CO2 Fluxes Recover Faster from Summer Drought in Sub-arctic Sphagnum Peatland than in Tundra Heath, While Being Similar after Winter Freezing

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 (CO 2 ) 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 CO 2 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 (53–69%) across ecosystems and treatments. Importantly, this delayed recovery shifted the summertime net ecosystem exchange toward CO 2 sources (− 0.2 to − 0.3 μmol m −2 s −1 in peatland and − 0.3 to − 0.7 μmol m −2 s −1 in tundra heath), while the controls remained a sink (0.5 and 0.3 μmol m −2 s −1 , respectively). The combined effect of drought and freezing caused extensive shoot mortality of the dominant dwarf shrub Empetrum hermaphroditum (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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