Netherlands Polar Data Center

Abstract

Extreme climatic events are increasing in frequency and intensity in the Sub-arctic, a rapidly warming region crucial to global carbon-climate feedbacks. Ecosystem carbon dioxide (CO₂) fluxes can be greatly impacted by extreme events through both immediate and long-term legacy effects. As event intervals shorten, new extremes may coincide with incomplete recovery from previous ones; yet the consequences of multiple events remain poorly understood. We examined the recovery of tundra CO₂ fluxes relative to untreated controls following extreme drought, winter freezing and their combination. Mesocosms from two major sub-arctic ecosystem types, Sphagnum peatland and tundra heath, were subjected to a seven-week summer precipitation exclusion (2022) and subsequent winter freezing (- 20 °C for two weeks). During summer 2023, drought recovery differed markedly between ecosystem types: Sphagnum peatland had stronger Gross Primary Productivity (GPP) recovery than tundra heath (63 and 41 %, respectively). Winter freezing had a similar legacy in both ecosystems (40 and 48 % recovery in peatland and tundra heath, respectively). The combined effect of drought and freezing suppressed recovery to the lowest levels (29 % in peatland and 19 % in tundra heath) but did not differ statistically from freezing alone. In contrast with GPP, ecosystem respiration recovered more strongly (53 to 69 %) across ecosystems and treatments. Importantly, this delayed recovery shifted the summertime Net Ecosystem Exchange toward CO₂ sources (- 0.2 to - 0.3   in peatland and -0.3 to -0.7  in tundra heath), while the controls remained a sink (0.5 and 0.3  , respectively). The combined effect of drought and freezing caused extensive shoot mortality of the dominant dwarf shrub <i>Empetrum hermaphroditum</i> (63 % in peatland and 91 % in tundra heath. Overall, our results reveal substantial and persistent legacy effects of (multiple) extreme events, potentially amplifying carbon-climate feedbacks.

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Keywords

• CO2 fluxes
• Drought
• Extreme events
• Freezing
• Peatland
• Recovery
• Sub-arctic ecosystems
• Tundra heath