Impact of extreme temperature events on CO2 uptake in a hemiboreal forest in North America

Forested areas are a primary contributor of the terrestrial carbon sink; however, extreme temperature events, including both warm and cool anomalies, have been shown to influence carbon dioxide (CO₂) uptake in forested ecosystems. With the frequency and severity of these extreme temperature events increasing due to climate change, the CO₂ uptake response of forested ecosystems to these extreme temperature events may become increasingly impactful. Hemiboreal forests are of particular interest due to their unique geographical location in which the forests’ tree species are situated in their climatic limit; thus, hemiboreal forests have an increased susceptibility to the effects of climate change due to their location in the transition zone between temperate and boreal forests.

To quantify the response of forest-atmosphere CO₂ exchange of hemiboreal forest ecosystems to extreme temperature events, we analysed daily eddy-covariance net CO₂ fluxes, quantified by the net ecosystem exchange (NEE), measured at Howland Forest (Maine, U.S.A.) and the derived component fluxes gross primary production (GPP) and ecosystem respiration (R_eco). Using the 29-year dataset of daily CO₂ fluxes and corresponding meteorology, we derived daily CO₂ flux and temperature anomalies to assess the impact of extreme temperature events (i.e., days with air temperature greater than 2 standard deviations above/below the mean air temperature for that day of year).

Our results indicate that, on average, net CO₂ uptake is reduced in response to both extreme warm and cool events in the hemiboreal forest. We observed a statistically significant decrease in net CO₂ uptake (corresponding to positive NEE anomalies) during extreme warm and cool events in seven and five of the nine non-winter months (i.e., March to November), respectively. Particularly in the summer months, both extreme warm and cool events were associated with less CO₂ uptake. The NEE response in the non-winter months resulted from reduced GPP during both extreme warm and cool events as well as elevated R_eco during extreme warm events. Future analyses will investigate the impact of the frequency and magnitude of extreme temperature events on monthly and annual CO₂ budgets of this hemiboreal forest ecosystem.

These results demonstrate a decrease in net CO₂ uptake in response to extreme temperature events with potentially negative effects on the CO₂ sink strengths of hemiboreal forest ecosystems; this may reinforce a positive feedback loop with increasing air temperature decreasing CO₂ uptake in forested ecosystems, contributing again to increasing air temperatures. Our research helps to gain a more thorough understanding into the role that forested ecosystems play in terrestrial CO₂ sequestration in today’s changing climate.