Twelve-year response of coast redwood to precommercial thinning treatments

Abstract

Six precommercial thinning treatments and two types of control treatments were established in 9- to 11-year-old, even-aged coast redwood (Sequoia sempervirens (D Don) Endl.) forests in coastal California to study the impact of thinning on stand and individual tree level volume growth. Species composition was 74% redwood and 23% coast Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco). Treatments were replicated across seven stands and monitored for 12 years. Data analysis included both analysis of variance (ANOVA) to compare treatment means and mixed-effects modeling to examine broader trends and important variables affecting increment. The ANOVA used target treatment densities as the predictor variable, whereas the mixed models used actual posttreatment densities. Results showed great variation within treatments, largely because high variability in spatial patterns of regeneration resulted in difficulty achieving target initial treatment densities. Ingrowth from sprouting redwood and, to a lesser extent, broadleaved trees and seedlings of other conifers also contributed to large variation in density levels. Results indicate tree increment generally decreased with increasing density and stand increment increased with density. Total stand increment, including both initial trees and ingrowth, was highest at the 4 × 4-ft spacing and lower in control treatments and thinning treatments that left lower densities. Both analysis procedures provide useful information to guide evaluation of thinning studies and complementary results to guide management regimes.