Restoration thinning enhances growth and diversity in mixed redwood/Douglas-fir stands in northern California, U.S.A.

摘要

We examined early responses to different forest restoration thinning treatments to evaluate their efficacy in accelerating tree growth rates, changing species composition, and enhancing structural diversity while minimizing stem damage by black bears and reducing slash depth. Treatments were conducted in 15-25-year-old mixed redwood/Douglas-fir stands and consisted of high-density, low-density, and localized release thinning, as well as a control. Four years post-treatment, data collected from 60 large plots in 20 stands spread across the Mill Creek and Rock Creek watersheds in Del Norte County, California, indicated that spatial variability created from thinning treatments was reflected in diameter growth. Trees having no crown competition had on average a 33% greater basal area increment than trees surrounded by neighboring trees. Thinning treatments enhanced redwood representation by preferentially removing other species, but also promoted rapid growth which increased incidences of bear damage. Approximately 25% of redwood trees sustained bear damage in thinned stands. The localized release thinning created a mosaic of circles flanked by unthinned patches within which trees grew slowly. Localized release created more heterogeneity in tree size but did not generate significantly greater fuel bed depths, result in more bear damage, or represent any sacrifice in understory light or plant abundance and species diversity than a more conventional approach to thinning.