The Role of Tree Species, the Herb Layer, and Watershed Characteristics on Nitrogen Cycling in a Central Appalachian Hardwood Forest
Date of Graduation
Davis College of Agriculture, Natural Resources and Design
Division of Forestry and Natural Resources
Mary Beth Adams
Little is known about the forest herbaceous layer’s contributions to nitrogen (N) cycling under varying levels of N deposition, at different stages of forest succession, in watersheds with different aspects, and under different tree species. The objectives of this study were to quantify nitrogen reductase activity (NRA) of two tree species, red maple and sugar maple, and surrounding common herb-layer species at the tissue (foliage, roots) and plot level and to assess how these species affect the concentrations of nitrate, ammonium, and organic N in soil water collected in lysimeters beneath these trees. Different watersheds located at the Fernow Experimental Forest in West Virginia were used in this study: a 50-year-old stand experimentally treated with N fertilizer (WS3), a 50-year old stand left to regrow after herbicide applications (WS7), a 110-year-old stand with a northeast aspect (WS13), and a 110-year-old stand with a south aspect (WS10). This study demonstrated that maple NRA does not respond to additional fertilizer or differ between stands of different ages or different watershed aspects. Some herb-layer species’ NRA, however, did increase with nitrogen fertilization, but did not vary with stand age or overstory tree species. This indicates that the herb-layer may be an important sink for nitrogen. Spring NRA was higher than summer NRA for herb-layer species, indicating that those species may be taking advantage of high light availability before trees leaf out. Root NRA responded to fertilization whereas foliar NRA did not for certain species. Root NRA may then be a more reliable indicator of increased nitrate processing in trees and herb-layer plants. NRA expressed per unit of area (NRAA) of the herb-layer contributed 9 to 41% of total (tree plus herb-layer) foliar NRAA, despite the herb-layer making up only a small proportion of foliar biomass in a forest. Stand age, but not overstory tree species or fertilization, influenced nitrate soil water concentrations. The younger stand in this study had higher soil water nitrate concentrations than the older stand which may be related to a larger contribution of arbuscular mycorrhizal tree species to total basal area in the younger than the older stand. The unexpected lack of a fertilization effect on soil nitrate may have been due to low sample size. This project contributed new knowledge about the herbaceous layer’s role in nitrate uptake in a mixed hardwood forest. While most herb-layer plants studied seem to prefer ammonium, the herb layer contributes to nitrate assimilation disproportionally to its biomass in the forest and provides a vernal dam to nitrate loss not only by its early presence but also by increased spring NRA relative to summer. Playing an important role in forest N cycling, the herb-layer should be considered when predicting forest growth responses to increased CO2 concentrations and other perturbations.
Eisenhut, Sian E., "The Role of Tree Species, the Herb Layer, and Watershed Characteristics on Nitrogen Cycling in a Central Appalachian Hardwood Forest" (2021). Graduate Theses, Dissertations, and Problem Reports. 8223.