Co-assembly of plant and fungal root endophytes determine plant diversity and exotic plant success

Background/Question/Methods

Assembly history can be a major factor determining the structure and function of communities. Most assembly experiments manipulate introduction order within a single trophic level, not across multiple levels. For example, plant introduction order is often manipulated to understand plant community assembly, but without also manipulating introduction of other groups like soil microbes. This knowledge gap is significant given the increasing importance of plant-soil feedbacks for understanding plant succession and for successfully implementing ecological restoration. Restoring native plant communities has proven difficult, particularly at disturbed sites dominated by exotic plants. We manipulated introduction order of plant species and root fungal endophytes in a factorial greenhouse experiment to determine if plant and endophyte assembly histories interact to 1) affect plant diversity and productivity or 2) alter the success of exotic versus native plant species. We also measured fungal communities to help explain how interactive effects may have occurred. Plants and fungi were isolated from undisturbed FL rosemary scrub, disturbed scrub, and pastures sites that are restoration targets. One of three C4 grass species (Aristida gyrans, Schizachyrium niveum, and the exotic Rhynchelytrum repens) as well as fungi from one of the three site types were introduced two weeks before all others.

Results/Conclusions

After 6 months, plant community diversity was significantly lower when either the exotic plant, R. repens, was introduced first or fungi isolated from disturbed sites were introduced first. Plant productivity did not vary with fungal introduction order, but was significantly lower when R. repens was introduced first. When introduced first, R. repens germinated poorly except if native fungi were also introduced first and still grew poorly in those cases. Similarly, the native plant S. niveum also germinated poorly when introduced first with native endophytes, but not with other fungi or when introduced second. Both results indicate negative feedback when either plant was introduced first with fungi from sites where it dominates, and that a two week “head-start” by another plant can ameliorate these effects. In contrast, A. gyrans germinated poorly when introduced first regardless of fungal introduction order. Fungal community differences among introductions were marginally significant, but explained less than 7% of variation. These data demonstrate that plant and fungal introduction order can independently or interactively affect plant community diversity and exotic success. Our results suggest that within target restorations, exotic plants may generate negative feedback through fungi, but native plants can rapidly ameliorate these effects possibly leading to re-invasion.