Description

Western United States land managers are conducting fuel reduction and forest restoration treatments in forests with altered structural conditions. As part of the National Fire and Fire Surrogate (FFS) study, thinning and burning treatments were evaluated for changing forest structure. Shifts between pretreatment and posttreatment diameter distributions at seven western FFS study sites were determined by assessing live tree diameter frequency distributions and the 10th and 90th percentile and mean diameter. Diameter distributions were based on 31,517 live trees within 76 pretreatment units and 25,061 live trees within 85 posttreatment units. Cross-site comparisons were made using meta-analysis. Values for 10th percentile diameter increased at two sites, values for 90th percentile diameter increased at six sites, and values for mean diameter increased at five sites (P < 0.05) after active treatments (thin, burn, or thin + burn) compared with control sites. Across the seven western FFS study sites, the overall effect size of the thin treatment increased for 90th percentile and mean diameter, the overall effect size of the burn treatment increased for 10th percentile, 90th percentile, and mean diameter, and the overall effect size of the thin + burn treatment increased for 10th percentile, 90th percentile, and mean diameter. This work indicates that although each of the active treatments was effective in shifting diameter distributions toward larger trees, no single treatment or entry will probably mitigate nearly a century of fire exclusion and fuel accumulation in dry coniferous forests of the western United States.