Evaluating LPJ-GUESS Simulations of Wildfire-Vegetation-Climate Interactions in the Pacific Northwest

Disturbances such as wildfire play an important role in forest dynamics. Dynamic global vegetation models (DGVMs) are used to investigate how forests may respond to potential future changes in disturbance regimes, including changes in forest structure and taxa distributions. Here we evaluate the ability of an ecosystem model, LPJ-GUESS, to simulate long-term wildfire-vegetation-climate interactions in the Pacific Northwest. Many DGVMs, including LPJ-GUESS, integrate the effects of climate and atmospheric CO₂ concentrations on vegetation and simulate the responses of vegetation to changes in wildfire regimes. We ran LPJ-GUESS using simulated climate data for paleo, historical, and future time periods from CCSM3 TraCE (22 ka to 1989 CE) and CCSM4 CMIP5 (850-2100 CE; past1000, historical, rcp45, and rcp85 experiments) climate simulations downscaled to our Pacific Northwest study area. We also calculated wildfire-related climatic variables (e.g., moisture indices) to identify potential climatic controls on the simulated vegetation and wildfire changes. We compared the simulated vegetation and wildfire data with observed paleovegetation data from pollen and plant macrofossil records and with observed wildfire data recorded in charcoal records from the Pacific Northwest. The LPJ-GUESS simulations generally reproduce observed vegetation transitions from cold plant functional types to more temperate plant functional types over the past 22 kyr. In the parts of the study area with a summer-dry climate regime, simulated fire frequencies tend to increase over time as these areas become warmer and drier in summer, although the fire frequency response is mediated by changes in the amount and type of simulated biomass, which varies across the study area. We suggest ways to improve comparisons of simulated biomass and fire frequencies with observed vegetation and charcoal influx data. We also discuss the implications of our results for the future of Pacific Northwest forests.