# Plant Functional Traits, but Not Community Composition, Are Affected by Summer Precipitation and Herbivory in an Old‐Field Ecosystem

## TL;DR

This study shows that summer precipitation and insect herbivores interact to influence plant traits, but not community composition, in an old-field ecosystem.

## Contribution

The study experimentally demonstrates how precipitation and herbivory interact to affect plant functional traits, not just community composition.

## Key findings

- Grass biomass was higher without insects only under reduced summer precipitation.
- Plant functional diversity and traits related to resource use and herbivore resistance varied with precipitation and insect presence.
- Common plant species' functional traits were most affected by the combined effects of precipitation and herbivory.

## Abstract

Both precipitation and herbivores can independently control plant community composition and ecosystem function. However, few studies have experimentally examined the potential interactive effects of altered precipitation and herbivores on plant communities and plant traits. Here, we manipulated summer precipitation and insect presence in an old‐field ecosystem and quantified their interactive effects on plant community structure and functional traits. Overall, the effect of an insect herbivore on the plant community was contingent on the precipitation treatment. There were no experimental effects on total plant biomass or plant species richness, but grass biomass was higher in the absence of insect herbivores only in reduced summer precipitation plots. Furthermore, plant functional diversity and the community‐averaged trends of several plant functional traits related to resource use and herbivore resistance varied systematically with reduced precipitation and insect presence. We demonstrate that the effect of reduced precipitation on plant biomass, functional diversity, and the community‐averaged trends of plant functional traits can be mediated by the presence of insects. Our findings further suggest that the functional traits of the common plant species in the community are the most affected by the combined manipulation of altered summer precipitation and insect presence.

We demonstrate that the effect of reduced precipitation on plant biomass, functional diversity, and the community‐averaged trends of plant functional traits can be mediated by the presence of grasshoppers. Our findings further suggest that the functional traits of the most common species in the community are the most affected by the combined effects of reduced summer precipitation and the presence of a generalist herbivore.