Evaluating the efficacy of structural complexity enhancements to achieve old-growth conditions in Pacific Northwest coastal forests

After nearly two centuries of logging, only a small percentage of original old-growth forests in the Pacific Northwest (PNW) remains. Old-growth forests are characterized by heterogeneous forest structures with large and mature trees, multi-story canopies, and complex understories composed of snags, logs, and downed woody material. To jumpstart succession processes and restore old-growth forests, structural complexity enhancements (SCE), including prescribed burning and variable-density thinning, have been developed and used more frequently in forest management. Compared to traditional thinning methods (thin from below), SCE have been documented to provide greater ecosystem-wide benefits (Harmon et al. 1990, Gunn et al. 2014).

To test the effectiveness of SCE techniques in the PNW, the long-term effects of different management methods—prescribed burning, variable-density thinning, and traditional thinning—on previously logged coastal forests of the South Slough National Estuarine Research Reserve (NERR) located in Charleston, Oregon were investigated. Using the Forest Vegetation Simulator (FVS), 11 forest stands of different age classes were simulated from 2015-2125 under four management scenarios: no action (control), traditional thin from below, variable-density thin, and prescribed burn. The main variables of interest were structural complexity and carbon sequestration. Forest structure complexity (large trees, large snags, downed wood) was quantified using the Old Growth Habitat Index. Results showed that there were no significant differences in carbon sequestration among treatments except for the control. Thinning and variable-density thinning were similar across all structural elements and were most effective at accelerating large tree growth. The control treatment created the most downed wood and large snags. Among treatments excluding the control, prescribed burns created the most snags and downed wood. Findings suggest that multiple variable-density thin treatments or a combination of treatments may need to be implemented in order to effectively promote structural complexity.