Pre-fire fuel reduction treatments influence plant communities and exotic species 9 years after a large wildfire

Questions: How did post-wildfire understorey plant community response, including exotic species response, differ between pre-fire treated areas that were less severely burned, and pre-fire untreated areas that were more severely burned? Were these differences consistent through time? Location: East-central Arizona, southwestern US. Methods: We used a multi-year data set from the 2002 Rodeo-Chediski Fire to detect post-fire trends in plant community response in burned ponderosa pine forests. Within the burn perimeter, we examined the effects of pre-fire fuels treatments on post-fire vegetation by comparing paired treated and untreated sites on the Apache-Sitgreaves National Forest. We sampled these paired sites in 2004, 2005 and 2011. Results: There were significant differences in pre-fire treated and untreated plant communities by species composition and abundance in 2004 and 2005, but these communities were beginning to converge in 2011. Total understorey plant cover was significantly higher in untreated areas for all 3 yr. Plant cover generally increased between 2004 and 2005 and markedly decreased in 2011, with the exception of shrub cover, which steadily increased through time. The sharp decrease in forb and graminoid cover in 2011 is likely related to drought conditions since the fire. Annual/biennial forb and graminoid cover decreased relative to perennial cover through time, consistent with the initial floristics hypothesis. Exotic plant response was highly variable and not limited to the immediate post-fire, annual/biennial community. Despite low overall exotic forb and graminoid cover for all years (<2.5%), several exotic species increased in frequency, and the relative proportion of exotic to native cover increased through time. Conclusions: Pre-treatment fuel reduction treatments helped maintain foundation overstorey species and associated native plant communities following this large wildfire. The overall low cover of exotic species on these sites supports other findings that the disturbance associated with high-severity fire does not always result in exotic species invasions. The increase in relative cover and frequency though time indicates that some species are proliferating, and continued monitoring is recommended. Patterns of exotic species invasions after severe burning are not easily predicted, and are likely more dependent on site-specific factors such as propagules, weather patterns and management.