Thinning restores ungulate foraging habitat in historically logged forests

Abstract

Legacies of land use can persist for decades, thereby impacting populations, communities, and ecosystems long after the original disturbance has concluded. The coastal rainforests of western North America were fundamentally transformed by commercial logging throughout the 20th century, resulting in depauperate second-growth forests that provide limited understory production and foraging habitat for herbivores. The Tongass National Forest in Alaska, USA, is the largest contiguous tract of coastal temperate rainforest in the world, but nearly 200,000 ha of second-growth forest have created a need to restore understory plant communities and foraging habitat for ungulates like Sitka black-tailed deer (Odocoileus hemionus sitkensis), a regional indicator of forest health and key subsistence resource for local and Indigenous communities. We leveraged a 16-year adaptive management experiment—the Tongass-Wide Young Growth Studies (TWYGS) program—to test the effects of precommercial thinning on forage availability and use by deer in second-growth forests. We measured plant communities, presence–absence of browse, and slash debris in 14,908 quadrats across 730 plots and found that plant community composition, understory forage biomass, digestible energy, and digestible protein were all significantly higher in thinned plots versus controls. Precommercial thinning also doubled browse probabilities relative to unthinned stands, and plots treated within 35 years of stand initiation experienced the highest gains. Deer selected for both plant quantity and plant quality, as browse was positively associated with both digestible energy and digestible protein. Conversely, slash debris generated by precommercial thinning reduced browse probability by an average of 11.3%, but these effects attenuated as slash decomposed over the course of the study. We found no effects of landscape composition on relative browse probability, indicating that fine-scale resource quality and accessibility were the strongest drivers of habitat use by deer over nearly two decades of sampling. Collectively, our results show that precommercial thinning is a valuable management tool that increases forage and deer habitat use in second-growth coastal rainforests. This study highlights the enduring legacies of forest disturbance and underscores the value of adaptive management experiments with long-term monitoring to optimize habitat management for wildlife.