What’s so positive about positive feedback? Leveraging nucleation dynamics to enhance restoration in tallgrass prairie systems

Theo K. Michaels, Benjamin A. Sikes and James D. Bever, Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS, Theo K. Michaels, Benjamin A. Sikes and James D. Bever, Kansas Biological Survey, University of Kansas, Lawrence, KS

Background/Question/Methods
Leveraging nucleation theory for restoration requires identifying local scale positive feedback mechanisms and the patch size needed to generate a nucleation event. Tallgrass prairies are a model system for testing the principals of nucleation. Evidence suggests that these systems exist in monotypic patches of early or later successional stages whose transition depends on positive plant-soil feedback generated between plants and beneficial arbuscular mycorrhizal (AM) fungi. Consistent with this dynamic, following large-scale agricultural disturbance, degraded grasslands can be resistant to the reestablishment of late-successional native plant species because of the disruption of these mutualisms. In this study we asked if late successional prairie patches could generate the feedback dynamics necessary for nucleation. We transplanted prairie monoliths of small, medium and large size patches into three recipient sites: a recently disked disturbed field, a non-native dominated old field, and a restored prairie. To detect the presence and spread of local positive feedback we planted sterile test plants of different successional status and AM fungal dependence: Rudbeckia hirta (early succession) and Liatris pycnostachya (late succession). Test plants were planted inside, adjacent to, and four meters from the monoliths. Survival and fitness measures were collected over three years from 2018-2020.
Results/Conclusions
R. hirta survival was greatest in disturbed and old field sites (p = 0.004), and was virtually absent in the restored prairie by 2019. In the other sites, R. hirta survival (p = 0.014) and flowering (p = 0.005) increased with distance from the monoliths. These results support expectations that early successional species are poor competitors in late successional systems but can thrive in disturbed sites. In contrast, larger patch sizes significantly increased 2018 establishment of L. pycnostachya (p = 0.046). In 2019, survival increased with distance from the monoliths in disturbed and old field sites, but decreased with distance in the restored prairie (p = 0.004). By 2020, however, L. pycnostachya survival was highest adjacent to the monoliths, and decreased away from the monoliths (p = 0.046). Net probability of bolting also increased when adjacent to the monoliths (p = 0.0356). These results are consistent with local positive feedbacks at the patch edge, where L. pycnostachya can benefit from beneficial AM fungi in the absence of competition or pathogens found within the monoliths. Combined, our results are consistent with local positive plant-soil feedbacks driving nucleation dynamics that accelerate transitions between alternative stable states in tallgrass prairie successional systems.