The predicted increase in drought frequency with climate change and its impact on boreal tree species are growing concerns. In this study, we assessed how three boreal tree species in western Quebec used stem water storage during a three-week drought. We estimated stem water deficit from radial variations using point dendrometers installed on 50 mature trees (10 trembling aspen, 20 jack pine, and 20 black spruce) located on clay (all three species) and sandy sites (only conifers). Air temperature/humidity and soil moisture measurements were used to model their effect on stem water storage. Our results reveal contrasted stem water storage use across species during drought. Before the drought, jack pine maintained higher relative stem water storage at night than both black spruce and trembling aspen (all paired p < 0.05) but became more water-depleted during the drought (all p < 0.1). Similarly, black spruce showed the same pattern as jack pine when compared to trembling aspen, both before and during the drought (p < 0.05). Acclimation of jack pine and black spruce to moderately dry conditions on sandy soils was evident, as relative tree water deficit remained low and stable across 20%–60% relative soil extractable water, but it increased sharply below 20%. Our research highlights that boreal trees may substantially mediate drought stress using water storage pools. Our findings emphasize the need to consider stem water storage when determining which boreal tree species will be more affected by increasingly frequent droughts.

Abstract The area of primary boreal forest continues to decline due to anthropogenic disturbance, often targeting forests that provide the highest economic returns. This selective use of forests raises the question of whether the remaining primary forests occur within a subset of the environmental conditions present in their region of occurrence. We investigated whether and how the environmental conditions of primary boreal forests in Finland and northwestern Russia (Arkhangelsk, Karelia, Komi, and Murmansk) differ from those of their surrounding forests. To do this, we randomly selected 50 primary forests from each region and used openly available spatial data to quantify a set of variables describing topography, land cover, and accessibility for these primary forests and their surrounding forests. The remnant primary forests had different environmental characteristics compared with the surrounding forests in each study region. In terms of topography, the primary forests had either a higher absolute elevation or a higher topographic position than the surrounding forests. In Finland, the distance to rivers was also significantly higher in primary forests than in surrounding forests. The proportion of wetlands was high in the primary forests of Finland and Karelia, suggesting a high proportion of primary forests on organic soils. For all variables, the magnitude and occasionally even the direction of the difference between primary and surrounding forests varied between regions. In Finland and European Russia, the distribution of the remnant primary forests does not represent the full environmental variability present in their region of occurrence. This suggests that these forests do not only occur ?high and far,? but within a subset of the environmental conditions present in these high and far regions. From a conservation perspective, primary forest attributes should be restored regionally, taking into account the diversity of environmental conditions that exist within the region.