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 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 under nutrient enriched conditions, but are sensitive to drought conditions. We tested the hypothesis that H would be higher in native grasses than in invasive grasses, and be associated with root mass and growth responses, by determining H and measuring plant growth traits in two native (Pascopyrum smithii and Elymus canadensis) and two invasive (Bromus inermis and Agropyron cristatum) grasses. Individuals were grown in sand culture across a range of five N:P ratios and 2 levels of water availability (control and drought) for 6 weeks. Number of leaves, number of dead leaves, and number of tillers were determined at harvest. Total aboveground, belowground, and standing dead biomass were determined after drying and weighing samples.
Results/Conclusions
We find support for the hypothesis that HN is higher in native than invasive plants. Water treatments, water-N:P interactions, and provenance (i.e. “native” or “invasive”)-N:P interactions had significant effects on HN. Drought treated native and invasive plants had higher HN (4.02 and 3.46 respectively) than well-watered plants (3.21 and 2.36 respectively). Total plant biomass was best explained by nutrient treatment, water treatment, and HN, although this model was only marginally better than another containing nutrient treatment, water treatment, and species provenance. Root mass fraction (RMF) was not explained by provenance but best explained by treatment N:P. The lack of significant differences in RMF between native and invasive and well-watered and drought treated plants does not support the hypothesis that root extensivity explains higher H plants’ ability to withstand drought. Number of leaves and tillers and total aboveground, belowground, and standing dead biomass were negatively correlated with HN in well-watered plants but were positively correlated with HN in drought treated plants. Our results show higher HN in native plants, and that the expression of HN is mediated by environmental conditions as well as correlated with growth traits.