Background/Question/Methods Plant microbial symbionts are known to affect host plant growth with cascading effects on ecosystem functioning. Epichloë amarillans is a systemic endophyte in Ammophila breviligulata, a native ecosystem engineering grass that is one of the first species to colonize open sand dune habitats, initiating primary succession. Our previous research has demonstrated that Epichloë can increase vegetative growth of Ammophila, consequently slowing colonization of new plant species in dunes. Here, we follow-up to ask whether Epichloë continues to slow primary succession by reducing tiller die-off in Ammophila. We also examine whether soil resource availability, specifically nitrogen, alters the effects of Epichloë on successional trajectories. We tested these questions in a long-term field experiment in Lake Michigan sand dunes. In 2010, we planted Ammophila with or without Epichloë into a 90-plot factorial experiment and have been measuring tiller growth and colonizing plant species diversity for 10 years. Starting in 2016, we added a nitrogen addition treatment, applying slow-release nitrogen fertilizer to a subset of plots at 3 levels: control, low (0.5 g NH4+/m2, N-dep rates in the Chicago region), and high (10g NH4+/m2, releasing plants from any N limitation). We expected that N-addition would enhance the strength of Epichloë as a symbiont, further increasing Ammophila dominance. Results/Conclusions We found that Epichloë has no effect on Ammophila tiller density decline but continued to slow colonization by new plant species, most likely due to a legacy effect from increased Ammophila tiller growth early in the experiment. Nitrogen addition further exaggerated these effects of Epichloë by decreasing colonizing plant diversity where Epichloë was present, but not where it was absent. The strongest suppression of succession occurred under conditions of high nitrogen combined with Epichloë presence. Our results suggest that plant symbionts can have long-lasting effects on successional trajectories of communities, and that changes in resource availability due to anthropogenic global change may further exacerbate these effects.
