Nutrient availability does not explain invasion and dominance of a mixed grass prairie by the exotic forb Centaurea diffusa Lam.

Abstract

Diffuse knapweed ( Centaurea diffusa) is a noxious non-native forb that is currently invading and dominating western North American grasslands. We established a series of fertilization, reverse fertilization, and removal experiments in grasslands east of the Colorado Front Range, USA, to examine the response of C. diffusa and the rest of the grassland community to changes in nutrient availability. We asked whether changes in the availability of nitrogen, phosphorus or both affect weedy non-native species, and C. diffusa in particular, differently from the native species; and whether C. diffusa appears to control resource availability to a greater degree than native species. Neither addition nor reduction of available nitrogen, phosphorus, or both caused consistent changes in C. diffusa seed germination, rosette or mature plant density, biomass, seed head production, or seed production. Removal of C. diffusa from the community resulted in no significant change in rates of nitrogen cycling or pools of available nitrogen or phosphorus. Removal of C. diffusa had little effect on the relative cover or species richness of the remaining community, but removal of the remaining community caused a large increase in C. diffusa biomass. The results suggest that C. diffusa requires relaxation or removal of competition to invade grasslands, that it is tolerant of a wide range of nutrient availability, and that once established, it functions similarly to the native prairie grasses. Heterogeneity in cover and competitive interactions, generally thought to enhance species coexistence and overall diversity, also provide increased opportunity for invasion.