed their Holocene fire history and documented their successional pathways at the stand scale in two climatically-contrasted regions (western continental versus eastern humid climates of boreal Québec, Canada) based on botanically-identified and 14C-dated soil charcoal remains. Since mid-Holocene, western closed-crown conifer forests were resilient to fire disturbance, but hardwood trees declined significantly. Yet in the eastern region, closed-crown conifer forests similar to that observed today seems to have remained in place since its origin. In both regions, moderate fire frequency leads to a recurrence dynamics that favors renewal of forests stands with the same composition. However, balsam fir – paper birch stands follow a successional pattern characterized by the establishment of paper birch, with subsequent increasing abundance of balsam fir. This allows the maintenance of forests composed of these species, whose dominance fluctuates according to the time elapsed since the last fire. The succession from black spruce to balsam fir stands seems to be an uncommon process that requires a prolonged fire-free period. Although most closed-crown forests are resilient postfire ecosystems which have persisted over several millennia, their resilience is precarious as evidenced by the transformation of some forests into lichen woodlands after fire. [ABSTRACT FROM AUTHOR]

Natural root grafts (anastomoses) are morphological unions formed between roots of different trees. Common root systems allow translocation of water, nutrients and photosynthesis products between grafted trees, affecting their growth and their physiology. As carbohydrates are redistributed among grafted trees, the formation of a common root system could reduce the negative effect of intraspecific competition for light or soil resources within stands. The aim of this study was to investigate the role of root grafting on intraspecific competition and growth of balsam fir Abies balsamea. We studied inter‐tree relationships in three natural balsam fir stands of the boreal forest of Quebec (Canada) that contained an average 36% of grafted trees. At each stand, ring width and basal area of trees were measured using dendrochronology techniques. We used mixed linear models to test the effect of root grafting and intraspecific competition on annual basal area increment of trees. Trees before grafting had higher growth rates than trees once grafted. Thus, root grafting did not improve tree growth. Growth of grafted trees was more negatively affected by intraspecific competition than growth of non‐grafted trees. Thus, grafted trees cannot be considered as better competitors than non‐grafted trees. Under high intraspecific competition, growth of larger grafted trees was less affected than that of smaller trees suggesting that they were able to divert resources at their advantage within a union. Our study demonstrated that grafted trees acted on each other's growth and provides support for the idea that grafted trees respond to competition for resources more as a community rather than as individual trees. [ABSTRACT FROM AUTHOR]

The use of wood ash as a soil amendment in afforestation and reforestation efforts is increasing. While most studies suggest benefits or neutral results on tree growth and survival, a few studies indicate adverse effects. Hybrid larch, jack pine and white spruce were studied at three northwestern Quebec plantation sites after they received wood ash at two application rates. Soil chemical properties, foliar nutrients and seedling growth and mortality were monitored over a period of eight years. The response of soil to ash application was mostly observed in the forest floor and was more pronounced in year 3 than year 8, likely due to the acidifying nature of the boreal soils studied. Jack pine growth increased linearly with wood ash application rates, white spruce growth showed an inconsistent and delayed positive response under the higher application rate, and hybrid larch growth and survival were either increased or decreased under the lower application rate depending of site but decreased at all sites under the higher application rate. The divergence in growth response between tree species underlines a trade-off between species with rapid acquisition of resources (e.g., pine, larch) to species that use more conservative strategies and store nutrients in their tissues for longer periods (e.g., spruce). In the case of hybrid larch, it accumulated larger amounts of Mn in its needles under the higher application rate and thus, the high bioavailabil - ity of Mn appears to have been detrimental to its survival and growth. Its higher sensitivity to Mn addition from ash is likely due to its highly acquisitive (nutrients) nature compared to other coniferous species as well as the initial levels of available Mn levels in the soil. The contrasted growth responses reported here under similar growing conditions highlight the importance of identifying suitable species, sites and application rates to maximize the benefits of wood ash amendments for future tree plantations in the boreal forest. [ABSTRACT FROM AUTHOR]

Evaporation of intercepted rain by a canopy is an important component of evapotranspiration, particularly in the humid boreal forest, which is subject to frequent precipitation and where conifers have a large surface water storage capacity. Unfortunately, our knowledge of interception processes for this type of environment is limited by the many challenges associated with experimental monitoring of the canopy water balance. The objective of this study is to observe and estimate canopy storage capacity and wet canopy evaporation at the sub‐daily and seasonal time scales in a humid boreal forest. This study relies on field‐based estimates of rainfall interception and evapotranspiration partitioning at the Montmorency Forest, Québec, Canada (mean annual precipitation: 1600 mm, mean annual evapotranspiration: 550 mm), in two balsam fir‐white birch forest stands. Evapotranspiration was monitored using eddy covariance sensors and sap flow systems, whereas rainfall interception was measured using 12 sets of throughfall and six stemflow collectors randomly placed inside six 400‐m2 plots. Changes in the amount of water stored on the canopy were also directly monitored using the stem compression method. The amount of water intercepted by the forest canopy was 11 ± 5% of the total rainfall during the snow‐free (5 July–18 October) measurement periods of 2017 and 2018. The maximum canopy storage estimated from rainfall interception measurements was on average 1.6 ± 0.7 mm, though a higher value was found using the stem compression method (2.2 ± 1.6 mm). Taking the average of the two forest stands studied, evaporation of intercepted water represented 21 ± 8% of evapotranspiration, while the contribution of transpiration and understory evapotranspiration was 36 ± 9% and 18 ± 8%. The observations of each of the evapotranspiration terms underestimated the total evapotranspiration observed, so that 26 ± 12% of it was not attributed. These results highlight the importance to account for the evaporation of rain intercepted by humid boreal forests in hydrological models. [ABSTRACT FROM AUTHOR]

Aims: In the boreal mixedwood, fire initiates forest succession; however, over time other disturbances such as insect outbreaks cause pulses of mortality and opportunities for recruitment of shade‐intolerant species. Questions: What are the respective roles of time since fire and insect outbreaks in driving directional, vs cyclical and retrogressive succession? Do assessments from direct measurements and chronosequence approaches converge? We hypothesise that the chronosequence approach will accurately characterise large‐scale compositional patterns especially in younger stands and that direct measurements will better describe small‐scale, non‐directional changes in succession. Location: Boreal mixed wood of northwestern Quebec (79°1′ W, 48°30′ N). Methods: We sampled 469 plots over an 18‐year interval (in 1991 and 2009) in mixed hardwood/conifer forests to observe in situ the changes in stands having originated from seven fires covering a 249‐year chronosequence. Results: The combination of the remeasurement and chronosequence analyses validates predictions of forest succession. Our results indicate that time since last fire is the dominant factor explaining forest succession for the first 150 years after fire and globally at the scale of the entire forest, although smaller‐scale disturbances such as spruce budworm outbreaks can change the course of forest succession, especially at stand scales. Conclusion: While time since fire is the dominant driver of forest succession in younger forests, secondary disturbances, such as spruce budworm outbreaks, can advance or reverse forest succession in older forests. This study also serves as a validation of the overall patterns described in spatial chronosequence approaches at the landscape level in fire‐dominated systems but highlights that local succession may deviate from the overall pattern especially in older forests or in forests affected by non‐stand re‐initiating intermediate disturbances. [ABSTRACT FROM AUTHOR]

Questions: Which role did historical anthropogenic disturbances play in modifying the natural fire regime? To what extent have they shaped current forest? Do those disturbances have lingering impacts in the present‐day landscape? Are certain tree species related to former land use? Location: eastern Quebec, Canada. Methods: Spatial data on landscape structure, burnt areas, settlements, and forest patches were vectorized on an archival map dating back to 1938. For each landscape class, the total area, the number of polygons, and the proportion of the total landscape occupied by the largest polygon were analyzed according to elevation and to the Euclidean distance from the "settlement" polygons. An index of the spatial link between the landscape classes was calculated, based on the proportions of the perimeter of the polygons of each class shared with each of the other classes. A Kolmogorov–Smirnov test for pooled data was used to obtain the frequency distributions of landscape classes as a function of distance. The association between settlement fires and present‐day vegetation, and more specifically Populus and Betula stands, was tested by superimposing the most recent ecoforest map on the 1938 land‐use map. Distance bands on either side of the 1938 settlement front were delineated to calculate the proportion of each distance class occupied by present‐day aspen and birch stands. Results: Anthropogenic fires generated a recognizable landscape pattern of land use. Burnt areas were mostly located within 2 km from a settlement. Most burnings observed on the 1938 map were human‐induced, based on their spatial connection with the settled areas. Lingering impacts of these 20th‐century fires on present‐day forests were identified using the peculiar spatial distribution of tree species. The presence and spatial distribution of aspen in the present‐day landscape is tightly associated with previously burnt areas. Conclusions: Past land‐use strongly altered the natural fires regime and associated tree species. Current land‐use could potentially lead to increased degraded forest landscapes in the near future. [ABSTRACT FROM AUTHOR]

In Canada, recent catastrophic wildfire events raised concern from governments and communities. As climate change is expected to increase fire activity in boreal forests, the need for a better understanding of fire regimes is becoming urgent. This study addresses the 1972–2015 spatial distributions of fire cycles, mean fire size (FireSz) and mean fire occurrence (mean annual number of fires per 100 000 ha, FireOcc) in eastern Canada. The objectives were to determine (1) the spatial variability of fire-regime attributes, (2) the capacity of FireSz and FireOcc to distinguish homogeneous fire zones and (3) the environmental factors driving FireSz and FireOcc, with some emphasis on lightning strikes. Fire cycles, FireSz and FireOcc greatly varied throughout the study area. Even within homogeneous fire zones, FireSz and FireOcc were highly variable. FireSz was controlled by moisture content in deep layers of the soil and by surficial deposits, whereas FireOcc was controlled by moisture content in top layers of the soil and by relief. The lack of a relationship between FireOcc and lightning-strike density suggested that the limiting effect of lightning-strike density on FireOcc could be operating only under certain circumstances, when interacting with other environmental factors. A large variability of fire cycles, mean fire size and fire occurrence was observed in eastern Canada. Although the ignition source of natural fires, lightning strikes did not limit fire occurrence. Instead, it was controlled by climate and relief, whereas the mean fire size was controlled by climate and surficial deposits. [ABSTRACT FROM AUTHOR]

Human-caused habitat disturbances and climate change are leading threats to biodiversity. Studying the impacts of human activities on wildlife from a behavioral perspective is a relevant starting point to understand the mechanisms underlying population and species resistance and resilience to disturbances. In this study, we assessed the effects of weather (temperature and precipitation), habitat disturbances (roads and clearcuts), and natural habitat types on the space use patterns of a threatened boreal population of woodland caribou (Rangifer tarandus caribou). An extensive data set of 288,665 relocations from 50 adult females outfitted with GPS collars over 7 years in the boreal forest of Québec, Canada was used to evaluate residency time in natural and disturbed habitats for five distinct biologically defined periods. The most parsimonious linear mixed-effects model for each period showed that individuals stayed longer in more natural habitat types. During calving and summer, residency time decreased with increasing road density, whereas residency time decreased with increasing temperature during winter and spring. We found no evidence of a synergistic effect between daily weather and human disturbances on movement behavior of caribou, but consider their respective influence as additive. We also showed large individual variation in residency time, except during the calving period. Lower individual variation in residency time during calving may be explained by strong evolutionary constraints on behavior faced by females to ensure protection and survival of their offspring. Based on our results, we suggest keeping large patches of suitable and roadless habitat for caribou to favor the spacing-out antipredator strategy exhibited by females during calving. By tracking individuals over several complete annual cycles, we showed variation in the effects of daily weather and human disturbances on residency time across biological periods. Our study highlights that the inclusion of daily weather variables helps better understand space-use patterns of a threatened species. [ABSTRACT FROM AUTHOR]

Currently, there is no consensus regarding the way that changes in climate will affect boreal forest growth, where warming is occurring faster than in other biomes. Some studies suggest negative effects due to drought‐induced stresses, while others provide evidence of increased growth rates due to a longer growing season. Studies focusing on the effects of environmental conditions on growth–climate relationships are usually limited to small sampling areas that do not encompass the full range of environmental conditions; therefore, they only provide a limited understanding of the processes at play. Here, we studied how environmental conditions and ontogeny modulated growth trends and growth–climate relationships of black spruce (Picea mariana) and jack pine (Pinus banksiana) using an extensive dataset from a forest inventory network. We quantified the long‐term growth trends at the stand scale, based on analysis of the absolutely dated ring‐width measurements of 2,266 trees. We assessed the relationship between annual growth rates and seasonal climate variables and evaluated the effects of various explanatory variables on long‐term growth trends and growth–climate relationships. Both growth trends and growth–climate relationships were species‐specific and spatially heterogeneous. While the growth of jack pine barely increased during the study period, we observed a growth decline for black spruce which was more pronounced for older stands. This decline was likely due to a negative balance between direct growth gains induced by improved photosynthesis during hotter‐than‐average growing conditions in early summers and the loss of growth occurring the following year due to the indirect effects of late‐summer heat waves on accumulation of carbon reserves. For stands at the high end of our elevational gradient, frost damage during milder‐than‐average springs could act as an additional growth stressor. Competition and soil conditions also modified climate sensitivity, which suggests that effects of climate change will be highly heterogeneous across the boreal biome. [ABSTRACT FROM AUTHOR]

Research Highlights: This research provides an application of a model assessing the naturalness of the forest ecosystem to demonstrate its capacity to assess either the deterioration or the rehabilitation of the ecosystem through different forest management scenarios. Background and Objectives: The model allows the assessment of the quality of ecosystems at the landscape level based on the condition of the forest and the proportion of different forest management practices to precisely characterize a given strategy. The present work aims to: (1) verify the capacity of the Naturalness Assessment Model to perform bi-directional assessments, allowing not only the evaluation of the deterioration of naturalness characteristics, but also its improvement related to enhanced ecological management or restoration strategies; (2) identify forest management strategies prone to improving ecosystem quality; (3) analyze the model's capacity to summarize the effect of different practices along a single alteration gradient. Materials and Methods: The Naturalness Assessment Model was adapted to the Abies balsamea–Betula papyrifera forest of Quebec (Canada), and a naturalness assessment of two sectors with different historical management strategies was performed. Fictive forest management scenarios were evaluated using different mixes of forestry practices. The sensitivity of the reference data set used for the naturalness assessment has been evaluated by comparing the results using data from old management plans with those based on Quebec's reference state registry. Results: The model makes it possible to identify forest management strategies capable of improving ecosystem quality compared to the current situation. The model's most sensitive variables are regeneration process, dead wood, closed forest and cover type. Conclusions: In the Abies balsamea–Betula papyrifera forest, scenarios with enhanced protection and inclusion of irregular shelterwood cuttings could play an important role in improving ecosystem quality. Conversely, scenarios with short rotation (50 years) could lead to further degradation of the ecosystem quality. [ABSTRACT FROM AUTHOR]

Humid boreal forests are unique environments characterized by a cold climate, abundant precipitation, and high evapotranspiration. Transpiration ( E T ), as a component of evapotranspiration (E), behaves differently under wet and dry canopy conditions, yet very few studies have focused on the dynamics of transpiration to evapotranspiration ratio ( E T / E ) under transient canopy wetness states. This study presents field measurements of E T / E at the Montmorency Forest, Québec, Canada: a balsam fir boreal forest that receives ∼ 1600 mm of precipitation annually (continental subarctic climate; Köppen classification subtype Dfc). Half-hourly observations of E and E T were obtained over two growing seasons using eddy-covariance and sap flow (Granier's constant thermal dissipation) methods, respectively, under wet and dry canopy conditions. A series of calibration experiments were performed for sap flow, resulting in species-specific calibration coefficients that increased estimates of sap flux density by 34 % ± 8 % , compared to Granier's original coefficients. The uncertainties associated with the scaling of sap flow measurements to stand E T , especially circumferential and spatial variations, were also quantified. From 30 wetting–drying events recorded during the measurement period in summer 2018, variations in E T / E were analyzed under different stages of canopy wetness. A combination of low evaporative demand and the presence of water on the canopy from the rainfall led to small E T / E . During two growing seasons, the average E T / E ranged from 35 % ± 2 % to 47 % ± 3 % . The change in total precipitation was not the main driver of seasonal E T / E variation, therefore it is important to analyze the impact of rainfall at half-hourly intervals. [ABSTRACT FROM AUTHOR]

In this work, we examine the effects of commercial thinning on stand volume and individual stem form in nine naturally regenerated black spruce (Picea mariana (Mill.) B.S.P.) stands. We compared these study sites with controls in the commercial boreal forest of northern Quebec, Canada. At stand level, dendrochronological data provided insight into changes in stand volume ten years after thinning. Analysis of a subsample of six individual trees from each commercially thinned stand and three individual trees from the controls illustrated the role of thinning on stem shape development. Although average volume increased for residual stems in thinned stands slightly more than in the controls (110% versus 106%), the treatment effect stand-level volume increment or stand-level total volume harvested (ten years after treatment) was not statistically significant. Moreover, at tree level, thinning did not significantly affect stem volume increment. However, radial growth increment significantly increased after treatment. Trees from commercially thinned stands showed a significantly higher growth increment along the lower first two-thirds of the stem. Response to thinning at tree level correlated strongly with the size and number of harvested competitors around a residual stem. We conclude that commercial thinning modified wood allocation rather than wood volume and did not affect taper and stem shape. These patterns of post-cutting growth are explained by wood allocation following thinning. After commercial thinning, growth increment is favored at the expense of height growth. As the treatment effect was found at the stem scale rather than at the stand scale, further research is needed in regard to commercial thinning treatments to investigate how to increase productivity at the stand scale. [ABSTRACT FROM AUTHOR]