Facilitative effects of dune ecosystem engineer Ammophila breviligulata increase with density and association with an aboveground symbiont

Background/Question/Methods

All plant species interact with microbial symbionts throughout their life history. These symbioses can shift interactions between plant host species and their competitors along a gradient between facilitation and competition. Based on the spatial segregation of their associations with plant hosts, microbial symbionts can be split into two groups: aboveground and belowground symbionts. The effects these two groups of microbial symbionts on interactions between plant hosts and their competitors within the same community have not been well-explored. Additionally, few studies have examined how host density interacts with the effects of these symbionts. In this study, we examined how the association of a ecosystem engineering grass species Ammophila breviligulata (hereafter Ammophila) with aboveground and belowground microbial symbionts affected the growth of Ammophila and pair-wise interactions with four dune plant species (Artemisia campestris, Elymus canadensis, Lathyrus japonicus, and Calamovilfa longifolia). In June 2011, we established mesocosms in a dune blowout in northern lower Michigan with three symbiont treatments (endophytic Epichloë sp., old soil inoculum, and control) crossed with five density treatments (spanning 1 to 16 individuals of Ammophila). After three months of growth, we measured the height, calculated the standardized change in growth, and measured survival of target species.

Results/Conclusions

Artemisia campestris showed higher growth in presence of the Ammophila/Epichloë symbiosis than in the treatments without symbionts with a trend of intermediate growth in the soil inoculum treatment. Interestingly, this increase in growth was independent of the density of Ammophila. Due to unforeseen and repeated mortality of L. japonicus and C. longifolia, we terminated experiments for these species a month early. Density of Ammophila increased the survival of C. longifolia, but the symbiont treatments did not affect its survival. While the presence of Epichloë did not seem to have an effect on the growth Ammophila, the asexual reproduction of Ammophila showed significant negative density dependence. There were no significant effects of either the symbiont treatment or density on the growth of E. canadensis or the survival of L. japonicus. These results suggest that density of host plants and associations with symbionts alter intra- and inter-specific interactions. Growth in the mesocosms will be monitored for a second growing season to better understand the effects of density and symbionts on plant community interactions.