COS 8-2
Differential effects of mule deer, American elk, and cattle on aspen regeneration after fire
Introduction of livestock and increases in wildlife populations have the potential to alter herbivory patterns in plant communities. Aspen(Populus tremuloides) is an important component of forests across much of North America, and altered herbivory regimes are thought to explain loss of aspen. The objective of this study was to examine the differential effects of three ungulate species (deer, American Elk and cattle) across the time on plant regeneration success. We hypothesized that there are fundamental differences in the impact of ungulate species on forest regeneration after fire. We installed 10 sets of fencing treatments in four 2012 fires across three national forests in Central and Southern Utah. Each location has 4 fencing types on 30 x 30m plots: 1) full ungulate exclosure 2) exclosure of elk and cattle (allows deer-only), 3) cattle fencing (allowing deer and elk), and a control plot with no fence. We characterized ungulate browsing patterns of post-fire regenerating aspen in August, September and November of 2013. Measurements included: percent of browsed meristems, percent defoliation, height and density. We estimated relative ungulate abundance through cameras and fecal counts.
Results/Conclusions
Our results show clear detrimental effects of multiple ungulates on the regeneration of aspen forests after fire. 74% of aspen meristems were browsed in control plots, vs 46% and 41% in the cattle+elk exclosure, and cattle exclosure, respectively (0% in full exclosures). Aspen in control plots were defoliated twice as much as in cattle fencing, and exclusion of elk and cattle. Defoliation in each treatment was 76%, 39%, and 39% respectively, while full ungulate exclusion had only 5% defoliation (p<0.001). Density was higher in control plots (p=0.03), likely due to browse of apical buds which stimulates further basal shoots from the underlying root system, instead of vertical growth. This contributes to bush like morphology of aspen and reduces the capability of vertical escape from browse. Aspen height did not increase from during the growing season in control plots, while it increased an average of 6cm across all other treatments (p=0.006). This suggests that in control plots, co-occurring ungulate browsing limits the vertical growth of aspen, and potential recruitment. Variability in location likely contributed to the results, where areas with high ungulate densities had detrimental effects on aspen regeneration while areas with low densities experienced successful regeneration.