Evaluating the efficacy of thinning and prescribed fire for restoring mature forest characteristics in the Eastern Cascades, USA

Abstract Historical fire exclusion, intensive logging of large trees of fire-tolerant species, and even-aged management have contributed to widespread decline of mature/old-growth coniferous forests across the Western USA. To investigate silvicultural approaches for reducing disturbance risk while accelerating restoration of mature/old-growth forest characteristics, we used 20-year data from the Goosenest Adaptive Management Area study in Northern California, USA. Established to evaluate the alternatives for reducing fuels and accelerating the development of late-seral characteristics in ponderosa pine (Pinus ponderosa)/white fir (Abies concolor) mixed-conifer forests, the Goosenest Adaptive Management Area study consists of (i) an untreated Control, (ii) a Big Tree treatment using thinning from below to favor large trees of any species, (iii) a Pine Emphasis treatment combining thinning from below with radial thinning, group selection harvesting, and artificial regeneration to favor ponderosa pine while increasing structural complexity, and (iv) a Pine Emphasis with Fire treatment with postthinning prescribed burns added to (iii). Our objectives were to determine whether and how (i) treatments altered stand structure and composition and (ii) the dynamics of ecologically valuable large trees and snags varied by treatment. To accomplish this, we built generalized linear mixed models to compare treatment effects on stocking, structural complexity, composition, growth dominance, large-tree dynamics, and large-snag dynamics. We found that all the treatments initially lowered stocking relative to the Control, but Pine Emphasis with Fire was notably more successful in maintaining the stocking/fuel reduction. Both Pine Emphasis treatments maintained vertical and horizontal structural complexity. Pine Emphasis with Fire was most effective in restoring composition toward ponderosa pine dominance. Treatments did not discernably alter growth dominance, large-tree recruitment/mortality, or the large-snag recruitment/decay condition. Our results indicate that the Pine Emphasis with Fire treatment shows the greatest potential for accelerating restoration of mature/old-growth characteristics in ponderosa pine/mixed-conifer forests, while reducing longer term risk from fire and other disturbances.