LPJ-GUESS Simulated Vegetation Changes from 21-0 ka for the H. J. Andrews Experimental Forest Long Term Ecological Research Site (Oregon, USA)

The time period from 21-0 ka spans the last glacial-to-interglacial transition and includes periods of relatively large and abrupt climate changes (e.g., at the beginning of the Bølling-Allerød interval at ~14.7 ka). It is also the time period during which the iconic old-growth conifer forests of the Pacific Northwest (USA) region became established. Here we examine simulated vegetation changes for 21-0 ka for the H. J. Andrews Experimental Forest (HJA) Long Term Ecological Research (LTER) site, a 6,400 ha watershed in the Cascade Range (Oregon, USA). We use LPJ-GUESS, a dynamic global vegetation model, to simulate changes from 21-0 ka in vegetation, wildfire regimes, and related variables. Input climate data for LPJ-GUESS are from the CCSM3 TraCE simulations. We calendar-correct the TraCE data using PaleoCalAdjust and downscale the data to the HJA site location. The simulated vegetation and wildfire changes are evaluated in the context of the observed paleo vegetation (i.e., pollen and plant macrofossils) and wildfire (i.e., charcoal) data from the Pacific Northwest region and are also compared with historical data collected at the HJA site. For 21-0 ka, the simulated air temperature generally increases for all months and simulated precipitation generally decreases in spring and summer months, resulting in warmer and drier conditions during the growing season. In response to higher temperatures, the simulated vegetation shifts from boreal needleleaf evergreen plant functional types (e.g., Picea spp.) to more temperate needleleaf evergreen plant functional types (e.g., Pseudotsuga spp.). Warmer and drier conditions also lead to simulated increases in wildfire frequency in the region, starting around 17 ka. We describe the climate-vegetation-wildfire interactions, with a particular focus on the conditions promoting the establishment and persistence of the regions old-growth conifer forests.