Post-fire management in severely-burned Western forests often faces competing objectives, namely: short-term risks of increased surface runoff and erosion caused by post-fire precipitation events over newly-bare soil, versus long-term recovery of native herbaceous vegetation and conifers. The short-term risks often lead to various erosion control measures, including post-fire seeding with herbaceous plants that can quickly germinate and establish to eventually reduce soil loss. However, post-fire seeding applications always intentionally or accidentally contain exotic species. While exotic species may partially address short-term erosion control objectives if they establish within months, they may impede long-term recovery objectives if they persist in the environment and compete with both short-term and long-term native vegetation recovery. We utilize a unique 16-year dataset collected from 49 vegetation transects in the Jemez Mountains of New Mexico following the 1996 Dome Fire, which burned through ponderosa pine-mixed conifer forest, to assess 1) how the diversity and abundance of exotic species change over time in response to seeding treatment and burn severity, 2) which exotic species have transient establishment and which appear to persist long-term, and 3) how the diversity and abundance of exotic species changed following re-burn of the original fire by the 2011 Las Conchas Fire.
Results/Conclusions
A total of 20 exotic plant species were identified over the course of the study. Mean exotic species richness was generally between 2-4 species per 50m transect, with minimal differences detected between treatments in years prior to the Las Conchas Fire. Abundance (percent cover) of exotic species was significantly higher in seeded transects than unseeded transects the first year after the Dome Fire, and generally remained higher for the next eleven years, regardless of burn severity. The dominant exotic species shifted over time however: Lolium multiflorum dominated during the first two years post-Dome Fire in seeded plots only, but disappeared from all transects within twelve years, while Bromus inermis increased significantly from low abundance the first two years post-Dome Fire to significantly higher abundance after twelve years, particularly in seeded transects. Following the Las Conchas re-burn, richness and cover of all exotic species, and cover of Bromus inermis specifically, was higher in seeded than unseeded transects that had previously burned at high severity. Seed mixes consisted primarily of native species and Lolium multiflorum, with Bromus inermis present at low abundance as a contaminant. These findings illustrate some of the ecological risks associated with introduction of exotic plants through post-fire seeding.