Professor Cornell University, New York, United States
While several studies have been published suggesting that plant soil feedbacks (PSFs) are predictive of species-abundance patterns, the overall role of PSFs in affecting coexistence dynamics between plants and their interactors remains unclear. Moreover, a number of recent studies suggest that PSFs differ between greenhouse and field experiments, casting doubt on the extrapolation of results from simplified greenhouse experiments to explain broad ecological patterns. A meta-analysis of PSF studies found that PSFs can have additive or even synergistic effects with competition, possibly doubling the effect of competition measured in sterile soil. On average, stronger competitors appear to experience more negative feedbacks than weaker competitors, suggesting that PSFs could act as a stabilizing factor promoting coexistence (and increasing local biodiversity). We conducted a competition experiment in the greenhouse to investigate how successional changes in the soil microbiome affect the outcome of competition between tall goldenrod (Solidago altissima) and several commonly co-occurring species (Symphyotrichum spp). Plants were inoculated with soil from our long-term successional chronosequence experiment in Brooktondale, New York. We hypothesized that successional shifts in PSFs would negatively impact the competitive ability of S. altissima, acting as a stabilizing mechanism.
We found that both early and late successional soil microbe inoculants had negative effects on plant performance (as compared with a sterilized control) while successional changes (from early to late succession) were generally neutral or slightly positive. However, these effects were asymmetrical between species such that soil inoculation generally favored the three competitively subordinate Symphyotrichum species over tall goldenrod. This effect was especially pronounced for Sy. novae-angliae, which actually overtook goldenrod as the strongest competitor when grown in the late successional soil background. Unlike the other species, Sy. novae-angliae did not show reduced growth in either microbial inoculation treatment when grown alone, demonstrating an unusual insensitivity to PSFs. Thus, successional changes in the soil microbial community appear to act as a stabilizing mechanism by reducing the relative competitive ability of tall goldenrod. Our results are broadly consistent with prior studies showing that mid- to late-successional species perform better in late-successional soil as compared with early-successional soil and that dominant competitors are more likely to suffer disproportionately from negative PSFs. Our results also reinforce prior work suggesting that it is important to consider the interactive effects of PSFs with other stressors such as competition to understand how PSFs shape ecological communities.