Forests in the western US provide many vital ecosystem services, such as erosion control, wildlife habitat, watershed services, recreation, and act as a massive and vital carbon sink. Forests in the Pacific Northwest (PNW) US are some of the most carbon-dense forests in the world, and their ability to store and sequester carbon for centuries suggests these forests play a key role in climate mitigation. Climate change is a global phenomenon which not only directly impacts forests through changes in climatic patterns, but also indirectly impacts forests through increases in occurrence of disturbances (e.g. wildfire, insect outbreaks). Interacting factors between direct and indirect impacts of climate change on PNW forests leads to challenges in predicting the future forest carbon sink potential. Natural disturbances like wildfire and insect outbreaks have been increasing in occurrence throughout the western US due to shifting climatic influences. Both natural and human disturbances, such as logging and fuels reductions, may decrease the resilience and sustainability of ecosystem services and carbon storage. Intensified disturbance regimes decrease the carbon sink by increasing mortality, and potentially decreasing recovery. Here, we explore the consequences of direct and indirect impacts of climate change on the carbon balance in PNW forests.
Results/Conclusions
Direct climate change impacts on forest carbon storage and sequestration across the PNW are challenging to quantify as findings differ by forest type, scale, time frame, and approach. Case-studies of climate change, management, and disturbances allow more controlled evaluation of these interacting effects. For example, a large-scale management plan in the PNW, the Northwest Forest Plan (NWFP), decreased harvest by 50-60% on 10 million acres of federal forests. Protected areas increased in aboveground carbon stocks (AGC) while unprotected areas decreased by 10%. AGC stocks were 39% higher in protected forests than unprotected forests. Comparable forests without large-scale protection (i.e. Idaho, Montana) had continued decreases in AGC. Tree mortality from fire in PNW forests have been climbing, with peak emissions during high fire years. High-fire years correlate with hot and dry seasons, which are becoming more common with climate change. However, harvest mortality has been 32% higher than fire mortality in western forests. While fire and harvest do reduce AGC stocks, harvest does so at a much higher rate due to complete removal of C stocks from the ecosystem. Continued evaluation of PNW forests will help to reveal how climate change will directly and indirectly impact forest carbon stocks and sequestration.