THE IMPACTS OF TERRESTRIAL INVASIVE PLANTS ON STREAMS AND NATURAL AND RESTORED RIPARIAN FORESTS IN NORTHERN NEW ENGLAND

Non-native invasive species are a major cause of ecosystem degradation and impairment of ecosystem service benefits in the United States. Riparian areas are at high risk for invasion because they are among the most human-disturbed ecosystems in the world. Forested riparian areas provide us with many ecosystem services and are vital to streams and rivers as they increase habitat complexity and available resources for organisms of many trophic levels. In this study, I quantified the impacts of terrestrial invasive plant invasions by Japanese knotweed and woody invasive plant species on riparian forest structure, stream physical habitat, soil structure, and soil functioning in northern New Hampshire. In addition, I assessed the effects of restoring native trees to disturbed riparian sites and their ability to resist invasive plants in central Vermont. Invaded plots had greater stems per hectare but were associated with reduced basal area of native trees. Invaded plots consistently had greater canopy closure upland from the stream (5m from the stream edge) but provided less shade at the stream channel with larger open canopy angles, therefore increasing the amount of solar radiation entering the stream. Native tree sapling densities were generally reduced in invaded sites when compared to non-invaded sites. Significantly less organic material was available at invaded sites, with less course woody debris (CWD) and less litter and duff on the forest floor. Invaded plots also had greater amounts of bare exposed mineral soil and higher amounts of embedded stream substratum. When comparing planted vs. non-planted riparian sites in Vermont, we found that non-planted sites had three times the amount of invasive plants and 43% greater stem density. The results of this study may assist conservation efforts of riparian forests to further understand the distribution of invasive plants and how to minimize the risk of invasion. This study will also provide insight on what ecosystem functions may be altered by invasive plants and may need to be restored