Fire severity and ungulate herbivory shape forest regeneration and recruitment after a large mixed-severity wildfire

Fire severity and herbivory by wild and domestic ungulates can strongly influence regeneration and recruitment of plant species. Importantly, the effects of herbivores on recruitment of plants is predicted to vary depending on the level of fire severity that an area experiences and habitat use of wild and domestic ungulates. In particular, two foundational plant species, quaking aspen (Populus tremuloides) and Gambel oak (Quercus gambelii), are widespread fire-adapted trees that support unique ecological communities; yet little is known about how fire severity and herbivory by ungulates potentially interact and shape their regeneration and recruitment. We studied aspen and Gambel oak regeneration and recruitment in relation to fire severity, topography, and habitat use by wild ungulates (elk and deer) and domestic livestock (cattle) across a forested region experiencing a large mixed-severity wildfire (10 years post fire). We predicted that plant regeneration and recruitment would increase with fire severity, but that recruitment would also be influenced by habitat use of ungulates. For aspen and Gambel oak, there was a strong positive relationship between regeneration and recruitment with fire severity. In addition, as the proportion of saplings that were browsed increased, plant recruitment decreased. However, the effects of herbivory on recruitment of resprouting trees in our study depended on the level of fire severity and habitat use of ungulates. Regeneration and recruitment of aspen and Gambel oak were greatest in areas experiencing moderate to high fire severity, and were absent or at low levels in areas that were unburned or experienced low fire severity (which would reduce availability of stems to be browsed). However, in higher fire severity areas there was variability in recruitment, depending on intensity of herbivore habitat use. Our study indicates that some fire-adapted plant species, such as aspen and Gambel oak, are most likely to recruit and promote the integrity and resilience of ecological communities in areas where (1) the positive effects of mixed-severity fire occur over sufficiently broad spatial extents and (2) the negative effects of herbivory by ungulates are reduced, especially in areas of lower fire severity, or where high-severity fires are smaller in size.