Simulating shrubs and their energy and carbon dioxide fluxes in Canada's Low Arctic with the Canadian Land Surface Scheme Including biogeochemical Cycles (CLASSIC)

Abstract

Abstract. The Arctic is warming more rapidly than other regions of the world leading to ecosystem change including shifts in vegetation communities, permafrost degradation and alteration of tundra surface-atmosphere energy and carbon (C) fluxes, among others. However, year-round C and energy flux measurements at high-latitude sites remain rare. This poses a challenge for evaluating the impacts of climate change on Arctic tundra ecosystems and for developing and evaluating process-based models, which may be used to predict regional and global energy and C feedbacks to the climate system. Our study used 14 years of seasonal eddy covariance (EC) measurements of carbon dioxide (CO2), water and energy fluxes and winter soil chamber CO2 flux measurements at a dwarf-shrub tundra site underlain by continuous permafrost in Canada's Southern Arctic ecozone to evaluate the incorporation of shrub plant functional types (PFTs) in the Canadian Land Surface Scheme Including biogeochemical Cycles (CLASSIC), the land surface component of the Canadian Earth System Model. In addition to new PFTs, a modification of the efficiency with which water evaporates from the ground surface was applied. This modification addressed a high ground evaporation bias that reduced model performance when soils became very dry, limited heat flow into the ground and reduced plant productivity through water stress effects. Compared to the grass and tree PFTs previously used by CLASSIC to represent the vegetation in Arctic permafrost-affected regions, simulations with the new shrub PFTs better capture the physical and biogeochemical impact of shrubs on the magnitude and seasonality of energy and CO2 fluxes at the dwarf-shrub tundra evaluation site. The revised model, however, tends to overestimate gross primary productivity, particularly in spring, and overestimated late winter CO2 emissions. On average, annual net ecosystem CO2 exchange was positive for all simulations, suggesting this site was a net CO2 source of 18 ± 4 g C m−2 year−1 using shrub PFTs, 15 ± 6 g C m−2 year−1 using grass PFTs, and 25 ± 5 g C m−2 year−1 using tree PFTs. These results highlight the importance of using appropriate PFTs in process-based models to simulate current and future Arctic surface-atmosphere interactions.

Vegetation type is an important predictor of the arctic summer land surface energy budget

“…The potentially important but still uncertain role of Arctic vegetation for climate feedbacks has been highlighted before 18,19 . Previous studies have shown that local estimates of surface energy fluxes are improved if land surface components of Earth system models account for differentiated Arctic PFTs at several high-latitude validation sites and -regions 18,47,57 . Here, we provide quantitative evidence of the importance of vegetation types for predicting Arctic surface energy budgets at circumpolar scale and support recent calls for refined accounting of high-latitude vegetation types and associated vegetation functions in land surface components of Earth system models 18,19,58 .…”

Simulating shrubs and their energy and carbon dioxide fluxes in Canada's Low Arctic with the Canadian Land Surface Scheme Including Biogeochemical Cycles (CLASSIC)

“…Code and data availability. The CLASSIC code v1.0.1 including shrub and sedge plant functional types is archived on Zenodo (https://doi.org/10.5281/zenodo.4301108; Meyer et al, 2020a), and the eddy covariance and meteorological measurements made at the Daring Lake dwarf-shrub tundra site (DL1) between 2004 and 2017, which were used to drive and validate CLASSIC, are available on Zenodo as well (https://doi.org/10.5281/zenodo.4301133, Meyer et al, 2020b).…”

Evaluating the performance of the Canadian Land Surface Scheme Including Biogeochemical Cycles (CLASSIC) tailored to the pan-Canadian domain

“…This growth may offset some of the anticipated C emissions from the warming and thawing of arctic permafrost soils (Miner et al, 2022;Schuur et al, 2015;Schuur & Mack, 2018). At present, the representation of vegetation within Canada's boreal forests, and tundra ecosystems is limited in regional-scale simulations, restricting our ability to disentangle the impacts of these various processes and make projections (Friedlingstein et al, 2019;Melton et al, 2020;Meyer et al, 2021;Sulman et al, 2021;Wullschleger et al, 2014).…”

“…Moreover, the PFTs used in LSMs have historically been developed to represent global patterns of vegetation and their associated traits (Bonan et al, 2002;Box, 1996;Melton et al, 2020;Wullschleger et al, 2014). Region-specific PFTs can enhance model realism, more accurately represent the diversity of vegetation on the landscape, and include more informed parameterizations that act to reduce regional biases (Curasi et al, 2022;Epstein et al, 2001;Mekonnen et al, 2021;Meyer et al, 2021;Peng et al, 2014;Rezende et al, 2016;Rogers, 2014). For these region-specific PFTs to improve model performance and robustness they require sufficient data or expert knowledge to inform their parameterization and specify their distribution.…”

“…Currently, CLASSIC and indeed we are not aware of any other LSMs that include PFTs tailored to explicitly represent Canada's boreal forests, tundra sedges, and shrubs (Melton et al, 2020;Meyer et al, 2021;Wullschleger et al, 2014).…”