Higher fall precipitation rather than high summer temperature negatively impacts Thuja occidentalis growth at the boreal-temperate ecotone

Projecting the growth of culturally and economically significant trees is vital for our understanding of forest vulnerability to climate change. Trees growing near the distributional limits of their species are especially susceptible to environmental changes. To assess the vulnerability of the boreal tree Thuja occidentalis , northern white - cedar, to expected climatic changes, we conducted a dendrochronological analysis of trees growing at the North American boreal - temperate ecotone. We collected samples from ten stands of different soil water availability and tree densities. When analyzing both early and latewood increments, we found negative associations between early treeradial growth and increasing precipitation during the autumn of the year prior to ring formation, and to a lower extent between increasing maximum temperatures during the summer of ring formation. There was a trend, although not significant, of stronger negative associations between earlywood growth and autumn precipitation at sites that were wetter, and a weaker association in areas with higher heterospecific tree density. The negative associations with summer temperatures were stronger in areas with higher heterospecific tree density. These results indicate that thinning would be beneficial for this species in stands with lower soil moisture, but detrimental in areas with high water tables, underscoring the importance of considering local conditions in management aimed at mitigating the impacts of adverse growing conditions.