Wild ungulates compound herbicide-mediated simplification of early successional plant communities in forest plantations

Abstract

Understanding the degree to which management interacts with ecological processes to drive the structuring of biodiversity remains a central challenge in ecology. For instance, the effects of herbivores on post-disturbance vegetation development may depend on the degree to which anthropogenic interventions have altered plant communities.We hypothesized that by simplifying plant species composition at local scales, intensive forest management practices should amplify herbivore-mediated changes in early-successional vegetation development and diversity at multiple levels of biological observation (i.e., alpha, beta, and gamma diversity). To test this hypothesis, we implemented a six-year experiment that manipulated post-timber harvest vegetation establishment via a gradient in vegetation management intensity (i.e., herbicides), and quantified compositional changes with and without wild-ungulate herbivores.Increasingly intensive herbicide treatments suppressed broadleaf vegetation and reduced plant diversity at alpha and gamma levels, while the effects of wild-ungulates were most evident in measures of community change and beta diversity for a commonly applied treatment type. Following the broad-spectrum and follow-up herbaceous treatment combination, herbivory contributed to the suppression of vegetation cover, slowing plant community development by 21%, and resulting in 27% less among-plot heterogeneity (i.e., lower beta diversity). We found no effects of herbivory alpha diversity, but the divergence in communities subjected to herbivory resulted in disparate community composition and lower gamma diversity.Herbicide and herbivore-mediated alterations in community composition was due the suppression of broadleaf species and subsequent release of common herbivory-tolerant herbaceous species. Although herbivory also slowed vegetation development without herbicides, the high diversity and abundance of broadleaf forages likely buffered untreated sites from the herbivore-mediated alterations of plant composition at alpha, beta, and gamma levels.The variation in effects of herbicide and herbivory in this experiment highlight the importance of testing for interactions between natural and anthropogenic agents on vegetation development and quantifying diversity responses beyond local scales. These results also suggest that large herbivores may contribute to management-induced simplification of young forests, with potential implications for successional processes and regional biodiversity conservation.