Physiological responses to warming in Acadian Forest Region conifers and their implications for forest modelling under climate change

Abstract

Forest ecosystems in eastern North America are expected to experience substantial physiological stress under future climate warming and increasing atmospheric dryness. However, uncertainty remains regarding how species-specific physiological traits influence responses to these environmental changes and how they are represented in forest models. This thesis integrates controlled warming experiments with process-based modelling to improve understanding of forest responses to climate change in the boreal–temperate ecotone.

First, I investigated how geographic range and shade tolerance influence tree species responses to warming in the boreal–temperate ecotone. Using custom-built phytotrons with 12 warming levels, I measured growth, mortality, and physiological responses of seven tree species differing in southern range limits and shade tolerances. Shade-tolerant species, eastern hemlock and balsam fir, exhibited greater foliar damage and mortality under high temperatures, whereas light-demanding jack pine showed comparatively higher tolerance despite its northern distribution. These findings highlight the importance of considering functional traits when predicting warming responses.

Second, I examined species-specific photosynthetic responses to warming in balsam fir and white spruce, two boreal conifers commonly classified as the same plant functional type. Under +6 °C warming, white spruce maintained higher stomatal conductance, transpiration, net assimilation, and photosynthetic capacity than balsam fir, which exhibited reduced stomatal conductance and carbon uptake. These results demonstrate substantial physiological divergence between these species and highlight limitations of grouping species into broad plant functional types.

Third, I assessed the impact of incorporating new empirically derived physiological parameters into the process-based model iLand. Modifying the vapour pressure deficit response parameter for balsam fir reduced predicted growth and gross primary production under historical and future climate scenarios, improving alignment with experimental observations and highlighting the importance of using species-specific physiological thresholds for improving model accuracy.

Together, this thesis demonstrates that forest responses to warming are strongly mediated by species-specific physiological traits, particularly stomatal regulation and carbon balance, and that incorporating empirically derived, species-specific thresholds into models is essential for improving predictions of forest dynamics under climate change.