Heavy crown thinning in redwood/Douglas-fir gave superior forest restoration outcomes after 10 years

Abstract

Forest restoration thinning has the potential to enhance structural complexity and accelerate development of large trees important to wildlife, aesthetics, and wildfire resistance. These are key objectives for restoration of even-aged secondary forests within Redwood National Park in Humboldt County, California, USA. We evaluated tree growth and stand structure 10 years after two thinning methods were applied at two intensities in a 40-year-old mixed redwood (Sequoia sempervirens (Lamb. ex D. Don) Endl.))/Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) stand. Heavy thinning enhanced diameter growth of redwood and Douglas-fir trees more than light thinning. Crown thinning generally enhanced structural diversity more than low thinning, and structural diversity increased progressively over the 10 years following thinning. Understory plant richness fluctuated between measurement years. Heavy thinning enhanced understory shrub cover. The fastest-growing trees in heavily-thinned stands were much more likely to sustain bear damage, especially redwood trees. Overall, different thinning methods and intensities induced a different suite of outcomes, yet none restored redwood dominance, but all treatments enhanced some other ecosystem values important for old-growth restoration such as large overstory trees, understory plant and shrubs, and elements of structural complexity including tree-size variability, snags, down logs, and trees exhibiting stem or top damage.