Stoichiometric Homeostasis in Two Native and Two Invasive South Dakotan Grasses

Increased nutrient availability has been widely linked to the success of invasive plants, however a general mechanism explaining these observations is lacking. Stoichiometric homeostasis (H), which is the regulation of internal nutrient concentrations, has been used to explain changes in plant community diversity under alterations in nutrient availability. One hypothesis holds that plants with high regulation (larger H) decrease in abundance in nutrient enriched conditions but are stable in nutrient deficient and drought conditions, likely due to extensive root systems. Additionally, plants with low regulation (lower H) increase in abundance under nutrient enriched conditions but are sensitive to drought conditions. I tested the hypotheses that H would be higher in native grasses than in invasive grasses, that H would be modulated by environmental conditions, and that differences in H would be associated with differences in growth and biomass allocation. I calculated H and measured plant growth and growth traits in two native (Pascopyrum smithii and Elymus canadensis) and two invasive (Bromus inermis and Agropyron cristatum) grasses grown in two experiments. Both experiments contained a range of N:P fertilizer supply concentrations and the first experiment contained a two-level drought treatment while the second experiment contained a two-level mycorrhizal inoculation treatment. In the first experiment, I found support for the hypothesis that H is higher in native than invasive plants, that environmental conditions (i.e. water availability) affect the value of H, and that differences in H were associated with differences in growth. In the second experiment, there was no successful mycorrhizal inoculation, resulting in no differences in H between mycorrhizal treatment groups. There were significant differences in total growth between the second experiment native and invasive grasses, despite there being no significant differences in H. Differences in H values between control-treated grasses in the two experiments may be due to differences in greenhouse temperature and light conditions. These results show first, significant differences exist in H between invasive and native grasses, with invasive grasses expressing lower values of H, second, environmental conditions effect the expression of H, and third, that differences in the expression of H are matched by differences in growth