Plant community dynamics following hazardous fuel treatments and mega-wildfire in a warm-dry mixed-conifer forest of the USA

The greater than 200,000 ha Wallow Fire of May-July 2011 burned through mixed-conifer and ponderosa pine forests in Arizona and New Mexico in the southwestern U.S. This “mega-fire” set the stage for an opportunistic study to examine understory plant community responses to pre-fire fuel reduction treatments in areas dominated by dry mixed-conifer forests. In 2016, five years after the fire, we remeasured nine pairs of treated and untreated sites that were installed in 2012 and compared understory characteristics including species cover, richness and community composition. Native plant cover was significantly higher (20% total cover) in areas that were treated for fuel reduction before the fire compared with untreated areas (17% cover), despite the variability in fire severity and tree mortality. Non-native plant cover was not significantly different between treated and untreated units. Herbaceous understory cover was negatively correlated (p < 0.05) with total tree basal area and tree canopy cover in treated as well as untreated units, and positively correlated with tree mortality and basal area mortality in the untreated units. In the treated units, species richness and diversity were negatively (r = −0.62 to −0.63) correlated with insolation and elevation. Non-metric multidimensional scaling ordinations along with univariate results suggested that community composition was driven by increased insolation following loss of tree canopy. Cover of species that benefit from high light and warmer temperatures, such as some shrubs and perennial graminoids, increased from 2012 to 2016. Overall, fuel reduction treatments led to persistent increases in cover of native species, lending weight to the usefulness of hazardous fuel reduction treatments in promoting native plant cover.