Trade‐offs among management objectives in mature Douglas‐fir forests of the Pacific Northwest

Abstract

Mature conifer‐dominated forests are an important component of the Pacific Northwest landscape, and the conservation of species associated with late‐successional forests has been a primary management focus in these forests for decades. Increasingly, these forests are also valued as carbon stores, with considerable climate change mitigation potential. However, there are also increasing concerns about the effects of climate change, particularly drought, on late‐successional forests. Despite the complexity of balancing these diverse management concerns, few studies have examined the compatibility of biodiversity conservation, carbon storage, and drought adaptation. We used a spatially and temporally synchronous empirical dataset from mature Douglas‐fir (Pseudotsuga menziesii) stands representing three alternative management strategies, passive management (“unmanaged”), thinning, and retention harvest, to examine trade‐offs among management objectives related to drought adaptation, carbon storage, and the conservation of early‐successional and late‐successional forest songbirds. Although previous studies have evaluated drought adaptation in Douglas‐fir, none have focused on mature stands. Therefore, we also examined tree resistance and resilience to the 2001 drought. Trees in retention harvest stands displayed significantly higher drought resistance and resilience than trees in thinned or unmanaged stands, but no differences were observed between trees in the latter two management conditions, potentially due to the long (average of 22 years) period between treatment and drought in our thinned stands. Despite this, thinned stands provided a better multiobjective compromise than unmanaged or retention harvest stands in our trade‐off analysis. Across all mature stands, trade‐offs were largest for objective combinations that involved early‐ or late‐successional forest birds. While our analysis supports the consistency of managing late‐successional forest birds and carbon storage, trade‐offs between early‐successional birds and carbon storage were much larger. Given projected changes in climate, the substantial trade‐offs that we observed between drought adaptation and late‐successional forest birds are notable and imply that achieving these two objectives will be challenging at the stand scale. Our results suggest that a diversity of management approaches, incorporating both active management and reserve‐based strategies, may be necessary to foster a combination of drought adaptation, carbon storage, and biodiversity conservation goals in these forests.