Early-Stage Thinning for the Restoration of Young Redwood—Douglas-Fir Forests in Northern Coastal California, USA

Among forested parks and reserves of the Pacific Coast of the United States, the restoration of late-successional conditions to second-growth stands is a management priority. Some traditional silvicultural treatments may help achieve this objective. We evaluated early-stage thinning as a restoration treatment to facilitate the growth and development of young (33- to 45-year old), homogeneous, and second-growth stands of coast redwood (Sequoia sempervirens) and Douglas-fir (Pseudotsuga menziesii). Targeting both stand-level responses and dominant (focal) tree responses for analysis, we compared structural attributes of adjacent thinned and unthinned stands, 12–17 years after thinning. Thinned stands displayed enhanced metrics of tree vigor, growth, and mechanical stability, thereby improving response to future restoration treatments and broadening the range of potential stand conditions. We conclude that early-stage thinning has been successful as a preliminary restoration treatment because it accomplished many initial goals of forest restoration, while retaining sufficient tree numbers to buffer against possible attrition from future disturbances. 1. Introduction Throughout the range of coast redwood (Sequoia sempervirens), facilitating the acquisition of late-successional forest structures and compositions in second-growth forests has become a primary management objective. It is estimated that less than 10 percent of original old-growth redwood forests remain [1], the majority of which have been conserved in parks and reserves. In order to continue promoting the extent of late-successional habitat on a landscape scale, focus has switched to the area’s second-growth forests [2]. In many cases, ecological restoration of these former commercial timberlands may require some forms of silvicultural treatment [3]. In young stands acquired for such conservation purposes, early-stage thinning has been proposed as a potential restoration treatment. Some stand responses to early-stage thinning have been documented in pure stands of coast redwood [4–6] and pure stands of Douglas-fir (Pseudotsuga menziesii) [7–9]. However, analysis of early-stage thinning’s effects on the region’s more ubiquitous mixed redwood—Douglas-fir stands is lacking. Similar to O’Hara et al. [10], we use the term “early-stage thinning” to describe a forest restoration technique whose primary purpose is shifting composition imbalances and reducing stand densities in order to accelerate growth in young second-growth stands, with the ultimate long-term objective of acquiring old-growth forest