The response of Pacific Northwest evergreen coniferous forests to climate change: the case of Douglas fir explored with a dynamic vegetation model.

Background/Question/Methods
Climate warming is already affecting ecosystems in the Pacific Northwest (PNW). Decrease in ecosystem productivity and increase in mortality of some plant species induced by drought and disturbances have been reported. What will be the long-term response of vegetation to change in soil water content, in atmospheric CO2 concentration and in fire disturbances? We use the process-based dynamic vegetation model CARAIB which includes all these essential components to simulate present and future plant species distribution and functioning. We previously evaluated the ability of the vegetation model to reproduce the regional water and carbon cycling over the historical period using global Plant Functional Types. But, since individual species with a narrower bioclimatic spectrum than parent PFT might be more vulnerable to climate change, we now apply the vegetation model at the level of representative PNW species. Here, we focus on Douglas fir (Pseudotsuga menziesii), one of the most important and valuable timber trees of the world. Species physiological and structural parameters were progressively adapted from initial PFT traits to specific traits found in local or global trait databases (e.g., TRY database). For the historical period, the vegetation model is driven with the 1979-2014 meteorological dataset UIdaho MACA METDATA with a 1/24-degree (~4-km) spatial resolution and a daily temporal resolution. We simulate future conditions until 2100 using the UIdaho MACAv2-METDATA dataset, which includes downscaled CMIP5 projections from numerous GCMs.
Results/Conclusions
The model ability to reproduce the current spatial and temporal variations of PNW water and carbon stocks and fluxes has been formerly demonstrated using available datasets (e.g., National Biomass and Carbon Dataset, MODIS remote-sensed products, etc.). In the present research, we show that the model is in good agreement with the observations in regard to species occurrences (FIA plots) and functioning (eddy covariance sites). The modeled gross primary production (GPP) matches very well the measured GPP in Douglas-fir stands. Uncertainties remain regarding projected temperature and precipitation changes. Some climatic scenarios like the one generated by the CCSM4 model (which is among the best performers for historical PNW climate) project a warming and an emphasis of current precipitation regime, wetter winter and drier summer (especially in the lowlands). According to CARAIB, the resulting lower soil water content might not strongly change the spatial distribution of Douglas fir, but might impact its productivity. We also evaluate mortality induced by changes in drought frequency as well as in fire risk.