Plant Invasion: An Experimental and Mathematical Look at Plant-Soil Feedbacks

This dissertation examines potential impacts and mechanisms of plant invasion. I report results from field studies, greenhouse and growth room studies, and mathematical models. Chapter One describes a field experiment using Fallopia japonica. In this system I explored how Fallopia japonica alters soil N, affects plant diversity, and how clonal connectivity influence invasion. I hypothesized that Fallopia japonica would reduce available N in the soil, and in doing so facilitate its own growth. There was no difference in available N inside and outside of Fallopia japonica patches. However, clonal connectivity did contribute to invasiveness, and invasion reduced plant diversity. My second chapter uses a mathematical model to explore the importance of the relationship between soil community density and soil community impacts on plant growth, a relatively under-reported relationship in the literature on plant-soil feedbacks. Chapter three experimentally explores plant-soil feedbacks generated by Alliaria petiolata and Cynanchum louiseae and the impact of feedbacks on the growth of native species, Acer rubrum, Acer negundo and Asclepias syriaca. I created a gradient of invasive plant-conditioned soils to determine the relationship between soil community density and soil community impacts on plant growth. However, there was no significant negative feedback on native plant growth with invasive-conditioned soil, nor were there any differences in native plant growth due to concentration of invasive-conditioned soil. Finally, chapter four describes competition and growth experiments with the non-invasive Arabidopsis thaliana. Previous research has shown that Arabidopsis accessions generate unique and reproducible rhizosphere bacterial communities. My research asked whether these unique communities have distinct effects on plant growth and competition. I selected four Arabidopsis accessions and grew each as a focal plant in competition with its own accession and with each of the other three accessions. I replicated this in three soil types: sterile, non-sterile, and preconditioned. Rhizosphere communities did not uniquely affect Arabidopsis growth. However, all four Arabidopsis accessions grew larger in sterile soil. This dissertation adds to our knowledge of three invasive plants in New England, Fallopia japonica, Alliaria petiolata, and Cynanchum lousiaea, and more generally discusses how plant-soil feedbacks contribute to invasiveness and community composition