The dynamic consequences of invasion: negative plant-soil feedbacks on natives increase over the time course of invasion

Background/Question/Methods   Exotic species can negatively impact native community members, directly through interference competition, or indirectly by altering interactions between native species and other species such as pathogens and mutualists. Soil microbial communities have been shown to respond to invasive species, yet are relatively stable and may take time to respond to perturbations. For this reason, microbe-mediated effects of invasives on natives may take time to develop and change throughout the invasion process. Few studies have investigated how species interactions between natives and exotics change during invasions. Acer platanoides was introduced into the US from Europe and has since invaded intact forests, lowering understory diversity and inhibiting native tree species regeneration. We hypothesize that A. platanoides invasion will decrease seedling survival in the native A. saccharum by changing the soil microbial community through plant-soil feedbacks (PSF), and this relationship will intensify over the course of an invasion. We collected soil samples from beneath the canopies of both species co-occurring in Michigan forests that had been invaded by A. platanoides for varying time periods. In the greenhouse, we inoculated seedlings of both species with the microbial communities to determine how the soil community affects seedling survival and growth.

Results/Conclusions   The microbe-mediated negative effects of A. platanoides on survival of the native congener A. saccharum increased with increasing invasion age (negative correlation between invasion age and A. saccharum survival; r = -0.806, R2 = 0.65, p = 0.032). This result was not related to A. platanoides density, suggesting that age of invasion drives this pattern. Contrastingly, invasion age did not significantly influence survival of A. platanoides seedlings, indicating that A. platanoides' PSF impacts native species but not on conspecific regeneration. Overall, A. platanoides seedlings had increased growth (more and larger leaves) when grown in soil communities collected from the native A. saccharum, but A. saccharum seedlings had reduced growth when grown in conspecific soil (p = 0.044). These results suggest that invasive species may have increased performance in exotic ranges by their ability to modify the soil microbial community in a manner that suppresses the growth of native species. We show the microbial community cultivated by an invader alters the performance of a native plant species, and this effect increases over the course of an invasion. In future work, we plan to identify changes in microbial community composition and the relative abundances of mutualists versus antagonists in response to invasion to identify potential mechanisms.