Relative importance of competition and epichloid endophyte mutualism for plant community composition and ecosystem processes within the Great Lake dunes
Traditionally, the theoretical underpinnings of plant community ecology have been dominated by the study of antagonistic species interactions, mainly competition and herbivory. However, mutualistic species interactions may have similar or even stronger effects on plant community composition as antagonisms, but are less well studied. Class 1 fungal endophytes (family Clavicipitaceae) live within the above-ground tissues of ~20%–30% of grass species, as well as in other monocots and morning glories. These epichloid endophytes may provide a range of benefits to host plants in exchange for carbon and shelter within the host tissue. We tested the importance and interactive effects of an epichloae mutualism and interspecific competition on plant community composition and ecosystem processes within a Great Lakes dune plant community. During summer 2012, we established a field experiment using a randomized 2×2 factorial design manipulating epichloid endophyte (Epichloë sp.) presence in Ammophila breviligulata (present or absent) and intra/interspecific plant interactions (Monoculture = only intraspecific interactions or Mixture = interspecific + intraspecific interactions). Each plot is 3m×3m and contains 36 A. breviligulata plants and 9 individuals each of five common dune plant species. Allometric equations were used to estimate aboveground biomass for each plant species.
Both the presence of Epichloë and manipulated interspecific interactions had significant effects on plant community composition. By 2014, the presence of the endophyte had reduced the diversity of the plant community by 10% (F1,47 =8.2, P=0.006) while interspecific interactions (Mixture) had a positive effect on plant diversity relative to intraspecific interactions alone (Monoculture), increasing diversity by 15% (F1,47=14.1 P<0.001). Results from 2014 indicate that that the effects of interspecific competition (RII effect size = 0.071±0.040 95% CI) slightly outweigh the effects of endophyte mutualism (RII effect size = -0.057±0.039) in altering plant diversity. Using NMDS and PERMANOVA (Primer) we analyzed the flux of nutrients through the soil in Mixture plots with and without Epichloë. The presence of the Epichloë had a nearly significant effect on the profile of nutrients flux in the soil (pseudo-F1,17=2.3 P=0.055). Reduced magnesium, calcium, and total nitrogen in the plots with Epichloë contributed 30% to the dissimilarity between endophyte treatments. These effect on the nutrient flux could drive increasing negative effects of Epichloë on plant community diversity in the coming years. Microbial mutualists are an important driver, and comparable strength to interspecific interactions, of community composition and ecosystem processes.