Increasing Prevalence of Plant‐Fungal Symbiosis Across Two Centuries of Environmental Change

ABSTRACT Species' distributions and abundances are shifting in response to ongoing global climate change. Mutualistic microbial symbionts can provide hosts with protection from environmental stress that may promote resilience under environmental change; however, this change may also disrupt species interactions and lead to declines in hosts and/or symbionts. Symbionts preserved within natural history specimens offer a unique opportunity to quantify changes in microbial symbiosis across broad temporal and spatial scales. We asked how the prevalence of seed‐transmitted fungal symbionts of grasses ( Epichloë endophytes) has changed over time in response to climate change, and how these changes vary across host species' distributions. Specifically, we examined 2346 herbarium specimens of three grass host species ( Agrostis hyemalis , Agrostis perennans , Elymus virginicus