Pre-wildfire fuel treatments affect long-term ponderosa pine forest dynamics

Abstract

The 2002 Rodeo–Chediski fire, the largest wildfire in south-western USA history, burned over treated stands and adjacent untreated stands in the Apache–Sitgreaves National Forest, setting the stage for a natural experiment testing the effectiveness of fuel reduction treatments under conditions of extraordinary fire severity. In seven pairs of treated–untreated study sites measured 2 years after the fire, thinning was strongly associated with reduced burn severity. Treated areas had more live trees, greater survival, and reduced fire intensity as indicated by crown base height and bole char. Ponderosa pine regeneration was patchy but more dense in treated areas. We assessed decade- to century-long effects of the pre-wildfire fuel treatments using the Forest Vegetation Simulator (FVS). Differences between treated and untreated areas were projected to persist for several decades after the fire in terms of stand structure characteristics and for at least 100 years in terms of species composition, with ponderosa pine making up ~60% of basal area in treated areas but only 35% in untreated areas. Future ecosystem development may take the trajectory of recovery to a ponderosa pine/Gambel oak forest or of a shift to an alternative stable state such as an oak-dominated shrubfield, with untreated areas more apt to undergo a shift to a shrubfield state. Current management decisions about fuel treatments have multi-century legacies.