The definition of the non-growing season matters: a case study of net ecosystem carbon exchange from a Canadian peatland

Climate change is a threat to the 500 Gt carbon stored in northern peatlands. As the region warms, the rise in mean temperature is more pronounced during the non-growing season (NGS, i.e., winter and parts of the shoulder seasons) when net ecosystem loss of carbon dioxide (CO2) occurs. Many studies have investigated the impacts of climate warming on NGS CO2 emissions, yet there is a lack of consistency amongst researchers in how the NGS period is defined. This complicates the interpretation of NGS CO2 emissions and hinders our understanding of seasonal drivers of important terrestrial carbon exchange processes. Here, we analyze the impact of alternative definitions of the NGS for a peatland site with multiple years of CO2 flux records. Three climatic parameters were considered to define the NGS: air temperature, soil temperature, and snow cover. Our findings reveal positive correlations between estimates of the cumulative non-growing season net ecosystem CO2 exchange (NGS-NEE) and the length of the NGS for each alternative definition, with the greatest proportion of variability explained using snow cover (R (2) = 0.89, p < 0.001), followed by air temperature (R (2) = 0.79, p < 0.001) and soil temperature (R (2) = 0.54, p = 0.006). Using these correlations, we estimate average daily NGS CO2 emitted between 1.42 and 1.90 gCO(2) m(-2), depending on which NGS definition is used. Our results highlight the need to explicitly define the NGS based on available climatic parameters to account for regional climate and ecosystem variability.