How do microbial symbionts alter competition in the Great Lakes dune plant community?

Background/Question/Methods

Current plant community ecological theory neglects the effects of microbial symbiotic interaction on plant communities. These microbial symbionts can have broad-reaching effects on plant community interactions and composition. For example, plant microbial mutualists can increase the dominance of the host, which in turn can alter species diversity within the community depending on the host plant’s competitive ability. Microbial mutualists can have direct effect on competing plant species, such as increasing the host’s competitive ability for a given resource, or indirect effect on competing species, such as increasing host herbivore deterrence leading to increased herbivory of competing species. My research question is: How do above and below-ground symbionts alter density dependent competition and facilitation in the Great Lake Dunes? To examine this question I established a mesocosm experiment manipulating the density of Ammophila breviligulata and the presence of above and below-ground microbial symbionts. The density of A. breviligulata varied from zero to 16 individuals. The symbiont treatments were 1) endophyte: A. breviligulatainoculated with the fungal endophyte Epichloë sp. 2) soil: soil was added from a later succession soil microbial community, and 3) control: no endophyte or soil added. Competitive responses were measured for four common dune plant species.

Results/Conclusions

Across all four plant species, the endophyte treatment significantly increased the competitive responses compared to the control while the soil treatment had an intermediate effect. However, there was a significant interactive effect of the symbiont treatment and density of A. breviligulata on plant performance where in the soil treatment increasing A. breviligulata density correlated with the facilitation of target species growth. Even though the endophyte treatment significantly increased competitive responses across all species, the endophyte-A. breviligulata symbiosis had a significant facilitative effect on the growth of Artemisia campestris compared to the soil treatment. Target species in the endophyte and soil treatments showed significantly negative competitive responses while competitive responses in the control treatment were not significant. Within the competitive responses of each species, Prunus pumilia showed a significant negative competitive response in the endophyte treatment and Elymus canadensis showed a negative competitive response across all treatments. All other species responses were non-significant. Fungal endophyte-plant symbiosis seems to have facilitative effects on some competing species and increases the competitive effect for others. These effects could scale up to alter succession and species composition within the dune community.