Exploring links between Bromus invasion, anthropogenic nitrogen enrichment, and wildfires using systematic review and meta-analysis
Introduced plant species have changed the structure, diversity, and fuel characteristics of natural communities in fire-prone ecosystems of the American West. The factors that have allowed these introduced species to become prevalent are complex, but increased anthropogenic impact in the form of habitat fragmentation and nutrient pollution including nitrogen (N) deposition have been suggested as contributing to these invasions. We hypothesize that anthropogenic N addition to natural systems may contribute to the success of invasive species, which in turn contributes to increased wildfires in the West. We systematically collected peer-reviewed studies published between 1995 and 2014 to address 1) how specific quantities of N enrichment alter the productivity and competitive interactions of invasive Bromus spp. (hereafter referred to as Bromus) and 2) how Bromus responds to and alters fire regimes. We used meta-analytic techniques to compare data across studies and test our hypothesis using data from the post-1995 literature, and are currently expanding our literature search to include papers published since 1980.
Based on our study inclusion criteria, 13 papers published post-1995 addressed how nitrogen alters Bromus productivity or competition. Across studies, N enrichment significantly increased Bromus productivity by 70%. There was significant heterogeneity in this response across studies, which did not correlate with the quantity of N added (p=0.6), indicating that the degree of N enrichment was not as important as were other environmental variables in determining Bromus response, e.g. precipitation. We were unable to make generalizations about how N enrichment alters competitive interactions, since study design explained 95% of the heterogeneity in effect sizes across studies.
We collected 16 papers that address the feedback between Bromus and fire. Bromus presence intensified fire regimes compared with native vegetation in 10 out of 14 effect sizes, although the lack of variance values in published data made generalization inadvisable. In conclusion, initial synthesis of the post-1995 literature suggests that anthropogenic N enhancement of invasive Bromus production may contribute to increasing fire intensity in the American West.
The views expressed in this abstract are those of the authors and do not necessarily represent the views or policies of the U.S. EPA.