Date of Graduation


Document Type


Degree Type



Eberly College of Arts and Sciences


Sociology and Anthropology

Committee Chair

William Peterjohn

Committee Co-Chair

Mary Beth Adams

Committee Member

James McGraw

Committee Member

Brenden McNeil

Committee Member

Richard Thomas


The effects of nitrogen (N) deposition have been studied in many ecosystems, and one general pattern of response that has emerged from these studies is a decline in species richness. Globally, anthropogenic deposition of N has more than doubled the ambient input of N in terrestrial ecosystems, and the rate at which humans produce reactive N continues to increase annually. As a result of both the current and projected future rates of N input, N deposition is a threat to biodiversity worldwide and is projected to contribute, along with other global change factors, to species extinction. To prevent the loss of biodiversity from N deposition, it is critical to understand the mechanisms by which N deposition causes species loss. In this dissertation I examined some of the direct and indirect mechanisms by which N can influence species richness in the forest herbaceous layer. In Chapter 1 I outline the effects of N on the herbaceous layer and introduce the long-term fertilization studies at the Fernow Experimental Forest that I used to study these effects. In Chapter 2, I tested the rigor of an undocumented method for estimating cover in the forest herbaceous layer. The method was very precise and, when calibrated, potentially accurate for comparison of cover among sites. In Chapter 3, I investigated if the interaction between N fertilization and light led to the dominance of Rubus spp. in stands of an aggrading forest. Results indicated that added N significantly enhanced the cover of Rubus spp. in the forest herbaceous layer only at intermediate and high light levels. In Chapter 4, I tested the contribution of two hypothesized mechanisms for species losses (non-random and random species loss) in N-fertilized, N-fertilized and limed, and unfertilized plots. Both mechanisms influenced species richness, with non-random loss becoming the main mechanism over time. I observed that advantages were conferred to some nitrophilic species---particularly Rubus spp.---whereas disadvantages were observed in non-nitrophilic species. Additionally, my results indicate that the effect of N on some herbaceous species could be mitigated with the addition of lime. In Chapter 5 , I examined a potential indirect effect of N fertilization on the herbaceous layer by investigating differences in storm damage experienced by trees growing in N-fertilized, N-fertilized and limed, and unfertilized plots. Trees growing in N-fertilized plots were more susceptible to damage from a wind-storm and the extent of damage depended on the species of tree. There was also evidence that the addition of lime could mitigate the susceptibility of trees to storm damage in N-fertilized plots. As a result of this study, I hypothesized in Chapter 6 that differential damage to trees among fertilized and unfertilized treatments could result in differential changes in the herbaceous layer, either by increasing the frequency or size of canopy gaps, or by altering the species composition of trees. Overall, this research offers strong support for the idea that both direct and indirect mechanisms will influence species richness in the forest herbaceous layer under the projected increases in global N deposition.