Hill prairies are small prairie remnants on the bluffs of large rivers in the Midwestern U.S. These ecosystems are endangered due to encroachment of woody vegetation from the surrounding forests. We have previously reported that encroachment by shrubs and trees (primarily Cornus drummondii and Juniperus virginiana) alters the microbial community composition of hill prairie soils by replacing prairie-favoring microbes with microbes more typically found in the forest. While plant-microbe interactions can be important drivers of plant community change, we do not know if shrub-induced changes to hill prairie microbial communities play a role in further woody encroachment through “plant-soil feedback” loops. To investigate this, we collected soil from open areas of hill prairies and from areas under heavy encroachment by C. drummondii, and we used this soil as live or steam-sterilized inoculum in a series of greenhouse trials involving C. drummondii, J. virginiana, and eight different native prairie plants. We quantified the effect of soil microbes by comparing the growth of each plant in live vs. sterilized soil, and we compared the effect of native prairie microbes with shrub-associated microbes to understand the role of microbial communities in shrub encroachment.
In general, plants benefited from the presence of live soil microbes, although three plant species (Bouteloua curtipendula, Kuhnia eupatorioides, and Monarda fistulosa) grew better in sterilized soil than in soil containing live microbes. With only two exceptions (Andropogon gerardii and Lespedeza capitata) we found that native prairie plants had a better response to soil microbes originating from shrub-encroached soils than from those originating from open prairie soils. In other words, most prairie plants received a greater benefit (or less harm) from shrub-associated microbes than from microbes that predominate in pre-encroachment soils. Interestingly, we also found that C. drummondii received a much larger growth benefit from microbes found in pre-encroachment soils than from shrub-associated microbes. Taken together, these patterns suggest that the plant-soil system in hill prairies is characterized by “negative feedback:” plants receive greater benefits from soil microbes present in other portions of the prairie than from microbes present in their “home” areas. This negative feedback could help accelerate C. drummondii encroachment into open areas of the prairies, but it might also make shrub-encroached areas vulnerable to re-establishment of native prairie vegetation.