A WashU Dissertation by: Rachel Ellen Bucknell (Dept. of Biology & Biomedical Sciences)
Abstract: We are currently in the midst of an ecological crisis with many species and ecosystems threatened with extinction. Less than 1% of the original extent of the incredibly biodiverse tallgrass prairie ecosystem remains and other grassland ecosystems such as the limestone cedar glade complex that is home to many endemic plant species are threatened by development, woody plant encroachment, and trash dumping. Restoration practitioners have been acting to restore these degraded ecosystems for over a century and rare plant reintroductions have steadily increased over the past few decades, yet many plant restorations and reintroductions are often not successful. Prairie restorations are often significantly less biodiverse than remnant prairie communities and attempts at reintroduction of rare plant populations often fail to create sustainable, reproductive populations. Restoration ecologists have recently posited that restoration and reintroduction sites may be lacking in essential pathogenic and beneficial microbial communities necessary for establishing plant species and allowing for plant coexistence. Recent research has demonstrated that late-successional plant species typically only found in remnant prairie communities are dependent on specific mycorrhizal fungal species that increase their ability to uptake nutrients and provide other benefits such as drought and pathogen resistance. Thus, the use of mycorrhizal inoculum in restorations is increasing. Furthermore, greenhouse studies testing the impacts of soil microbes on plant species have become common, however these studies are rarely performed in a way that allows for a true test of coexistence. The studies also rarely take into account the role of plant competition and plant phylogenetic relatedness in the structuring of plant communities. I had three major goals for my dissertation. My first goal was to gain a better understanding of how soil microbes affect tallgrass prairie plant species and how microbes and plant phylogenetic relatedness could impact plant community coexistence. I achieved this through performing a fully reciprocal plant-soil feedback experiment on 18 tallgrass prairie plant species that ranged in relatedness from closely to distantly related. I found that negative plant-soil feedbacks are common among tallgrass prairie plant species, and that those plant species pairs that were more distantly related experienced stronger coexistence-inducing negative plant-soil feedbacks. I also performed DNA metabarcoding on soil from all 18 species and found that soil fungal communities differed significantly among plant species, a prerequisite for plant-soil feedbacks to affect coexistence among plant species. My second goal was to move my study of the impacts of soil microbial communities beyond the greenhouse to an outdoor research garden experiment. This experiment was structured with background plant communities that ranged from closely related to distantly related and tested the impacts of soil microbial communities from a remnant prairie and an old field, similar to where prairie restorations typically occur. I tested how plant phylogenetic relatedness and soil microbial community origin affected the establishment of four hard-to-establish tallgrass prairie plant species. I found that the hard-to-establish Asclepias sullivantii grew significantly larger when in plant communities more distantly related to itself, while the hard-to-establish Antennaria neglecta was not affected by plant community relatedness but grew larger when the background community was smaller. I found no soil effects in the first two years of the experiment on either species. The other two species included in my experiment, Melanthium virginicum and Sporobolus heterolepis have not established in high enough numbers within the first two years of my experiment to enable analysis of establishment. My final goal was to determine whether the federally endangered Astragalus bibullatus, a rare legume known only to exist in a few glade sites in central Tennessee, is limited in reintroductions by a lack of essential mutualistic soil microbes such as arbuscular mycorrhizal fungi or rhizobial bacteria. I performed a greenhouse experiment testing the impacts of soil microbial communities from five sites where A. bibullatus is historically present and four potential reintroduction sites where the species is historically absent. I found that A. bibullatus grew significantly larger and developed significantly more root nodules when in soil microbial communities from historically present sites compared to soil microbial communities from historically absent sites. These results indicate that the reintroduction sites are likely lacking in these essential rhizobial bacteria that increase the species’ ability to uptake nitrogen from the soil and confer other benefits such as drought resistance. Overall, all aspects of my dissertation research illustrate the need to move beyond the traditional focus of simply increasing plant diversity in restorations, to include restoring key microbial taxa missing at these sites as well as structuring plant communities based on plant phylogenetic relatedness to increase plant community coexistence, either through plant-soil feedbacks or plant competition.
Citation or DOI: Becknell, Rachel Ellen, “The Role of Soil Microbes in Improving Tallgrass Prairie Restoration and Rare Plant Reintroduction” (2021). Arts & Sciences Electronic Theses and Dissertations. 2396. https://doi.org/10.7936/vjex-7366.