Modeling the seasonality of belowground respiration along an elevation gradient in the western Chugach Mountains, Alaska

Abstract

Belowground respiration is typically the largest flux of carbon from terrestrial ecosystems to the atmosphere, making up >70% of total respiration in boreal forests. Recent work has shown that belowground respiration continues during the snow-covered season in boreal ecosystems, but few studies have made complementary measurements during the snow-free season and it remains uncertain what proportion of annual belowground respiration occurs during winter. Traditional models of the relationship between temperature and respiration assume fixed temperature sensitivity, but it has become clear that the apparent temperature sensitivity of belowground respiration increases as soils approach 0°C. Use of fixed temperature sensitivity to model carbon budgets of northern ecosystems may, therefore, yield misleading results. We measured belowground respiration monthly over 2 years in four ecosystems along an elevation gradient in south-central Alaska. Three models, representing different hypotheses about the relationship between temperature and respiration, were confronted with the data. A logistic model, which allows the temperature sensitivity to vary inversely with temperature, and a variation of the Q10 model, which allows the temperature sensitivity to vary seasonally, performed well at all sites and produced similar estimates of seasonal and annual belowground respiration. The traditional Q10 model performed poorly at all sites and overestimated respiration during the snow-covered season. Annual belowground respiration was generally greater than in ecosystems of interior Alaska, where winters are colder and summers are warmer and drier. Belowground respiration during the snow-covered season made up 6–15% of the annual total—a small, but sensitive, component of annual carbon budgets.