Plant invasions can alter ecosystems past a threshold, resulting in a state that is difficult to reverse or restore, yet the mechanisms causing threshold dynamics are not well understood in most ecosystems. In addition, threshold dynamics of invasion are likely to be influenced by other elements of global change. During a period of significant drought in California, we examined how the amount and the spatial distribution of nitrogen (N) deposition influenced threshold dynamics of invasive, annual grasses on the decline of the native, perennial grass Stipa pulchra. We hypothesized that invasion (and the decline of native populations) would be rapid once the abundance of the invasive species reaches a critical threshold (Hypothesis 1), and that high nitrogen availability would reduce the critical invasive abundance threshold (Hypothesis 2). We also hypothesized that in heterogeneous resource environments, native species would escape competition in low N patches, thus increasing the critical invasive abundance threshold in heterogeneous, compared to homogeneous, environments (Hypothesis 3). We tested these hypotheses in a controlled field experiment from 2012-2014 by manipulating the amount and spatial heterogeneity of nitrogen availability across a range of invasive/native – dominated grassland communities.
Invasion by Bromus spp. led to significant declines in the abundance of Stipa pulchra (Hypothesis 1), with steeper declines in plots with N addition; supporting that N deposition reduced the invasion threshold (Hypothesis 2). Contrary to predictions, invasion thresholds were the lowest in heterogeneous N treatments (Hypothesis 3); which may occur because the high N patches in heterogeneous treatments may promote the growth of Bromus diandrus. In addition, soil moisture was reduced by invasion and by the addition of N, suggesting that competition for water was likely a driver of the significant, rapid decline of S. pulchra. Thus, the interactive effects of N deposition and drought conditions may lead to a potential for invasive species to expand their ranges, and also for native species to decline due to the reduction in critical invasion thresholds.