Genotype and environment modulate the response of trembling aspen to simulated ungulate browsing

Background/Question/Methods

Quaking aspen (Populus tremuloides) populations are declining throughout western North America, due to factors such as excessive browsing by ungulates, forest succession and climate change. We investigated the concurrent effects of aspen genotype and soil fertility on the responses of this species to simulated ungulate browsing. In a 2-year common garden study including 13 aspen genotypes (8 replicate trees per treatment per genotype), we subjected trees to combinations of high or low soil fertility and imposed artificial browsing on half of the trees in each genotype/fertility group. Artificial browsing was done once per year by removing 75% of current annual growth, using needlenose pliers. During the first year of the experiment, trees experienced substantial heat stress due to abnormally warm weather, and we incorporated this “environmental treatment” into our analysis. At the end of the study, we assessed the independent and interactive effects of artificial browsing, aspen genotype,  soil fertility, and heat stress on tree mortality, size ((basal diameter)² × height), and relative biomass gain, as well as on morphometrics such as branch number, leaf number, and leaf area.

Results/Conclusions

Artificial browsing led to decreased aspen fitness relative to that of unbrowsed control trees. The effects of browsing on fitness varied by genotype, and were accentuated by higher soil fertility and heat stress. Browsed seedlings cultivated in high-fertility soil had significantly higher death rates than did unbrowsed trees, with 100% of some genotypes in the browsed/high-fertility treatments dying during the natural heat-stress event. Across all genotypes and fertility levels, the surviving browsed trees were of 10% smaller size, had 37% more branches, 7% less leaf area, and gained 29% less biomass than did control trees at the end of the study. Browsing and elevated soil nutrients each led to reapportionment of biomass from roots to aboveground tissue, by 9% and 25%, respectively. Redistribution of biomass from roots to aboveground tissues may have decreased the ability of aspen to respond to subsequent stressors by restricting the capacity of the trees to absorb water. Our study suggests that both genetic and environmental factors modulate the response of aspen populations to ungulate browsing, and that intensive browsing decreases the capacity of aspen to endure other stressors such as extreme heat events.