Current and future effects of drought, fire and mitigation options on Pacific Northwest forests

Background/Question/Methods

Pacific Northwest forest diversity, productivity and sustainability are shaped by climate and anthropogenic influences, including legacy land use. These forests are among the highest in productivity and biomass globally. Climate change is expected to include warming, changes in precipitation regimes, and lengthen forest growing seasons which can exacerbate drought stress and contribute to disturbance from insects, pathogens and wildfire. These disturbance agents often occur in sequence, further complicating understanding of potential trajectories of change in carbon cycling.  In addition, natural disturbances can be amplified by anthropogenic activities, increasing vulnerability of forests. There is much uncertainty about how and where to mitigate these effects at the local to regional scales. To simulate possible effects of changing climate, N deposition, and forest mitigation options, ecoregion-specific parameterization and fire combustion factors in the NCAR CLM4 model, regional atmospheric forcing datasets, IPCC RCP4.5 and RCP 8.5 scenarios, and a life cycle assessment (LCA) were applied across the region. In addition, a physiology-based model and decision rule analysis produced estimates of species richness and presence/absence by the 21^st century.

Results/Conclusions

Simulations of future distribution of major forest species predict potential decline in the presence of ponderosa pine and increase in Douglas-fir in parts of the PNW by 2050, and a shift in species richness to the north. By the end of the 21^st century, CLM4 simulations suggest the regional net biome production (NBP) will increase by 58% despite fire increases, indicating that enhanced productivity from a warmer climate and CO₂ fertilization compensates for disturbance losses if current management intensity is continued. After simulating typical harvest and thinning treatments intended to sustain forest carbon, and applying a complete Life Cycle Assessment, none of the thinning scenarios reduced net emissions to the atmosphere by the end of the 21^st century.  The ecoregion-specific model-data integration and LCA provides a framework for regional analysis of future forest biogeochemistry, distribution, and sustainability, and potential effects of forest management mitigation options.