"Minimal-Impact" Restoration Treatments Have Limited Effects on Forest Structure and Fuels at Grand Canyon, USA

Restoration treatments were tested on the South Rim (ponderosa pine) and North Rim (mixed conifer) of Grand Canyon National Park, never-harvested forests where the historically frequent surface regime was interrupted in the late 19th century. Treatments were designed for "minimal impact" by limiting the size of trees to be thinned and minimizing mechanical equipment. Treatments included (1) thinning of small trees (diameter ≤ 12.7 cm) and prescribed fire; (2) thinning of small trees located close to large old trees and prescribed fire; (3) prescribed fire only; and (4) control. On the South Rim, density declined by 23–45% but basal area only by 9–14%. On the North Rim, density was reduced 32–68% and basal area declined 18–31%. Declines of 5–8% were observed in these same variables in the controls. Surface fuels were significantly reduced in all burned treatments (70–84% reduction in forest floor, 66–76% reduction in woody debris). However, canopy cover was nearly unchanged, and canopy fuel loads were not significantly reduced, although canopy base heights increased. The experiment accomplished the goal of minimally impacting the forest ecosystem but the effects of small-tree thinning plus burning were nearly indistinguishable from those of burning alone. All the treated units were left with modest gain in resistance to severe wildfires but in conditions still far removed from prefire-exclusion conditions.