Statistical upscaling of ecosystem CO₂ fluxes across the terrestrial tundra and boreal domain: Regional patterns and uncertainties

Abstract

The regional variability in tundra and boreal carbon dioxide (CO2) fluxes can be high, complicating efforts to quantify sink‐source patterns across the entire region. Statistical models are increasingly used to predict (i.e., upscale) CO2 fluxes across large spatial domains, but the reliability of different modeling techniques, each with different specifications and assumptions, has not been assessed in detail. Here, we compile eddy covariance and chamber measurements of annual and growing season CO2 fluxes of gross primary productivity (GPP), ecosystem respiration (ER), and net ecosystem exchange (NEE) during 1990–2015 from 148 terrestrial high‐latitude (i.e., tundra and boreal) sites to analyze the spatial patterns and drivers of CO2 fluxes and test the accuracy and uncertainty of different statistical models. CO2 fluxes were upscaled at relatively high spatial resolution (1 km2) across the high‐latitude region using five commonly used statistical models and their ensemble, that is, the median of all five models, using climatic, vegetation, and soil predictors. We found the performance of machine learning and ensemble predictions to outperform traditional regression methods. We also found the predictive performance of NEE‐focused models to be low, relative to models predicting GPP and ER. Our data compilation and ensemble predictions showed that CO2 sink strength was larger in the boreal biome (observed and predicted average annual NEE −46 and −29 g C m−2 yr−1, respectively) compared to tundra (average annual NEE +10 and −2 g C m−2 yr−1). This pattern was associated with large spatial variability, reflecting local heterogeneity in soil organic carbon stocks, climate, and vegetation productivity. The terrestrial ecosystem CO2 budget, estimated using the annual NEE ensemble prediction, suggests the high‐latitude region was on average an annual CO2 sink during 1990–2015, although uncertainty remains high.

Model simulations of arctic biogeochemistry and permafrost extent are highly sensitive to the implemented snow scheme in LPJ-GUESS

“…Overall, the simulated pan-arctic carbon fluxes are systematically lower than other published values (Efren et al, 2019;Rawlins et al, 2015;McGuire et al, 2012). Virkkala et al (2021) estimate an annual NEE in the range of −46 to +10 g C m −2 yr −1 , while our simulations have a mean estimate around −35.5 g C m −2 yr −1 (see Table 2).…”

“…Despite numerous field-based and modelling efforts to date, it is still uncertain whether the Arctic will act as a carbon source or sink in the future (McGuire et al, 2012;Virkkala et al, 2021). The predicted future carbon balance varies widely among models -between a loss of 641 Pg C A.…”

Plant uptake of CO2outpaces losses from permafrost and plant respiration on the Tibetan Plateau

“…The sensitivity analyses, manipulated field experiments, and model simulations all consistently show that the fixation of CO 2 by plants outpaces the potential loss of CO 2 from permafrost and accelerated plant respiration in current conditions. The fixation of CO 2 by vegetation has also led to an accumulation of soil CO 2 (6) in the surface layers across the Tibetan Plateau, as shown by 137 Cs- 14 C dating (44,45), 137 Cs-140 Pb ex dating (46), and a repeatsampling approach (6), reporting a net soil C accumulation of 25 to 113 g C m −2 y −1 . Together, these methods have reported an average net soil C accumulation rate of 53.6 g C m −2 y −1 based upon isotope dating, indicating that roughly half of the C newly fixed by plants is stored in soils.…”

“…A recent study showed that the Pan-Arctic permafrost is a net CO 2 source of 630 Tg C y −1 (12), albeit with strong uncertainties (range: 15 to 975 Tg C y −1 ). By contrast, a synthesis study and model simulations reported that the Arctic can act as a CO 2 sink at the ecosystem level (8,13,14), given that the uptake of CO 2 by plants is accelerating as a result of Arctic greening. These contrasting results reveal the large uncertainties in feedbacks to the present-day warming climate across Arctic permafrost ecosystems.…”

Impacts of Permafrost Degradation on Carbon Stocks and Emissions under a Warming Climate: A Review

“…An inconsistent regional estimation of carbon sink and/or source potentials under a warming climate has been reported for regions of degrading permafrost with varied ecosystem types. Virkkala et al (2021) estimated the boreal regions as a carbon sink based on a net ecosystem exchange (NEE) of −46 g C m −2 yr −1 , because of the photosynthetic offset of the released CO 2 (soil respiration) [81] and the Arctic tundra served as a carbon source at 10 g C m −2 yr −1 [82]. In the High-Arctic semi-desert regions, the carbon sink could strengthen by an order of magnitude under a warmer-wetter climate, but the summer carbon sink could be reduced by 55% under just a warming treatment [83].…”