Ungulate Browsing Maintains Shrub Diversity in the Absence of Episodic Disturbance in Seasonally-Arid Conifer Forest

Burak K Pekin,Bryan A Endress,Catherine G Parks,Bridgett J Naylor,Michael J Wisdom,Mari Moora

doi:10.1371/journal.pone.0086288

Abstract

Ungulates exert a strong influence on the composition and diversity of vegetation communities. However, little is known about how ungulate browsing pressure interacts with episodic disturbances such as fire and stand thinning. We assessed shrub responses to variable browsing pressure by cattle and elk in fuels treated (mechanical removal of fuels followed by prescribed burning) and non-fuels treated forest sites in northeastern Oregon, US. Seven treatment paddocks were established at each site; three with cattle exclusion and low, moderate and high elk browsing pressure, three with elk exclusion and low, moderate and high cattle browsing pressure, and one with both cattle and elk exclusion. The height, cover and number of stems of each shrub species were recorded at multiple plots within each paddock at the time of establishment and six years later. Changes in shrub species composition over the six year period were explored using multivariate analyses. Generalized Linear Mixed Models were used to determine the effect of browsing pressure on the change in shrub diversity and evenness. Vegetation composition in un-browsed paddocks changed more strongly and in different trajectories than in browsed paddocks at sites that were not fuels treated. In fuels treated sites, changes in composition were minimal for un-browsed paddocks. Shrub diversity and evenness decreased strongly in un-browsed paddocks relative to paddocks with low, moderate and high browsing pressure at non-fuels treated sites, but not at fuels treated sites. These results suggest that in the combined absence of fire, mechanical thinning and ungulate browsing, shrub diversity is reduced due to increased dominance by certain shrub species which are otherwise suppressed by ungulates and/or fuels removal. Accordingly, ungulate browsing, even at low intensities, can be used to suppress dominant shrub species and maintain diversity in the absence of episodic disturbance events.

Highlights

Both livestock and wild ungulates commonly occupy high elevation pastures during the summer months in many parts of the globe
Compositional responses to browsing At non-fuels treated sites, the change in shrub composition was much greater at un-browsed paddocks compared to browsed paddocks, regardless of browsing intensity or ungulate type (Figure 3)
Our results demonstrate that ungulates have an important role in maintaining plant diversity in western forest ecosystems in the absence of episodic disturbance events such as fire and/or mechanical fuels removal

Summary

Introduction

Both livestock and wild ungulates commonly occupy high elevation pastures during the summer months in many parts of the globe. In the western United States, most ranchers bring their livestock to high elevation summer pastures that are located within public forest lands [1]. The use of these inter-montane forests by large numbers of ungulates, primarily cattle and elk, poses a significant challenge to their sustainable management [2]. In order to reduce wildfire risk and protect forest resources, prescribed burning and other fuels reduction treatments such as mechanical thinning are regularly conducted in western forests [3].

Methods

Results

Conclusion