Nutrient retention and invasive plant permeability in forested riparian corridors

Williams, Christopher K.D'Amico, VincentForested riparian corridors exist because of inadvertent habitat fragmentation or purposeful establishment to protect water quality. Previous studies have focused on the extent of a buffer required to prevent nutrient (e.g. phosphorus) enrichment of surface waters through runoff. If forested corridors are to be effective nutrient buffers, it is important to understand the patterns of nutrient deposition in buffer soils and the long-term potential for nutrients leaching via shallow groundwater. The first section of this research focuses on patterns of phosphorus and secondary nutrient enrichment and saturation within forested buffers. By testing soil cores taken across the widths of forested riparian corridors bordering active agricultural fields, I found significant declines in phosphorus concentration and saturation levels after 10m. Phosphorus saturation levels were low to moderate, and none of the buffers I sampled showed a high risk of phosphorus leaching into shallow groundwater. I also found significant declines in base saturation and cation exchange capacity, as well as in the concentrations of calcium, manganese, and potassium as distance from the edge of the buffer increased. These changes suggest that the declines in phosphorus and phosphorus saturation may be due in part to changes in nutrient cycling within the buffer and not entirely due to differential rates of phosphorus deposition. Regardless of the cause of the declining phosphorus concentrations, my data suggest that a forested buffer of 10 m is sufficient under most conditions for the protection of surface waters from phosphorus runoff, though wider buffers may be advisable if active management of the buffer is anticipated. Whether resulting from an effort to protect surface waters or as a natural consequence of human land use patterns, fragmented forested riparian corridors may be the only forest habitat available. As such, they are an opportunity to promote biodiversity within managed ecosystems, and a prime target for efforts to promote biotic integrity. Invasion by non-native plants is one threat to biotic integrity and native biodiversity that is of particular concern in fragmented landscapes. The second section of this research examines the role of corridor width in the exclusion of non-native invasive plants. Some invasive plants, being edge specialists or generalists, have been shown to be less competitive in forest interior habitats. By examining the prevalence of several non-native plants across the forested corridors of varying widths, I determined the extent to which increasing buffer width may help prevent invasive plant domination within corridors and especially along stream banks. While Japanese honeysuckle and multi-flora rose did not significantly decline across corridor widths, Oriental bittersweet and garlic mustard showed strong declines after 20 m. These findings suggest that modest increases in riparian corridor width can have a positive impact on the exclusion of some exotic species from forest interiors and may help to improve the biotic integrity of these habitats.University of Delaware, Department of Entomology and Wildlife EcologyM.S