The boreal forest represents the terrestrial biome most heavily affected by climate change. However, no consensus exists regarding the impacts of these changes on the growth of tree species therein. Moreover, assessments of young tree responses in metrics transposable to forest management remain scarce. Here, we assessed the impacts of climate change on black spruce (Picea mariana [Miller] BSP) and jack pine (Pinus banksiana Lambert) growth, two dominant tree species in boreal forests of North America. Starting with a retrospective analysis including data from 2591 black spruces and 890 jack pines, we forecasted trends in 30-year height growth at the transitions from closed to open boreal coniferous forests in Québec, Canada. We considered three variables: (1) height growth, rarely used, but better-reflecting site potential than other growth proxies, (2) climate normals corresponding to the growth period of each stem, and (3) site type (as a function of texture, stoniness, and drainage), which can modify the effects of climate on tree growth. We found a positive effect of vapor pressure deficit on the growth of both species, although the effect on black spruce leveled off. For black spruce, temperatures had a positive effect on the height at 30 years, which was attenuated when and where climatic conditions became drier. Conversely, drought had a positive effect on height under cold conditions and a negative effect under warm conditions. Spruce growth was also better on mesic than on rocky and sub-hydric sites. For portions of the study areas with projected future climate within the calibration range, median height-change varied from 10 to 31% for black spruce and from 5 to 31% for jack pine, depending on the period and climate scenario. As projected increases are relatively small, they may not be sufficient to compensate for potential increases in future disturbances like forest fires.

There is a pressing need for a better understanding of changing forest fire regimes worldwide, especially to separate the relative effects of potential drivers that control burned areas. Here we present a meta-analysis of the impacts of climate fluctuation and Euro-Canadian settlement on burned areas from 1850 to 1990 in a large zone (>100 000 km2) in northern temperate and boreal forests of eastern Canada. Using Cox regression models, we tested for potential statistical relationships between historical burned areas in 12 large landscapes (reconstructed with dendrochronological data) with climate reconstructions, changes in the Euro-Canadian population, and active suppression (all reconstructed at the decadal scale). Our results revealed a dominant impact of climate fluctuations on forest burned areas, with the driest decades showing fire hazards between 5 to 15 times higher than the average decades. Comparatively, the Euro-Canadian settlement had a much weaker effect, having increased burned areas significantly only during less fire-prone climate conditions. During periods of fire-prone climate, burned areas were maximum independent of fluctuations in Euro-Canadian populations. Moreover, the development of active fire suppression did not appear to reduce burned areas. These results suggest that a potential increase in climate moisture deficit and drought may trigger unprecedented burned areas and extreme fire events no matter the effects of anthropogenic ignition or suppression.

An increase in frequency, intensity and duration of drought events affects forested ecosystems. Trees react to these changes by adjusting stomatal conductance to maximize the trade-off between carbon gains and water losses. A better understanding of the consequences of these drought-induced physiological adjustments for tree growth could help inferring future productivity potentials of boreal forests. Here, we used samples from a forest inventory network in Canada where a decline in growth rates of black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.) occurred in 1988–1992, an exceptionally dry period, to verify if this growth decline resulted from physiological adjustments of trees to drought. We measured carbon and oxygen isotope ratios in growth rings of 95 spruces and 49 pines spanning 1985–1993. We used 13C discrimination (Δ13C) and 18O enrichment (Δ18O) as proxies for intrinsic water use efficiency and stomatal conductance, respectively. We studied how inter-annual variability in isotopic signals was linked to climate moisture index, vapor pressure deficit and annual snowfall amount. We found significantly lower Δ13C values over 1988–1990, and significantly higher Δ18O values in 1988–1989 and 1991 compared to the 1985–1993 averages. We also observed that a low climatic water balance and a high vapor pressure deficit were linked with low Δ13C and high Δ18O in the two study species, in parallel with low growth rates. The latter effect persisted into the year following drought for black spruce, but not for jack pine. These findings highlight that small differences in physiological parameters between species could translate into large differences in post-drought recovery. The stronger and longer lasting impact on black spruce compared to jack pine suggests a less efficient carbon use and a lower acclimation potential to future warmer and drier climate conditions.

In areas sensitive to forest management, paludification and successive disturbances in boreal forest can affect forest regeneration negatively, sometimes resulting in stand opening. As these negative effects on forest productivity are not fully considered in strategic management planning, a new landscape dynamics model integrating fire, paludification, forest harvesting, and regeneration failure was used to assess these impacts in a large forest management unit (10,828 km2) of northwestern Québec. Two reforestation scenarios, one based on the accessibility of the areas to be treated and the other aimed at restoring all burned and paludified areas to production were compared to one with no intervention. The success of the scenarios was evaluated using the predicted volume harvested, the proportion of closed or opened stands areas, an indicator of productivity; the cost of reforestation and the royalties associated with harvesting. Harvesting the paludified areas without reforesting would lead to a sharp increase in open stands areas (+17.3%). The strategy of reforesting accessible areas is the most promising for achieving sustainable forest management targets. The monitoring of maximum potential volume (MPV) and the closed forest area as indicators of landscape productivity provides the ability to anticipate problems earlier than with the conventional forest planning indicators.

Forest fires are a key driver of boreal landscape dynamics and are expected to increase with climate change in the coming decades. A profound understanding of the effects of fire upon boreal forest dynamics is thus critically needed for our ability to manage these ecosystems and conserve their services. In the present study, we investigate the long-term post-fire forest dynamics in the southern boreal forests of western Quebec using historical aerial photographs from the 1930s, alongside with modern aerial photographs from the 1990s. We quantify the changes in forest cover classes (i.e., conifers, mixed and broadleaved) for 16 study sites that were burned between 1940 and 1970. We then analyzed how interactions between pre-fire forest composition, site characteristics and a fire severity weather index (FSWI) affected the probability of changes in forest cover. In the 1930s, half of the cover of sampled sites were coniferous while the other half were broadleaved or mixed. Between the 1930s and the 1990s, 41% of the areas maintained their initial cover while 59% changed. The lowest probability of changes was found with initial coniferous cover and well drained till deposits. Moreover, an important proportion of 1930s broadleaved/mixed cover transitioned to conifers in the 1990s, which was mainly associated with high FSWI and well-drained deposits. Overall, our results highlight a relatively high resistance and resilience of southern boreal coniferous forests to fire, which suggest that future increase in fire frequency may not necessarily result in a drastic loss of conifers. View Full-Text

The carbon isotope ratio (?13C) in tree rings is commonly used to derive estimates of the assimilation?to?stomatal conductance rate of trees, that is, intrinsic water?use efficiency (iWUE). Recent studies have observed increased iWUE in response to rising atmospheric CO2 concentrations (C a), in many different species, genera and biomes. However, increasing rates of iWUE vary widely from one study to another, likely because numerous covarying factors are involved. Here, we quantified changes in iWUE of two widely distributed boreal conifers using tree samples from a forest inventory network that were collected across a wide range of growing conditions (assessed using the site index, SI), developmental stages and stand histories. Using tree?ring isotopes analysis, we assessed the magnitude of increase in iWUE after accounting for the effects of tree size, stand age, nitrogen deposition, climate and SI. We also estimated how growth conditions have modulated tree physiological responses to rising C a. We found that increases in tree size and stand age greatly influenced iWUE. The effect of C a on iWUE was strongly reduced after accounting for these two variables. iWUE increased in response to C a, mostly in trees growing on fertile stands, whereas iWUE remained almost unchanged on poor sites. Our results suggest that past studies could have overestimated the CO2 effect on iWUE, potentially leading to biased inferences about the future net carbon balance of the boreal forest. We also observed that this CO2 effect is weakening, which could affect the future capacity of trees to resist and recover from drought episodes.

Sustainable forest management implies successful regeneration after disturbances. Low N availability and competition can, however, limit tree establishment in boreal ecosystems. To develop silviculture strategies that maintain productivity in such context, we established a field trial in northern Québec, Canada. We evaluated if a companion N2-fixing species (Alnus alnobetula) promotes or hinders Picea mariana and Pinus banksiana establishment over six growing seasons. We tested if Alnus has a facilitation effect through nutritional processes and a competition effect through light interception. Foliar stable nitrogen isotope ratio (?15N?=?15N/14N, ‰) results confirmed that Alnus obtains a substantial part of its N through biological fixation and represents an N source in this system. Although we did not observe increased foliar N concentrations in either conifer species in the presence of Alnus, Pinus growth was nonetheless higher in presence of Alnus, whereas no difference was observed for Picea. In the plots where Alnus cohabited with the conifers, the former had a negative impact on seedling growth, suggesting a significant competition for light. Overall, the net effect of Alnus was positive for Pinus and neutral for Picea. Our results have significant implications for silviculture in N-limited systems, especially in the context of climate change that imposes increased levels of stress on regeneration.

Studying the response of wildlife to anthropogenic disturbances in light of their evolutionary history may help explain their capacity to adapt to novel ecological conditions. In the North American boreal forest, wildfire has been the main natural disturbance driving ecosystem dynamics for thousands of years. Boreal caribou (Rangifer tarandus caribou) is a threatened ungulate for which widespread decline has been associated with the rapid expansion of timber harvesting across its range. Although caribou may not be adapted to this new type of disturbance, cutovers share many similarities with wildfires by producing large landscapes of whole?stand removal associated with an increased predation risk for caribou. We hypothesized that caribou with more evolutionary experience of fire disturbance should better perceive the cues associated with disturbances and adjust their behavior toward human disturbance accordingly. Given the extensive distribution of caribou populations in the boreal forest, we assessed how their historical exposure to wildfires could explain their behavioral response toward both burned and cutover areas. Our results indicate that caribou from regions with high historical burn rates displayed a consistent avoidance of recent burns (<5 yr old), and that this behavior translated in a similar avoidance of recent cutover, providing support to the cue similarity hypothesis. On the contrary, caribou with less evolutionary experience of wildfires were more likely to select recently disturbed (<5 yr?old and 6–20 yr?old) habitats. In the context that timber harvesting and its associated road network has been linked to increased mortality in boreal caribou populations, we discuss how this naïve habitat use of clearcuts can be exacerbated by historical disturbance regimes and become maladaptive for this endangered species.

Climate changes are expected to progressively increase extreme wildfire frequency in forests. Finding past analogs for periods of extreme biomass burning would provide valuable insights regarding what the effects of warming might be for tree species distribution, ecosystem integrity, atmospheric greenhouse gas balance, and human safety. Here, we used a network of 42 lake-sediment charcoal records across a ~2000 km transect in eastern boreal North America to infer widespread periods of wildfire activity in association with past climate conditions. The reconstructed fluctuations in biomass burning are broadly consistent with variations in ethane concentration in Greenland polar ice cores. Biomass burning fluctuations also significantly co-varied with Greenland temperatures estimated from ice cores, at least for the past 6000 years. Our retrospective analysis of past fire activity allowed us to identify two fire periods centered around 4800 and 1100 BP, coinciding with large-scale warming in northern latitudes and having respectively affected an estimated ~71% and ~57% of the study area. These two periods co-occurred with widespread decreases in mean fire-return intervals. The two periods are likely the best analogs for what could be anticipated in terms of impacts of fire on ecosystem services provided by these forests in coming decades.

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.