Fungal endophyte and nitrogen enrichment alter functional traits and productivity in a dominant grass

Global change factors, such as anthropogenic nitrogen (N) enrichment, have been shown to have diverse and widespread effects on plants and ecosystem functioning, including effects on intraspecific trait variation. Plant symbionts such as Epichloë amarillans ( Epichloë , hereafter), are ubiquitous and can have substantial effects on host traits and fitness, yet little is known about how they interact with global change factors to affect host growth. We used an in situ, long‐term experiment to examine the functional traits and population‐level productivity of both Epichloë ‐inoculated and sham‐inoculated Ammophila breviligulata (American beachgrass, Ammophila hereafter) plants experiencing three levels of N enrichment. We found that Epichloë and N enrichment shifted Ammophila functional traits toward a faster, more acquisitive growth strategy at the tiller level. At the population level, we found that Ammophila biomass was 43% lower in Epichloë ‐inoculated plots than in sham‐inoculated plots under conditions of low N enrichment. When plots were exposed to high N enrichment, however, this pattern was reversed, with Epichloë ‐inoculated Ammophila plots having 30% greater biomass than sham‐inoculated plots. We demonstrate that a fungal endophyte and N enrichment had significant direct effects on intraspecific trait variation in a dominant grass species, while their interaction impacted population‐level biomass in complex ways that were not immediately obvious from tiller‐level trait measures. Examining plant‐microbe interactions along with clonal traits and demography may be important for linking individual‐level functional traits with population‐level effects on ecosystem processes under conditions of global change.