Ecosystem control over temperature and energy flux in northern peatlands

Large-scale changes in climate may have many unexpected effects on ecosyste ms, given the importance of climate as a control over almost all ecosystem attributes and their many internal feedbacks. In particular, the interactio ns among energy flux, plant dynamics, and soil carbon and nutrient cycling are poorly known. In this study, we examined biotic controls over soil temp erature and evapotranspiration (ET) in a climate change experiment in two p eatlands, 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 diffe rences in soil chemistry, nutrient availability, and plant communities. We removed 27 intact peat monoliths each from a bog and a fen in northern M innesota to construct a large mesocosm facility that allows for direct mani pulation of climatic variables in a replicated experimental design. The tre atment design was a fully crossed factorial with three infrared-loading tre atments, three water table treatments, and two ecosystem types (bogs and fe ns), with three replicates of all treatment combinations. Heating was achie ved with overhead infrared lamps, so soil temperature was a dependent varia ble. Increased infrared loading caused mean monthly soil temperature to increase by 1.6 degrees-4.1 degrees C at 15 cm depth during the growing season (May -October). The soil temperature response was strongly seasonal, with much g reater effects during the growing season than during the winter. Within the growing season, heating caused greater increases in soil temperature in Ma y-June in the bog plots and in May-June and September-October in the fen pl ots. Occasionally during the winter, increased infrared loading cooled the soil by up to 5.1 degrees C, probably due to melting of the insulating snow cover. Fen plots were on average 0.8 degrees-1.0 degrees C warmer than bog plots during the growing season, although bog plots had a steeper soil tem perature 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 lea st partially explaining the cooler soil temperatures in the bog plots due t o latent heat of evaporation. Additionally, warmer, wetter conditions incre ased 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 pla nt canopies accounted for much of the observed differences in ET between bo g and fen mesocosms, and hence soil temperature. We present a conceptual mo del of how above- and belowground ecosystem processes control energy fluxes in northern peatlands and suggest that such controls represent a general p henomenon that should be incorporated into climate change models that inclu de biotic feedbacks.