Functional and hydraulic traits plasticity of boreal tree species along a latitudinal climate and permafrost gradient in northwestern North America

Boreal forests cover about one third of the world's forested area and undergo rapid changes in composition, structure, and function in response to environmental changes. Here we investigated the inter- and intra-specific variability and plasticity of boreal tree functional and hydraulic traits along a 2000-km latitudinal climate and permafrost gradient. The study area is located in northwestern Canada and includes forests with no permafrost, over isolated, sporadic and discontinuous, to continuous permafrost, spanning from the southern- to the northern edge of the boreal forest ecozone. Focusing on the region's dominating boreal tree species, black spruce (Picea mariana) and larch (Larix laricina), we monitored growing-season sap flux density of ca. 200 individuals. Moreover, leaf functional traits (e.g., specific leaf area, leaf nitrogen concentration, maximum photosynthetic capacity) are also measured for selected individuals across the study domain. By jointly analyzing stem hydrodynamics, leaf functional traits, and the prevailing environmental and micrometeorological conditions along the gradient, we were able to provide a detailed quantification of black spruce and larch inter- and intra-specific trait variability across and within sites. Moreover, species-specific water use strategies were revealed and associated with tree morphological and architectural characteristics across sites. This analysis allowed us to obtain a better understanding of boreal forest plasticity and resilience to the ongoing environmental changes.