Pre-fire treatments have persistent effects on post-fire plant communities.

摘要

Wildfires characterized by large areas of high severity are increasingly occurring in ponderosa pine (Pinus ponderosa P. & C. Lawson) forests of the Southwest to extents that are out of the natural range of variability. Managers are now routinely applying thinning and/or burning treatments to reduce fire severity. To investigate the effects of pre-fire treatments and fire severity on post-fire vegetation recovery, we re-measured established plots on the 2002 Rodeo-Chediski Fire on the White Mountain Apache Tribal (WMAT) lands eight years post-fire and the Apache-Sitgreaves National Forest (ASNF) nine years post-fire.;On the WMAT lands we re-measured 70 plots stratified by fire severity (high, low) and pre-fire treatment (untreated, and cut/burned). We found significantly higher overall plant cover, exotic forb cover (although this was still low, <1%) and pine regeneration frequency in high severity areas, and highly significant overall differences in plant community composition and abundance between severity classes. Pre-fire treatment also influenced vegetation response within fire severity class. In particular, pine regeneration was more frequent in pre-fire treated areas than untreated areas for both severity classes, which we linked to a generally more open canopy in treated low severity fire areas and to a more heterogeneous neighborhood severity pattern in treated high severity areas.;On the ASNF portion of the study, we re-measured 80 plots in paired pre-fire thinned sites, which were less severely burned, and pre-fire unthinned sites that were more severely burned. Plant community composition and abundance in thinned and unthinned areas were converging nine years post-fire; however, persistent differences included significantly higher overall plant cover, as well as higher mean shrub cover in the untreated areas.;A low exotic species response was observed on both study areas, but we did detect an increased frequency of some exotic species compared to the initial observations. Studies throughout the Southwest have documented varying rates of exotic species invasions, suggesting there is no clear pattern between fire severity and exotic species response. Although high severity fire can increase growing space for exotic species, post-fire management practices, on-site propagules and weather patterns may be the more important drivers of exotic response.