The effects of changes in water and nitrogen availability on alien plant invasion into a stand of a native grassland species

Plant invasions are a major component of global change, but they may be affected by other global change components. Here we used a mesocosm-pot experiment to test whether high water availability, nitrogen (N) enrichment and their interaction promote performance of three invasive alien plants (Lepidium virginicum, Lolium perenne and Medicago sativa) when competing with a native Chinese grassland species (Agropyron cristatum). Single plants of the three invasive and the one native species were grown in the center of pots with a matrix of the native A. cristatum under low, intermediate or high water availability and low or high N availability. The invasive species L. virginicum and M. sativa grew larger, and produced a higher biomass relative to competitors than the native species A. cristatum did. Increasing water availability promoted biomass production of all species, but water availability did not change the biomass of the central plants relative to that of the competitors. Nitrogen addition also increased biomass production of all species, and it increased the biomass of the central plants more so than that of the competitors. The positive effect of N addition on the biomass of the central plants relative to that of the competitors increased with increasing water availability. However, compared to central plants of the native species, the positive effect of N addition on the relative biomass of L. virginicum decreased when water availability increased. These interactions indicate that future changes in water availability and N enrichment may affect the invasion success of different alien species differently.