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Simulating Shrubs and Their Energy and Carbon Dioxide Fluxes in Canada's Low Arctic with the Canadian Land Surface Scheme Incl

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Simulating Shrubs and Their Energy and Carbon Dioxide Fluxes in Canada's Low Arctic with the Canadian Land Surface Scheme Incl Biogeosciences, 18, 3263–3283, 2021 https://doi.org/10.5194/bg-18-3263-2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. Simulating shrubs and their energy and carbon dioxide fluxes in Canada’s Low Arctic with the Canadian Land Surface Scheme Including Biogeochemical Cycles (CLASSIC) Gesa Meyer1,2, Elyn R. Humphreys2, Joe R. Melton1, Alex J. Cannon1, and Peter M. Lafleur3 1Environment and Climate Change Canada, Climate Research Division, Victoria, BC, Canada 2Geography and Environmental Studies, Carleton University, Ottawa, ON, Canada 3School of Environment, Trent University, Peterborough, ON, Canada Correspondence: Gesa Meyer ([email protected]) Received: 9 December 2020 – Discussion started: 29 December 2020 Revised: 9 April 2021 – Accepted: 18 April 2021 – Published: 2 June 2021 Abstract. Climate change in the Arctic is leading to shifts The revised model, however, tends to overestimate gross pri- in vegetation communities, permafrost degradation and al- mary productivity, particularly in spring, and overestimated teration of tundra surface–atmosphere energy and carbon (C) late-winter CO2 emissions. On average, annual net ecosys- fluxes, among other changes. However, year-round C and en- tem CO2 exchange was positive for all simulations, suggest- −2 −1 ergy flux measurements at high-latitude sites remain rare. ing this site was a net CO2 source of 18 ± 4 g C m yr This poses a challenge for evaluating the impacts of climate using shrub PFTs, 15 ± 6 g C m−2 yr−1 using grass PFTs, change on Arctic tundra ecosystems and for developing and and 25 ± 5 g C m−2 yr−1 using tree PFTs. These results high- evaluating process-based models, which may be used to pre- light the importance of using appropriate PFTs in process- dict regional and global energy and C feedbacks to the cli- based models to simulate current and future Arctic surface– mate system. Our study used 14 years of seasonal eddy co- atmosphere interactions. variance (EC) measurements of carbon dioxide (CO2), water and energy fluxes, and winter soil chamber CO2 flux mea- surements at a dwarf-shrub tundra site underlain by continu- ous permafrost in Canada’s Southern Arctic ecozone to eval- Copyright statement. The works published in this journal are uate the incorporation of shrub plant functional types (PFTs) distributed under the Creative Commons Attribution 4.0 License. This license does not affect the Crown copyright work, which in the Canadian Land Surface Scheme Including Biogeo- is re-usable under the Open Government Licence (OGL). The chemical Cycles (CLASSIC), the land surface component of Creative Commons Attribution 4.0 License and the OGL are the Canadian Earth System Model. In addition to new PFTs, interoperable and do not conflict with, reduce or limit each other. a modification of the efficiency with which water evaporates from the ground surface was applied. This modification ad- © Crown copyright 2021 dressed 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 wa- 1 Introduction ter stress effects. Compared to the grass and tree PFTs previ- ously used by CLASSIC to represent the vegetation in Arctic The terrestrial carbon (C) cycle of the Arctic is changing as permafrost-affected regions, simulations with the new shrub the region warms at more than twice the rate of the rest of PFTs better capture the physical and biogeochemical im- the world (IPCC, 2014; Post et al., 2019). Enhanced Arctic pact of shrubs on the magnitude and seasonality of energy soil C loss to the atmosphere and waterways is linked to per- and CO2 fluxes at the dwarf-shrub tundra evaluation site. mafrost degradation and thermokarst processes, deeper ac- tive layers, deeper snow, and more frequent and intense fires Published by Copernicus Publications on behalf of the European Geosciences Union. 3264 G. Meyer et al.: Simulating high-latitude shrubs at a dwarf-shrub tundra site (Hayes et al., 2014; Abbott et al., 2016; Myers-Smith et al., to project C flux trends into the future (McGuire et al., 2018; 2019, 2020; Belshe et al., 2013b; Turetsky et al., 2020). A Post et al., 2019). Model estimates of C budgets are espe- significant positive climate feedback effect is possible if even cially important in the Arctic, as year-round flux measure- a small proportion of the approximately 1035 ± 150 Pg C ments are difficult to obtain and hence remain rare. stored in the top 3 m of Arctic soils (Hugelius et al., 2014) is It is critical that land surface models are able to capture transferred to the atmosphere (Chapin et al., 2005; Schuur the important interactions between the Arctic tundra and the et al., 2009; Hayes et al., 2014). However, longer grow- atmosphere due to potential feedbacks on regional and global ing seasons and Arctic “greening” associated, in part, with climate. The Canadian Land Surface Scheme Including Bio- northward migration of the tree line and shrub expansion are geochemical Cycles (CLASSIC, the open-source community linked to increased growing season carbon dioxide (CO2) up- model successor to CLASS-CTEM; Melton et al., 2020) rep- take (Belshe et al., 2013a; Abbott et al., 2016; Myers-Smith resents the land surface exchanges of energy, water, and C in et al., 2011). the Canadian Earth System Model (CanESM) (Swart et al., There are large uncertainties as to whether the Arctic tun- 2019) and has been extensively evaluated on a global scale dra is currently an annual source or sink of CO2 (McGuire (e.g., Melton and Arora, 2016; Arora et al., 2018). CLAS- et al., 2012). Recent studies have highlighted the importance SIC already focuses on several physical processes relevant of CO2 emissions during the long winter and shoulder sea- to the high latitudes, including treatment of snow and soil son periods, which may offset CO2 gains by Arctic ecosys- freeze–thaw processes (Melton et al., 2019) and peatland C tems during the short growing season (Belshe et al., 2013a; cycling (Wu et al., 2016). However, CLASSIC does not have Oechel et al., 2014; Euskirchen et al., 2017; Arndt et al., a plant functional type (PFT) specific to tundra. Instead tun- 2020). Long-term trends in winter CO2 fluxes are generally dra vegetation is represented by C3 grass and/or trees de- difficult to ascertain due to a scarcity of year-round measure- pending on land cover mapping. In reality, Arctic vegetation ments, but several studies have suggested winter CO2 emis- is diverse, consisting of erect or prostrate, evergreen or de- sions are changing. Belshe et al.(2013a) found a significant ciduous shrubs, graminoids, herbs, moss, and lichen (Chapin increase in winter CO2 emissions over the 2004–2010 time and Shaver, 1985). Although there are challenges in inter- period using observations from six pan-Arctic sites. Natali preting satellite-based trends of Arctic greening or browning et al.(2019) up-scaled in situ measurements from about 100 (Myers-Smith et al., 2020), observed increases in greenness high-latitude sites using a boosted regression tree machine or productivity have been linked to shrub expansion through learning model to estimate a contemporary loss of 1662 Tg C infilling of patches, advances in shrubline, and increased per year from the Arctic and boreal northern permafrost re- height and abundance of shrub species (Myers-Smith et al., gion (land area ≥ 49◦ N) from October to April (2003–2017). 2011; Lantz et al., 2012). These kinds of changes in tun- This winter loss of CO2 exceeded the average growing sea- dra vegetation communities can affect snow and active-layer son CO2 uptake estimated using five process-based mod- depths, hydrology, surface–atmosphere energy exchange, nu- els by over 600 Tg C per year. In addition to belowground trient dynamics, and the terrestrial C balance of the Arctic microbial respiration during the winter months, diffusion of tundra (Myers-Smith et al., 2011). stored CO2 produced during the non-winter period could In order to further improve the representation of Arctic have contributed, to an unknown extent, to the observed win- surface–atmosphere interactions in CLASSIC, we evaluate ter CO2 emissions (Natali et al., 2019). Using hourly at- new dwarf deciduous and evergreen shrubs and sedge PFT mospheric CO2 measurements, early-winter respiration rates parameterizations with eddy covariance (EC)-based observa- from Alaska’s North Slope tundra region are estimated to tions of CO2 and energy fluxes at an erect dwarf-shrub tundra have increased by 73 ± 11 % since 1975 (Commane et al., site in Canada’s Southern Arctic. We also address an evapo- 2017). However, both growing season and winter CO2 fluxes ration (E) bias discovered in CLASSIC, which has important from Alaska’s North Slope were not well represented by the implications for appropriately simulating energy flux feed- majority of 11 Earth system models (ESMs) from the Cou- backs and water-stress impacts on growing season photosyn- pled Model Intercomparison Project Phase 5 (CMIP5) inves- thesis. For example, Sun and Verseghy(2019) found that soil tigated by Commane et al.(2017). Most models predicted the E was overestimated for mid-latitude shrublands during wet start of the growing season earlier than observed, underesti- periods in spring, which led to underestimation of evapotran- mated early-winter CO2 losses, and overestimated annual net spiration (ET) and photosynthesis in the summer. We com- CO2 uptake (Commane et al., 2017). pare our offline model simulations with C3 grass and tree pa- Field observations and process-based models are comple- rameterizations to highlight how shrubs uniquely affect CO2 mentary approaches to better understand Arctic CO2 sink and and energy exchange with the atmosphere. Finally, we use source dynamics. Field observations may be used to parame- CLASSIC to simulate winter CO2 fluxes and estimate the an- terize and evaluate process-based models (Zhang et al., 2014; nual CO2 budget for this tundra site, where winter flux mea- Commane et al., 2017; Park et al., 2018; Huntzinger et al., surements are challenging to obtain.
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