Herbivory can drive large shifts in plant communities but often these shifts are patchy across the landscape. Understanding what drives heterogeneous shifts is vital for assessing plant community resilience and identifying thresholds of change. Abiotic factors, including soil depth and texture, can affect the stress of a plant by altering the amount of soil water available for plants. What remains unknown is the degree to which these soil properties alter the response of a plant population to herbivory. Here we assessed if soil texture mediates the population response of big sagebrush (Artemisia tridentata) and associated bunch grasses to different types of large ungulate herbivory.
We sampled long-term (60+ years) herbivore exclosure sites located in big sagebrush dominated plant communities. We selected 17 exclosures located in loam soils and 15 exclosures in sandy loam soils. Each site contained three herbivory treatments: 1) no ungulate herbivory (DX), 2) wild ungulate herbivory only (LX), and 3) wild ungulate and cattle herbivory (OX). We measured sagebrush density by height and age class, bunch grass density, and ungulate use at each site. The effects of herbivore treatments and soil texture were tested with generalized linear mixed effects models.
Soil texture mediated the effects of ungulate herbivory on density and height/age structure of big sagebrush (interaction: p=0.03). In sandy loam soils, overall sagebrush density was significantly reduced in the presence of wild ungulate herbivory (DX= 5167 ± 954/ha; LX= 2993 ± 441/ha; OX= 2671 ± 561/ha). The presence of wild ungulate herbivory also shifted sagebrush into smaller size classes, significantly decreasing the density of 50-100cm tall individuals and increasing density of 0-15cm individuals. In contrast, in loam soils the overall density of big sagebrush was not significantly affected by herbivory. However, like in sandy loam soils, the sagebrush population shifted to smaller size classes in the presence of wild ungulate herbivory. The presence of cattle in loam soils was also associated with higher sagebrush seedling densities (OX = 640 ± 97/ha, LX=161 ± 48/ha, and DX=131 ± 33/ha). Soil texture also altered the response of two perennial bunchgrasses, Big Bluegrass (Poa secunda) and Indian Ricegrass (Achnatherum hymenoides), to the presence wild ungulates and cattle (interaction: p=0.04 and p=0.02, respectively). Our results illustrate that abiotic environmental factors can modify the resilience of plant communities to biotic factors like herbivory.