Tussock Tundra Methane Fluxes Are Heterogeneous and Sensitive to Spring Conditions: An NGEE Arctic Study at Council, AK
Authors
Sigrid Dengel1* (sdengel@lbl.gov), Ori Chafe1,2, Misha Krassovski3, Jeff Riggs3, Margaret S. Torn1, Colleen Iversen3
Institutions
1Lawrence Berkeley National Laboratory, Berkeley, CA; 2University of Oregon, Eugene, OR; 3Oak Ridge National Laboratory, Oak Ridge, TN
URLs
Abstract
Arctic tussock tundra is often treated as one plant community for modeling and projecting greenhouse gas fluxes, but small-scale heterogeneity in thaw, and thus hydrology, can have large impacts on greenhouse gas fluxes. The team measured methane (CH4), carbon dioxide (CO2), energy exchange, and above- and belowground properties between 2017 and 2023 at the Next-Generation Ecosystem Experiments Arctic site near Council, AK (AmeriFlux site US-NGC). The northern flux tower footprint was moderately dry while the southern section contained inundated thermokarst features. Thaw depth, soil moisture, and temperature were distinctively different between these sectors. While the soil thawed faster in the non-thermokarst areas in spring as thermokarst features were often covered with snow into June, by July thermokarst features reached a thaw depth of 1 m almost a month before those in the drier, upland areas. While there was little interannual variation in meteorological conditions in summer, variation in spring conditions dictated CH4 fluxes at the site. The highest CH4 fluxes occurred in 2019, which had a mild, wet spring (snowmelt 24 May; growing season CH4 fluxes were 23.5 nanomoles per m2 per second in the south, 12 nmol m-2 s-1 in the north). In contrast, the lowest fluxes were in 2022 when spring was cooler and drier (snowmelt 29 April; CH4 fluxes were 7.8 and 4.2 nmol m-2 s-1 in wet and dry areas, respectively). In all years, the influence of thermokarst CH4 hot spots was clear in footprint-averaged fluxes. The areas also had different timescales of response to environmental conditions, with faster response in wet areas. Thus, tussock tundra, which is currently modeled as a single vegetation community, has highly heterogeneous CO2 and CH4 fluxes in space, seasonally, and interannually. These differences are associated with thermokarst and weather impacts on subsurface moisture and temperature, pointing to the need for better representation of thermohydrology in land models.