Exotic Plant Species: Drivers and Impacts of Plant Invasions in an Eastern Deciduous Forest Community

Abstract

Humans are moving plants around the globe, both intentionally and accidentally, which may lead to the range expansion of broadly tolerant exotic invasive species. When these exotic species invade and become naturalized in an ecosystem, they can threaten the biodiversity and alter the function of the system, causing economic losses through damages to crops or structures or the loss of native species. In addition to transportation of invasive plants, humans have also caused disturbances to ecosystems that may create opportunities for non-natives at the expense of native species who are unable to adapt to the disturbance. One example of such disturbance is the substantial increase in the range and population density of white-tailed deer (Odocoileus virginianus) over the last century. This overabundant deer population can then have both direct and indirect effects on forest plant species and facilitate the success of invasive species.This dissertation intends to understand the effects of deer on the plant communities and the effects of exotic species on the litter-dwelling communities of a deciduous forest ecosystem in the Shenandoah Valley of Virginia. In Chapter 1 I used paired open and fenced plots to demonstrate that deer had indirect effects when they were present, increasing soil compaction, altering soil nutrient pools, and lowering nitrate fluxes. I found that deer had direct effects when they were present, causing greater herbivory damage resulting in reduced plant height and biomass, lowering the survival of all study species except for the exotic shrub Lonicera maackii, whose survival was not affected by deer herbivory. I found that invasive species became much more common in the herbaceous layer of the plant community when deer were present.iii In Chapter 2 I used a 22 factorial experiment to examine the separate and interactive effects of herbivory pressure from deer and the alteration to the shade environment through deer browsing. I found that deer had little direct (herbivory) or indirect (shade) effects on two native forest herbs (Arisaema triphyllum and Podophyllum peltatum). I found that a common invasive grass (Microstegium vimineum) benefitted from both indirect and direct effects of deer presence, increasing in size, reproduction, and abundance. I found that the invasive herb Alliaria petiolata had a more complicated vi was writing. Through your input I have developed greatly as a scientific writer, which will serve me well throughout my career.I would like to recognize Drs. Howie Epstein, Kyle Haynes, Michael Pace, Paolo D'Odorico, and Laura Galloway for serving as my committee members. Your input and suggestions helped me to develop a strong set of research projects. I have valued your input as I progressed through each step of my graduate studies. I am grateful for your patience while I have worked through the writing process for my dissertation.