Declining conifer productivity will drive future forest dynamics as climate changes in northern New England

Climate change is expected to decrease habitat suitability for conifers in the mixed species, temperate forests of New England in the northeastern United States. How existing forests will be affected during the transition from current to future growing conditions, however, is less clear and has important implications for commercially managed forests and the growing interest in forest carbon as a natural climate solution. We used the LANDIS-II forest landscape model to simulate climate change under two Representative Concentration Pathways (RCP 4.5 and 8.5) and interactions with timber harvesting under the predominant harvest regime across 4 million ha of managed forestland in Maine. Using a factorial design, we compared the relative and cumulative impacts of predicted changes in species' maximum annual net primary productivity or establishment probability on trends in live, aboveground biomass (2010-2100). In addition to area-wide scenario comparison, we evaluated variation in local outcomes (n = 356 townships or ~100 km² land units) to better inform adaptation at a scale relevant to forest management. Although broadleaf productivity was predicted to be generally resistant to regional increases in temperature and precipitation, projections suggested climate change will have a negative effect on area-wide biomass, driven predominantly by declines in conifer productivity. Nonetheless, conifers (specifically balsam fir [Abies balsamea] and red spruce [Picea rubens]) will likely remain the area's most productive species; consequently, across climate scenarios there was a positive correlation (r = 0.68-0.82) between percent change in local biomass density and percent change in conifer biomass. Simulations also suggested that even with climate change, regional harvest supply could be maintained over the next 70-80 years; however, current harvest rates and practices may become unsustainable across an increasingly large area as conifer productivity declines. In addition, species establishment probability is likely to decline for many broadleaf and conifer trees, which will be particularly impactful in harvested areas with insufficient regeneration. Overall, our results suggest the forests in northern New England will continue to be an aboveground carbon sink over this century, but without adaptive management to improve forest resistance and resilience to climate change, it will be a sink of diminishing capacity.