Linking black spruce xylem anatomy to local hydroclimate in two boreal fens

Black spruce (Picea mariana) growing in treed boreal fens in North America face significant challenges due to anticipated warming and moisture deficits. To assess site and climate influence on black spruce xylem anatomy, we investigated two treed fens at different elevations (740 m and 320 m a.s.l.) in Alberta, Canada. We examined key xylem traits – cell number, cell lumen area, and cell wall thickness – comparing the two sites to assess the effects of local conditions. Additionally, we correlated these anatomical features with long-term temperature, precipitation, and vapor pressure deficit (VPD) data to evaluate the impact of inter-annual climate variability. We observed larger cell lumens and thinner cell walls in trees at the lower-elevation fen. Xylem climate responses were clearer at the higher-elevation fen, characterized by a more stable water table. Here, previous year summer temperatures were negatively associated with cell number. High temperatures and VPD during the current year late spring and early summer reduced earlywood lumen size. Precipitation showed marginal associations at both sites. This study demonstrates the utility of quantitative wood anatomy in understanding environmental influences on tree xylem anatomy in treed fens. Differences in climatic responses between nearby sites highlight the importance of local hydroclimatic conditions in shaping xylem structure. Notably, observed sensitivity of cell lumen area to spring and early summer temperature and VPD suggests that climate warming could significantly impact the xylem structure and water transport efficiency of black spruce in treed boreal fens.