Functional diversity is a passenger but not driver of drought-related plant diversity losses in annual grasslands

1. Effects of climate change on plant community functional diversity are of interest since experiments have found functional diversity to predict ecosystem function. Functional diversity has been hypothesized to confer resilience to plant communities (as a “driver” of community change), but in unmanipulated natural communities, it might alternatively (or additionally) act as a “passenger” by responding to changes in plant diversity caused by extrinsic factors such as climate. 2. We examined trends in plant functional diversity in annual grasslands in Northern California over a 19-year period, during which a trend toward drier winters had previously been associated with the losses of drought-intolerant species. We tested whether functional diversity decreased over the period of the study (acting as a passenger), and also whether initial site functional diversity influenced the degree of community change over the study period (acting as a driver). 3. Initial community functional diversity...