Abstract: Climate change has altered species’ phenologies, has triggered changes in traits, and has led to shifts in abundance and distributions. Yet, we still know little about how biotic interactions alter these environmental responses due to lack of data on biotic interactions over long time scales across spatially heterogenous changes in climate. Mutualisms, reciprocally beneficial interactions, may provide resilience to changes in the environment, but at the same time increasing stress may lead mutualisms to breakdown. Epichloë fungal endophytes are vertically-transmitted symbionts of cool-season grass species. They can provide their grass hosts with tolerance to drought conditions, and several field-based surveys have found spatial patterns in endophyte prevalence structured by environmental gradients. However, these surveys are often limited to a single year and a spatial extent reachable during a single growing season. Herbaria preserve plant specimens collected through time across broad spatial scales. Because Epichloë are transmitted through seeds, they are preserved within the tissues of dried plant specimens. These specimens present an opportunity to ask how endophyte prevalence has responded to climate change. Specifically, we took samples of seeds from herbarium specimens of three grass host species collected over the last 195 years, quantified the presence or absence of fungal endophytes within the seeds with microscopy, and evaluated spatial and temporal trends in prevalence using spatial modelling.
Fungal endophyte prevalence has increased over the last two centuries across the three host species from 25% prevalence to 75% prevalence on average, however our modelling also reveals areas that are hotspots of change, which differ between species. These hotspots are associated with observed changes in climate. In particular, changes in prevalence within Elymus virginicus are primarily driven by increases in seasonal temperatures, while changes in prevalence for both Agrostis species are driven by declines in seasonal precipitation. Thus we provide novel evidence for adaptive biotic responses to ongoing climate change. We will build on this work by forecasting changes in endophyte prevalence under future climate projections.
