Respiratory loss during late-growing season determines the net carbon dioxide sink in northern permafrost regions

Abstract

Warming of northern high latitude regions (NHL, > 50 °N) has increased both photosynthesis and respiration which results in considerable uncertainty regarding the net carbon dioxide (CO2) balance of NHL ecosystems. Using estimates constrained from atmospheric observations from 1980 to 2017, we find that the increasing trends of net CO2 uptake in the early-growing season are of similar magnitude across the tree cover gradient in the NHL. However, the trend of respiratory CO2 loss during late-growing season increases significantly with increasing tree cover, offsetting a larger fraction of photosynthetic CO2 uptake, and thus resulting in a slower rate of increasing annual net CO2 uptake in areas with higher tree cover, especially in central and southern boreal forest regions. The magnitude of this seasonal compensation effect explains the difference in net CO2 uptake trends along the NHL vegetation- permafrost gradient. Such seasonal compensation dynamics are not captured by dynamic global vegetation models, which simulate weaker respiration control on carbon exchange during the late-growing season, and thus calls into question projections of increasing net CO2 uptake as high latitude ecosystems respond to warming climate conditions.

Permafrost Carbon: Progress on Understanding Stocks and Fluxes Across Northern Terrestrial Ecosystems

“…Q. Luo et al, 2012). Additionally, top-down atmospheric inversion models are constrained by atmospheric data where concentration changes are linked to flux and atmospheric transport and are often spatially coarser than the bottomup approaches (Bruhwiler et al, 2021;Byrne et al, 2023;Z. Liu et al, 2022).…”

“…Atmospheric inversion model ensembles are an integral part of determining global CO 2 and CH 4 budgets as they aggregate natural terrestrial and aquatic as well as anthropogenic sources over large domains (Friedlingstein et al, 2022;Saunois et al, 2020). Full ensembles have been less frequently used in the permafrost region where atmospheric inversions have a large model spread in CO 2 and CH 4 fluxes due to differing transport models, priors, and observations (Bruhwiler et al, 2021;Z. Liu et al, 2022).…”

“…Emissions occurring during this extended period can add up to a substantial annual flux, up to 50% of annual ER and CH 4 emissions (Celis et al., 2017; Hashemi et al., 2021; Treat et al., 2018b; Zona et al., 2016). At some sites, the non‐growing season CO 2 emissions currently offset or exceed growing season uptake and ultimately determine the annual C balance (Hashemi et al., 2021; Z. Liu et al., 2022; Watts et al., 2021). However, only ca.…”

Carbon uptake in Eurasian boreal forests dominates the high‐latitude net ecosystem carbon budget

“…We compared the resulting TCFM-Arctic budgets with regional flux estimates (SI Section 8) from an Arctic-boreal version of the Community Land Model Version 5 (CLM 5; Birch et al, 2021); satelliteinformed SMAP L4_C and MODIS (MOD17A2H) CO 2 flux products (Kimball et al, 2012;Running et al, 2004); statistically upscaled CO 2 estimates from Virkkala et al (2021) and FluxCom (Jung et al, 2020); statistically upscaled CH 4 estimates from Peltola et al (2019), and results from atmospheric inversions-Atmospherically-enhanced Inversion (ACI) models (Liu et al, 2020(Liu et al, , 2022 and the v10 Orbiting Carbon Observatory-2 inversion modeling intercomparison project (v10 OCO-2 MIP; Byrne, Baker, et al, 2022;Byrne, Liu, et al, 2022) experiments. Additionally, we evaluated our terrestrial CH 4 emission estimates against assimilation records from CarbonTracker-CH 4 (Bruhwiler et al, 2014).…”

“…In contrast to the boreal zone, a majority of the models evaluated here (including TCFM-Arctic) indicated the tundra domain as being, on average, neutral or a small source for NEE. However, adjusting the TCFM-Arctic tundra NEE budget to account for a potentially large underestimation of episodic CO 2 emissions during spring and autumn shoulder seasons (Arndt et al, 2020;Byrne, Baker, et al, 2022;Byrne, Liu, et al, 2022;Commane et al, 2017;Liu et al, 2022;Schiferl et al, 2022) and source (when soils are warm and less wet) (e.g., Euskirchen et al, 2014;Laine et al, 2019;Olefeldt et al, 2017;Rinne et al, 2020;Schulze et al, 1999) depending on water table depth and soil wetness.…”