Testing how plant competition alters plant mediated interactions

Background/Question/Methods

A plant’s success is contingent on the environmental conditions within which is embedded, but also interactions with neighboring plants and a suite of species above- and belowground. Neighbor interactions in the context of community assembly is of growing interest, as the relative effect of neighbor origin and density on multispecies above-belowground interactions is unclear. Understanding changes to plant-mediated interactions has both applied and theoretical value. We designed a common garden experiment to test how two plant neighbors differ in their effect on a focal plant’s interactions with associated species and symbionts, using Solidago altissima as the focal species, planted with itself, the native Cirsium discolor, the naturalized Achillea millefolium, and with both species. From stepwise manipulations of the focal plant’s neighbor, soil microbial community and access to pollinators, we measured changes in plant leaf and floral traits, herbivory, and pollinator diversity throughout the growing season. At the end of the growing season, we collected fine roots to identify changes in AM fungal colonization rates based on neighbor interactions, and soil samples and elucidate shifts in overall bacterial and fungal community assemblages using next generation sequencing; and harvested aboveground biomass.

Results/Conclusions

These data show that S. altissima’s performance is differently altered based on plant neighbor, and to some extent by its initial soil inoculum origin (S. altissima field soil vs wild soil). Specifically, we found changes to growth strategies, whereby under higher competition scenarios, S. altissima exhibits higher trait variability, whereas under lower competition, it has a more constrained trait response. These changes to resource allocation have had subsequent effects on S. altissima’s associated communities aboveground. Rates of herbivory on S. altissima changed significantly by neighbor, though not by soil origin, and correlated with leaf dry matter content. Furthermore, S. altissima had significantly higher abundance of pollinators when planted with itself compared to other neighbors, showing that S. altissima experienced resource trade-offs under competition, whereby it flowered earlier when planted with itself, and therefore bloomed for a longer period. Significant belowground shifts also occurred based on competition, and we expect these to be indirectly related to aboveground alterations. Our results show that even within a single growing season, significant changes to plant allocation and subsequent biotic interacts can occur based on plant neighbor competition. Furthermore, this research highlights the importance of understanding how indirect interactions can mediate or competitive interactions.