Prior to the twentieth century, mixed-conifer forests in California experienced a frequent, low-severity fire regime. As a result of twentieth century fire-suppression, fire frequency has decreased, while the proportion of high-severity fires have increased as a result of increased tree density and fuel loads. Fuel reduction treatments are increasingly implemented as a means to reduce wildfire severity. Although federal agencies have treated millions of hectares of forests, there has been little monitoring of ecological effects of these treatments post-wildfire. Evaluations of fuel treatment effects on fire severity and plant community composition have primarily focused on modeled fire behavior, or on field assessments of individual wildfires that burned through treatments. We use the largest study to date of post-wildfire fuel treatment effects to ask how interactions between treatment and wildfire affect 1) fire severity patterns, and 2) plant community composition. We sampled from 12 wildfires that burned between 2005 and 2011 through fuel treatments in mixed-conifer forest in California managed by the U.S. Forest Service. At each fire, we established transects that crossed the treatment boundary in both burned and adjacent unburned forest. Along each transect, we estimated fire severity, measured tree mortality and recruitment, and sampled understory plant diversity.
Treatments had strong effects on both fire severity and community composition. In 10 of the 12 fires we sampled, fire severity and tree mortality were significantly reduced in fuel treatments. This corresponded with a significant increase in litter depth in treated areas after fire, because tree crown torching was reduced in treatments, resulting in abundant needle cast. The effects of fuel treatments on understory community composition differed between burned and unburned areas. In the absence of fire, treated areas had significantly lower tree seedling abundance at most sites, while effects on shrub seedling abundance and shrub cover were mixed. However, after fire, treated areas at most sites had significantly greater tree seedling abundance compared to untreated areas, whereas shrub seedling abundance and cover was greater in untreated areas. Total community composition, described using a multivariate ordination, was most strongly affected by burning. Post-fire effects of treatment on total community composition were associated with differences in shrub cover between treated and untreated areas. This apparent post-fire shift away from tree recruitment and towards shrub recruitment in untreated forest suggests that by altering fire behavior, treatments may change the post-fire successional trajectory of forest vegetation.