Indirect effects of abundant deer drive unbrowsed plant species decline in temperate forests

Background/Question/Methods Herbivores can indirectly facilitate co-occurring but unpalatable plant species by browsing on their more palatable neighbors. This is expected to diminish inter-specific competition and result in better performance of the unpalatable species. Here we test the hypothesis that ungulate herbivory on palatable plant species indirectly facilitates co-occuring unpalatable species. First, using Arisaema triphyllum as a model unpalatable species, we quantified population performance metrics in seven forested sites in Pennsylvania that differ in mean annual levels of deer browse on palatable Trillium species. Second, in three sites in Virginia where browse levels on individual species are known, we determined individual plant size of five unpalatable species, including Arisaema, growing within paired long-term deer exclusion/deer access treatment plots. Finally, to test a second hypothesis that abiotic factors may be co-affected by high deer browsing in a site, we measured key abiotic factors at all natural forest sites and paired plots to gain insight into additional mechanisms of deer's indirect effects on unpalatable plants.
Results/Conclusions For our focal unpalatable forest herb, Arisaema triphyllum, we found surprising and insidious changes in its population structure and demography in sites where browse on the co-occurring Trillium species was high. Reduced growth rates, plant size, and seed rain, and increased male-biased sex ratios in Arisaema populations were all significantly correlated with deer browse on Trillium. In addition, size data for Arisaema and four other unbrowsed herbs growing in deer exclusion/access experimental plots supported these results; all species were significantly smaller in the deer access plots. Further, soil compaction was lower in both deer exclusion plots and in the natural sites with low browse intensities, suggesting a mechanism for Arisaema's performance declines. Our findings suggest that many unbrowsed species in forests overabundant deer could be negatively affected along with their palatable neighbors. Our study implicates high ungulate density in the cascade of plant species decline and highlights the urgency of this conservation issue.