Adaptation of trembling aspen and hybrid poplars to frost and drought: implications for selection and movement of planting stock in western Canada

This study contains a series of experiments to evaluate growth performance and survival of hybrid poplars (Populus spp.) and trembling aspen (Populus tremuloides Michx.) in boreal planting environments in western Canada. Ecophysiological traits related to drought resistance and winter survival were studied and compared with growth in long-term field trials, within and between these two plant groups. The results showed that trembling aspen is more resistant to drought stress and more water-use efficient than hybrid poplars, suggesting that these two groups employ different water-use strategies. Tree height was negatively correlated with branch vessel diameter in both plant groups and was highly conserved in aspen trees from different geographic origins. Hybrid poplars with wider xylem vessel were also more prone to freezing-induced embolism, suggesting that smaller vessel diameters may be an essential adaptive trait to ensure frost tolerance and long-term productivity of hybrid poplar plantations in boreal planting environments. For aspen, provenances ranging from northeast British Columbia to Minnesota were tested in a series of reciprocal transplant experiments across western Canada. The analysis found pronounced increases in productivity as a result of long-distance transfers in northwest direction. Commonly reported trade-offs between freezing tolerance and growth rate were not found in this study. Seed transferred from Minnesota to northeast British Columbia (2,300 km northeast and 11° latitude north), still outperformed local sources by 17 % in height had more than twice the biomass at age ten. Increased productivity as a result of northwest transfers was not associated with reduced survival. The results suggest that the potential benefits of northward movement of aspen populations in forestry operations outweigh the potential risks, especially in the context of climate change.