Post-fire seeding with ryegrass: implications for understory plant communities and overall effectiveness

Seeding following high-severity wildfires is motivated by the goals of increasing vegetative cover and decreasing bare soil in order to minimise soil erosion and exotic plant invasions. We compared the ground cover and vegetation response of seeded versus non-seeded areas located in the Warm Fire in northern Arizona, where post-fire seeding treatments with Italian ryegrass (Lolium perenne spp. multiflorum (L.)) were conducted in 4000 ha of high-severity burned areas. Over the course of the study, we observed no significant difference between seeded and non-seeded plots in percentage of bare soil, total vegetative cover or exotic plant cover. However, there were significant differences in plant community composition as revealed by PERMANOVA and Indicator Species Analysis. Two years post-fire there were significantly fewer ponderosa pine seedlings, and the cover of annual and biennial forbs was significantly lower in plots that were seeded with Italian ryegrass. In the third year, the cover of native bunch grasses was significantly lower in seeded plots. The differences we observed may be due to differences in pre-existing vegetation composition because of the geographic separation of the plots across the landscape. Our results illustrate the ineffectiveness of post-fire seeding in achieving the goals of increasing vegetative cover and decreasing the invasion of non-native plants, and we suggest that alternative post-fire remediation should be considered in the future.