Influence of Functional Traits, Shrub Neighbors, and Habitat Types on Plant Responses to Herbivores

Herbivores play important roles in determining the performance of individual plants and the abundance of plant populations. However, fundamental gaps remain in our ability to predict how plant species respond to the direct and indirect effects of herbivores. Although the influence of herbivores on plant species may be determined in part by the functional traits of plants and by interactions with neighboring vegetation, studies seldom consider the importance of both of these factors. In a coastal grassland in northern California, we used a large-scale, 14-year-old exclosure experiment to examine the responses of six plant species (Heracleum maximum, Cirsium vulgare, Ranunculus californicus, Iris douglasii, Baccharis pilularis and Lupinus arboreus) to a large mammalian herbivore - tule elk (Cervus elaphus nannodes)- across three habitat types (Baccharis-dominated, Lupinus-dominated, and open grasslands). For two species (Heracleum and Cirsium), we also examined the influence of neighboring shrubs in mediating the influence of herbivores. We found that elk had no effects on the abundance or cover of two plant species (Cirsium and Lupinus) but had a negative effect on Heracleum, Iris and Baccharis and a positive effect on Ranunculus. We hypothesize that elk affected these four plant species primarily through disturbance rather than consumption, with negative effects having arisen from disturbance that caused plant mortality and positive effects having arisen from disturbance that reduced the intensity of competition. For Iris and Baccharis, we also found that the effects of elk varied among habitats, as herbivores were influential in only one or two of the three habitat types. In general, elk did not affect plant functional traits and this did not vary by habitat type. Specific leaf area (SLA) was the strongest predictor of species responses to herbivores, with elk increasing the abundance of plant species with higher SLA and having no influence on plants with lower SLA. Leaf area and leaf dry matter content were weaker predictors of species responses to herbivores. Lastly, for Cirsium, we found that plant neighbors mediated its responses to herbivores. Cirsium grew taller when associating with neighboring shrubs and elk had a reduced negative effect on Cirsium when it associated with neighbors. Our results suggest that species with more resourceacquisition functional traits may respond more strongly to herbivores and that species responses may be mediated by plant-plant interactions. Our results found high variability in plant responses to herbivores and highlight the utility of integrating both functional traits and plant-plant int~ctions into studies predicting the response of plants to herbivores.