Jack pine's responses to climate change: increased water-use efficiency but evident growth limitations in dry environments.

Pinus banksiana Lamb. 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.