Simulating arctic plant functional types in a land surface model: Impacts on ecosystem carbon and nitrogen cycling

Benjamin N. Sulman and Colleen Iversen, Climate Change Science Institute and Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, Fengming Yuan, Environmental Sciences Division & Climate Change Science Institute, Oak Ridge National Laboratory, Oak Ridge, TN, Verity G. Salmon and Peter E. Thornton, Environmental Sciences Division and Climate Change Science Institute, Oak Ridge National Laboratory, Oak Ridge, TN, Amy L. Breen, International Arctic Research Center, University of Alaska Fairbanks

Background/Question/Methods
Accurate simulations of high latitude ecosystems are critical for confident Earth system model (ESM) projections of carbon (C) cycle feedbacks to global climate change. Land surface model components of ESMs, including the E3SM Land Model (ELM), simulate plant communities by grouping vegetation into like sets of plant functional types (PFTs). Many such models represent high-latitude vegetation using only two PFTs (shrub and grass). However, arctic plant communities vary over small spatial scales and are dominated by graminoid, shrub, moss, and lichen vegetation types that are not well represented in global models. We used a combination of plant community surveys and biomass measurements at the NGEE Arctic Kougarok Hillslope field site, located on the Seward Peninsula of Alaska, to replace the original ELM configuration for the first time with nine Arctic-specific PFTs. The newly-developed PFTs include: 1) nonvascular mosses and lichens, 2) deciduous and evergreen shrubs of various height classes, including a nitrogen (N)-fixing alder PFT, 3) graminoids, and 4) forbs. We configured the model to represent multiple plant communities occurring across the hillslope including tussock tundra, shrubland, and lichen-dominated communities, and simulated how C and N cycles evolved over the historical period through 2100.
Results/Conclusions
Compared to model simulations using the default arctic grass and boreal shrub PFTs, new arctic PFTs and parameterizations yielded more accurate simulations of total biomass, aboveground-belowground biomass partitioning, N fixation, and annual net primary production when compared with site measurements. The addition of N-fixing alder, forb, and lichen PFTs significantly increased vegetation growth and N availability, especially in alder-dominated plant communities. When projected into future climate conditions (RCP 8.5 through 2100), simulations with arctic PFTs showed increased vegetation C storage in communities dominated by alders and other potentially tall shrubs but not in communities dominated by dwarf shrubs and lichens. Our results highlight the importance of representing the diversity of vegetation growth forms and N fixation capacities for accurately simulating arctic ecosystems in terrestrial ecosystem models.