Date of Graduation


Document Type


Degree Type



Eberly College of Arts and Sciences



Committee Chair

Richard Thomas.


This study addressed the hypothesis that plants exposed to elevated N availability are more susceptible to drought and insect herbivory. I grew seedlings of gray birch (Betula populifolia), yellow birch (B. alleghaniensis), and red oak (Quercus rubra) for two years under varying levels of N and water availability in the field. Growth and biomass allocation were examined for the three species. Photosynthetic response was determined and feeding trials with gypsy moth larvae were conducted with both birch species. I found that the effects of nitrogen on growth and biomass allocation depended on species, while water had no significant effect on these measures. Both birch species showed increased total biomass, leaf area, and relative growth rate with nitrogen fertilization. Red oak seedlings showed decreased root/shoot ratio and root weight ratio with nitrogen fertilization. These changes could potentially increase transpirational water loss in birch seedlings and reduce water uptake in red oak seedlings, making these species more susceptible to drought. Photosynthetic rate, stomatal conductance, P WUE, PNUE, and photosynthetic capacity of gray birch were greater than yellow birch and these differences were independent of nitrogen and water supply. Gray birch foliage had greater water content, nitrogen concentration, and sugar/condensed tannin ratios than yellow birch. Gray birch also had lower C/N ratios and fewer condensed tannins than yellow birch. In this way, gray birch was a more nutritive food source for gypsy moth and larvae had higher growth rates when feeding on it, compared to yellow birch. When gypsy moth larvae were fed foliage from birch seedlings grown under the different nitrogen and water regimes, larval nutritional indices changed in a nonlinear and unpredictable manner. In summary, this study indicates that the interactive effects of water and elevated N availability on growth and photosynthetic response may impact competitive interactions between species with different life history traits. Furthermore, it is clear that if changes in species composition occur in forests receiving elevated N deposition and variations in rainfall events, they will impact feeding behavior and, thus, defoliation by generalist insect herbivores, like gypsy moth.