Forest floor carbon pools and fluxes along a regional climate gradient in Maine, USA

Abstract

Global carbon (C) reserves in soil are large compared with atmospheric stocks (in the form of CO2 and methane), so small changes in soil C storage will have a significant effect on atmospheric CO2 concentrations. In order to better understand the consequences of global climate change, it is essential that we define how soil C storage is influenced by changes in temperature and moisture that are expected as a result of global climate change. Forest floor carbon pools and fluxes were evaluated at 16 northern hardwood sites located within four distinct climate regions (Northern, Central, Southern and Coastal) in Maine. Mean annual air temperature at the sites ranged from 2.0°C in the Northern region to 6.2°C in the Coastal region and average annual precipitation ranged from 90 cm in the Northern region to 140 cm in the Coastal region.Leaf litter mass and leaf litter C flux were not correlated with temperature indices and did not vary among regions. However, they were positively correlated with annual precipitation, suggesting that litter production was controlled, in part at least, by precipitation but not by temperature.Northern sites stored more C in the forest floor than Coastal sites, and they experienced slower decomposition rates. Because soil and vegetation characteristics of these sites were similar, we attribute these trends to differences in climate. Indeed, C turnover time was correlated with latitude and temperature indices. Slower decomposition in the Northern region was attributed to a combination of lower specific activity at temperatures below 13°C, cooler average temperatures and a shorter frost-free season.Soil respiration at each site was positively correlated with temperature and the slope of the relationship increased with latitude, indicating that the ability of the soil biota to respire C varied with climate. A predictive equation is presented that accounts for the change in slope with latitude.Because C loss through soil respiration was more sensitive to temperature than C inputs from litter, any regional warming in the next century may lead to a decrease in forest floor carbon storage. However, if precipitation increases with temperature, then litter C flux may increase and offset the increase in soil respiration.