Chronic moose browsing impacts the architecture of balsam fir saplings

Background/Question/Methods

Chronic herbivory by browsing ungulates is hypothesized to alter the architecture of small trees and shrubs while reducing total forest biomass. Quantifying the impact of browsers on forest biomass would improve models of potential C storage in forest ecosystems. Moose are important browsers in boreal forests, which may be an important terrestrial C sink due to their extensive range. To quantify the impact of moose browsing on a common browse species, we sampled 90 balsam fir saplings from Isle Royale National Park, Michigan, USA. Forty-five saplings were collected from a region of high moose impact (the “west side”) and 45 from a region of low moose impact (the “east side”). Saplings were cut at ground level and separated into the aboveground components of main stem, branches, and foliage. We tested for the effects of moose browsing on sapling architecture by modeling allometric relationships between sapling diameter and each aboveground biomass component, total aboveground biomass, and total height for each side of Isle Royale. We hypothesized that west side trees would be shorter and contain greater branch biomass and less foliar biomass than east side trees of similar diameter.

Results/Conclusions  

Moose browsing reduced foliar biomass and tree height, but not total biomass, stem biomass, or branch biomass at similar diameters. Based on log-log models, diameter was a good predictor of each response variable (total aboveground biomass: R² = 0.913, stem biomass: R² = 0.884, branch biomass: R² = 0.882, foliar biomass: R² = 0.683, height: R² = 0.68). Although the slope of the height and foliar biomass curves differed by moose browse intensity, the intercepts did not. This suggests that the effect of moose on these variables increases with sapling age, as saplings continually miss opportunities for growth each time they are browsed. It is interesting that we observed a difference in height but not in total, stem, or branch biomass in saplings of similar diameter. This indicates that chronically browsed trees tend to be shorter with greater girth whereas unbrowsed trees have greater biomass because they are taller. The shorter trees with more girth are likely to be much older than taller trees of the same diameter. In conclusion, this study suggests that intense browsing by moose alters the architecture of balsam fir saplings and likely reduces growth rate and C storage in boreal forests.