Abstract
Siberia experienced an unprecedented strong and persistent heatwave in winter to spring of 2020. Using bottom–up and top–down approaches, we evaluated seasonal and annual CO2 fluxes of 2020 in the northern hemisphere (north of 30 °N), focusing on Siberia where the pronounced heatwave occurred. We found that, over Siberia, CO2 respiration loss in response to the pronounced positive winter temperature anomaly was greater than in previous years. However, continued warming in the spring enhanced photosynthetic CO2 uptake, resulting in the largest seasonal transition in net ecosystem CO2 exchange; that is, the largest magnitude of the switch from the net CO2 loss in winter to net CO2 uptake in spring until June. However, this exceptional transition was followed by the largest reduction in CO2 uptake in late summer due to multiple environmental constraints, including a soil moisture deficit. Despite a substantial increase of CO2 uptake by 22 ± 9 gC m−2 in the spring in response to the heatwave, the mean annual CO2 uptake over Siberia was slightly lower (3 ± 13 gC m−2yr−1) than the average of the previous five years. These results highlight the highly dynamic response of seasonal carbon fluxes to extreme temperature anomalies at high latitudes, indicating a seasonal compensation between abnormal uptake and release of CO2 in response to extreme warmth that may limit carbon sink capacity in high northern latitudes.
Highlights
Global surface air temperature has increased rapidly since the beginning of the 20th century (0.08 ◦C per decade; Hartmann et al (2013)), but almost twice as fast in the Arctic (Serreze et al 2009, Cohen et al 2014)
From exposure to an average 3.1 ◦C warmer air temperature in Q1 compared to the years 1979–2019, soil thawed ca. 5 days earlier relative to 1979–2019 across Siberia, with central Siberia showing the largest anomaly since the start of the observation period in 1979 (figure 1(b))
The resulting soil moisture anomaly was comparable to the previous years until May but lower from June until the end of the year (figure 1(e)), especially in central and eastern Siberia (figures S1(c) and S2(b))
Summary
Global surface air temperature has increased rapidly since the beginning of the 20th century (0.08 ◦C per decade; Hartmann et al (2013)), but almost twice as fast in the Arctic (Serreze et al 2009, Cohen et al 2014). Seasonal thawing and the arrival of persistent warm temperatures in spring trigger leaf bud burst and flowering (Arft et al 1999, Aerts et al 2006, Ernakovich et al 2014), and can induce early onset of photosynthesis (gross primary production; GPP) and net CO2 uptake (negative values of net ecosystem CO2 exchange; NEE) by terrestrial ecosystems (Piao et al 2008, 2020, Park et al 2016, Li et al 2018, Parazoo et al 2018, Liu et al 2020). Ecosystem respiration (Reco) is more responsive to warming than GPP (Piao et al 2008), which can be attributed to constraints on photosynthesis such as photoperiod, moisture, nutrient availability, and genetic or physiological controls linked to sink limitations in plants (Arft et al 1999, Aerts et al 2006, Ernakovich et al 2014, Liu et al 2020, Zani et al 2020, Zhang et al 2020). The response to warming may differ by ecosystem (Welp et al 2007, Bastos et al 2020a, Flach et al 2021) and species (Babst et al 2012, Niu et al 2014)
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