Pinus banksiana exhibits remarkable ecological adaptability, thriving across diverse environments in the Canadian boreal zone, including clay deposits, fast-draining glacial tills, and rocky outcrops. However, projected rising temperature and increasing vapor pressure deficit (VPD), could increase the species’ vulnerability, particularly in dry regions. In this study, we measured basal area increment (BAI) and physiological responses from isotopic fractionation across a soil gradient including three sites in the boreal mixed wood of western Quebec, Canada. The sites were a clay-rich soil (CLY, a humid site), an esker base (ESB, an intermediate site), and an esker top (EST, a sandy, well drained, dry site). Using tree-ring analysis and dual stable isotopes (δ13C and δ18O), we evaluated intrinsic water-use efficiency (iWUE) and leaf water enrichment (Δ18Olw). Our results revealed a significant correlation between Δ18Olw and VPD, indicating that stomatal regulation is the crucial physiological mechanism controlling P. banksiana’s response to environmental stress across the sites. This effect was most pronounced at the dry EST site, where higher iWUE and less negative δ13C values suggest greater stomatal limitation of CO2 uptake. Increased iWUE was associated with enhanced BAI in the humid CLY site and a negative iWUE-BAI relationship emerged at EST, suggesting carbon assimilation constraints under drier conditions. Our results reveal a physiological trade-off in P. banksiana across a soil moisture gradient, demonstrating that rising atmospheric demand may decouple water-use efficiency from growth in drier environments like the EST site. By integrating isotopic signatures with growth dynamics, our study identifies a potential ecological tipping point beyond which increased iWUE may no longer sustain carbon gain under intensifying climate stress.

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 Introduction Mining activities can lead to the formation of degraded, barren, or metal-contaminated ecosystems. Resource-poor ecosystems such as rocky outcrops are more sensitive to mining degradation, and their natural regeneration can be challenging due to soil erosion, lack of resources or seeds, and soil acidification. Objectives Our aim was to test the effectiveness of using locally collected bryophyte (Ceratodon purpureus [Hedw.] Brid.) mats as a restoration treatment to protect and promote the establishment of tree seedlings in mining-polluted rocky outcrops in Rouyn-Noranda (Canada). Methods The bryophyte restoration treatment inspired by natural succession processes was compared to a control, where only local soil was used as substrate, and to a liming treatment that increases soil pH. The three treatments were applied to sixty 1???1?m?units located on five outcrops at various distances (1.9?26.9?km) from the pollution source. Four tested tree species were each seeded at a density of 100?seeds/m2 on all units. Results The bryophyte treatment had a positive effect on the establishment success of Jack pine seedlings (Pinus banksiana Lamb.) with an establishment rate of 12% compared to 5 and 4% for liming and control treatments, respectively. Wind exposure had a significant negative effect on seedling establishment, potentially masking any negative effects of soil heavy metal concentration, which were not statistically significant. Conclusions Our strategy using bryophytes and mimicking natural succession has the potential to effectively regenerate trees in degraded rocky outcrops.

Assessing lead (Pb) and cadmium (Cd) concentration in tree rings may provide historical records of environmental contamination, capturing temporal changes and spatial distribution. This study examined heavy metal (HM) bioaccumulation in tree rings to assess the impact of smelting activities using urban trees. Tree rings from urban conifers were collected within a 5 km radius of the smelter to estimate spatiotemporal trends of Pb and Cd concentrations, evaluating the outcomes of contamination reduction measures and interspecies bioaccumulation patterns. Pb isotopic ratios (206Pb/207Pb, 208Pb/206Pb) were also measured to evaluate the Pb origin. Results showed no clear spatial pattern in relation to distance to the smelter, which may be due to the small sampling area. However, Cd bioaccumulation in urban pine was 4 to 7 times higher than in the distant site, 80 km away, indicating a local impact of industrial contamination in the urban area. Temporal analysis for pine showed a decrease of 0.27 mg/kg Cd (47%) between 1990 and 2020, reflecting the potential influence of contamination-reducing measures, while Pb concentrations in pine were 2.7 times higher (increased by 0.06 mg/kg) for the same period. Pine bioaccumulated more Cd than spruce, while spruce accumulated higher levels of Pb compared to pine. Isotope measurement confirmed that the copper smelter is the primary source of Pb. These findings underscore the complex nature of HM uptake in urban trees and suggest that further research is needed to understand the spatiotemporal effects on HM bioaccumulation patterns and which species are best suited for phytoremediation.