Sequoia sempervirens and Pseudotsuga menziesii tree growth and drought resistance following restoration thinning in secondary northern coast redwood forests of California

The growing impacts of climate change and uncertainty about forest vulnerability to these changes make understanding forest response to drought increasingly important. Many of the studies investigating forest response to drought focus on inland forest types rather than coastal forests. This study examined tree growth and growth response to drought in coastal forests at restoration thinning sites, evaluating responses to local climate, tree-level competition, and site-level factors. Tree cores were extracted at three restoration sites in Redwood National Park, California, from both thinned and unthinned stands. Douglas-fir (Pseudotsuga menziesii) and coast redwood (Sequoia sempervirens) trees > 20 cm diameter at breast height were sampled (n = 268). Thinned stands had lower tree density, individual level competition index, and higher stand basal area than control stands. We modeled the influence of competition, size, site, and species identity on tree growth as measured by basal area increment (BAI). Prior to a period of drought, Douglas-fir had greater BAI relative to redwood, and competition was negatively associated with tree growth for both tree species. There was variability among sites, with the stand closest to the coast and with the most recent thinning treatment showing the fastest growing trees. Relative to the pre-drought period (2010–2012), average BAI declined slightly during the drought period (2013–2015) for Douglas-fir and coast redwood; however, we found less evidence for this effect in coast redwood. Notably, the relationships between growth and competition did not change during the drought period for either species. These results suggest that reducing competition via restoration thinning treatment improves tree growth, and this advantage is maintained even during short (3-year) periods of drought.