Managers of second-growth forests at Redwood National Park and the Bureau of Land Management’s Headwaters Forest Reserve encourage the development of late seral forest characteristics using mechanical thinning, where competing vegetation is removed to promote growth of residual trees. Yet the ability to quantify and reliably predict outcomes of treatments such as these is hindered by the long time scales at which forests respond to thinning. Here we present analyses of tree growth at Redwood National Park (RNP) and Headwaters Forest Reserve (HDWT) from sites that have had > 5 years to respond to thinning treatments.

Compared to untreated stands, thinned stands had lower stem density (trees ha^-1) and basal area (m² ha^-1), primarily due to removal of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). Individual tree growth (basal area increment, BAI, m² yr^-1) was related to tree size (basal area, m²) and treatment history, with the highest growth rates observed in large trees. Both redwood (Sequoia sempervirens (D. Don) Endl.) and Douglas-fir appeared to have a small, but detectable, positive growth response to thinning treatments. Early results suggest a large degree of variation among sites, with possible systematic differences in growth responses between RNP and HDWT. Future work will focus on identifying site-level differences (site quality, local competition, slope, aspect, stand age, distance from the ocean) to improve our understanding of the growth response.