Ecosystem control over temperature and energy flux in northern peatlands

Large-scale changes in climate may have many unexpected effects on ecosystems, given the importance of climate as a control over almost all ecosystem attributes and their many internal feedbacks. In particular, the interactions among energy flux, plant dynamics, and soil carbon and nutrient cycling are poorly known. In this study, we examined biotic controls over soil temperature and evapotranspiration (ET) in a climate change experiment in two peatlands, a bog and a fen. Bogs are isolated from groundwater inputs (i.e., ombrogenous) and are acidic, whereas fens receive groundwater inputs (i.e., minerogenous) and are more alkaline. They also have many associated differences in soil chemistry, nutrient availability, and plant communities. We removed 27 intact peat monoliths each from a bog and a fen in northern Minnesota to construct a large mesocosm facility that allows for direct manipulation of climatic variables in a replicated experimental design. The treatment design was a fully crossed factorial with three infrared-loading treatments, three water table treatments, and two ecosystem types (bogs and fens), with three replicates of all treatment combinations. Heating was achieved with overhead infrared lamps, so soil temperature was a dependent variable. Increased infrared loading caused mean monthly soil temperature to increase by 1.6°-4.1°C at 15 cm depth during the growing season (May-October). The soil temperature response was strongly seasonal, with much greater effects during the growing season than during the winter. Within the growing season, heating caused greater increases in soil temperature in May-June in the bog plots and in May-June and September-October in the fen plots. Occasionally during the winter, increased infrared loading cooled the soil by up to 5.1°C, probably due to melting of the insulating snow cover. Fen plots were on average 0.8°-1.0°C warmer than bog plots during the growing season, although bog plots had a steeper soil temperature gradient with depth. Water level had no effect on soil temperature. Annual ET was 12-23% greater in the bog plots than in the fen plots, at least partially explaining the cooler soil temperatures in the bog plots due to latent heat of evaporation. Additionally, warmer, wetter conditions increased ET. In the fen plots, ET was more sensitive to water table depth than in the bog plots. Differences in the composition and seasonality of the plant canopies accounted for much of the observed differences in ET between bog and fen mesocosms, and hence soil temperature. We present a conceptual model of how above- and belowground ecosystem processes control energy fluxes in northern peatlands and suggest that such controls represent a general phenomenon that should be incorporated into climate change models that include biotic feedbacks