Disentangling the seasonal timing of aspen (Populus tremuloides) herbivory and differential impact of three ungulate species on aspen recruitment after fire
Ungulate herbivore regimes are changing globally. Humans alter the native ungulate populations through; the removal of apex predators, active management, and altered habitat conditions. Further, ungulate communities change through introduction of non-native species, especially cattle, which alter plant-animal interactions. We evaluate the impact of single and multiple ungulates on their habitat through examining the timing and differential use of aspen after fire by mule deer, American elk, and cattle. Disentangling the effect of these species and their effect on aspen over a growing seasons elucidates ungulate effects on successional processes. We monitored aspen height growth, and herbivore utilization during 2013-2014 growing seasons following 3 independent fires in central and southern Utah using differential fencing exclosures. These treatments include; deer-only fence, deer-elk only fences, complete exclosures and no fences. Our objective was to evaluate the effect of multiple and single ungulate species on the growth, and recruitment of aspen after fire across a growing season. We ask; 1) When is aspen being utilized? 2) What is the effect of individual ungulate species on aspen? 3) And is ungulate species mix and abundance influencing aspen recruitment?
Our results show clear additive effects of multiple ungulates on the regeneration of aspen forests after fire. Peak aspen use occurred between August and September. For example, aspen defoliation averaged less than 5% for open plots during June, but spiked to 45% during August. Vertical growth, an indicator of recruitment success, increased in all plots through July and then remained stable, except for a 30% reduction in height in no fence plots. This resulted in no height increase during the growing season in fenceless plots, compared to 10 cm of growth over season growth in deer and deer-elk fences, and 15cm in full exclosures (p=0.006). We found that 74% of aspen meristems were browsed in fenceless plots, vs 46% and 41% in the deer-only fence, and deer-elk only fence plots, respectively. Aspen in control plots were defoliated twice as much as in plots with cattle fencing, and exclusion of elk and cattle. Defoliation in each treatment was 76%, 39%, and 39% respectively, while full ungulate exclusion had only was less than 5% defoliation (p<0.001). We found that aspen recruitment can be suppressed by each ungulate species individually, and that effects among species are additive.