COS 178-5 - Modeling Carbon and Species Trajectories Under Contrasting Forest Management strategies in Southeast Alaska

Abstract: Some of the most carbon dense forests in the world are located in the perhumid temperate rainforest of southeast Alaska. With climate change, these high latitude ecosystems have warmed significantly over the last 100 years and, with continued climate warming, the fate of their carbon remains largely unknown. Historically, the forests of this region have been constrained to even-aged management of old-growth western hemlock and Sitka spruce. Newly-implemented forest management plans in the region shift harvesting to young-growth stands in previously managed boundaries (that tend to be more productive and at lower risk of disturbance), while leaving areas less productive and less suitable for storing carbon in conservation reserves. To assess how the newly implemented management will influence the future spatial distribution of aboveground productivity, carbon stocks, species composition and forest structure in Alaska's highly productive perhumid forests, we used LANDIS-II to simulate forest succession and forest management under climate change at a high spatial resolution (90 m). We modelled changes in aboveground and soil organic carbon stocks and forest composition with three different climate scenarios: historical climate (1979 to 2016), moderate warming (CMIP6, SSP3-7.0), and severe warming (CMIP6, SSP5-8.5) and four forest management scenarios: spatially constrained to historical harvests, harvesting without spatial constraints, harvesting that mimic natural disturbance patterns, and proposed commercial thinning treatments in young-growth stands. Our results suggest that the shift to newly-implemented management decreases carbon accumulation in above and belowground carbon pools and that carbon stocks will increase under historical climate but decrease under climate change. Constraining harvest locations to areas of previous management had a moderate effect on future carbon stocks, showing less carbon accumulation over time, compared to our unconstrained scenarios. Commercial thinning in reserved young-growth stands initially decreased carbon stocks, however increases in tree growth returned stands to pre-treatment carbon stock levels in 30 years. This modeling approach characterizes the overall trends and spatial patterns of carbon stocks due to forest management and climate change within forests of southeast Alaska.