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

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.

In Chapter 2 I used a 2 x 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 response, with greater recruitment when unshaded and in the presence of deer, but had lower survival, growth, and reproduction when deer were present.

To evaluate the role of deer as endozoochorous seed dispersers, I collected scat from three locations in the landscape of the Blue Ridge and Shenandoah Valley. I found that deer were transporting seeds from many different taxa via endozoochory, but very few of these seeds germinated. Of the seeds identified in the samples of deer scat, the typical invasive seeds were from plants with small, hard seeds and non-fleshy fruits, while the typical native seeds were from fleshy-fruited plants. The results of the germination trials suggest that, instead of being seed dispersers, deer are acting more as seed predators and are not benefitting many of the species whose seeds they consume.

Finally, I tested for the effects of leaf litter from three invasive plants (Ailanthus altissima, Lonicera maackii, and Rhamnus davurica) on the detrital-layer food web. I found that invasive litter was more nutritious, but decomposed much faster than native litter. I found that bacteria, fungi, and arthropods all preferred litter from the invasive species, suggesting that these exotic invasive species are beneficial, novel resources for the litter-dwelling community. However, due to the high rates of decomposition, this resource is short-lived and native leaf litter may provide the most stable resource for this food web.

Taken together, the results of my dissertation indicate that deer are having significant impacts on this forest system. By altering the physical environment, deer are creating opportunities for invasive species to grow and reproduce. Native species are unable to take advantage of these disturbances. The effects of deer herbivory also appear to create advantages for invasive species that are either more tolerant of herbivory than native species or that benefit from the removal of preferred native competitors. Once invasive species are established, they may have strong effects on the food webs of this forest system. These invasive species may create a novel food web in this forest that is characterized by cycles of high abundance of litter-dwelling organisms in the spring that become very low in abundance in the summer. These cycles can then affect the higher trophic levels of the system, forcing them to find uninvaded habitats late in the growing season to sustain their populations.