Characterizing aspen regeneration by abundance of ungulates, topography, and tree species composition

Background/Question/Methods

Herbivory regimes are changing globally. Loss of apex predators, increasing ungulate populations, and introduction of livestock create novel pressures for many ecosystems. In subalpine forests of the west, Aspen (Populus tremuloides) provides habitat and forage for both native and introduced ungulates, yet historically high ungulate populations may threaten aspen resilience. The objective of this study is to characterize the impact of ungulate herbivores on aspen regeneration and recruitment as a function of stand composition and topography at a regional scale.

            We chose paired 93 aspen and mixed aspen-conifer stands across 3 national forests in Utah. In each stand, we estimated animal abundance by fecal group counts, measured the height of aspen suckers (0-200cm), counted the density, and recorded percent browsed stems. Elevation, aspect, and slope, was extracted from a statewide 30mx30m DEM. We used model selection based on AICc to measure the importance of topography, stand type and animal abundance for aspen regeneration. 

Results/Conclusions

            We found that aspen regeneration density was positively correlated with elevation, aspen overstory density and was 25% higher in aspen stands. Height was negatively correlated with all ungulate species, shorter at northern aspects, 7.5% shorter in mixed stands, and increased with elevation and slope. Browse damage was positively correlated with all ungulate species, and higher in mixed stands but the relationships were relatively weak

            We conclude that ungulate species can have a negative impact on aspen regeneration and that these effects increase with animal density. Aspen regeneration in pure aspen stands, at high elevation, and on sloped terrain .are most successful while sparse aspen stands at low elevation and gentle slopes.