Ecosystem Control over Temperature and Energy Flux in Northern Peatlands

Abstract

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 1.6Њ-4.1ЊC 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 5.1ЊC, probably due to melting of the insulating snow cover. Fen plots were on average 0.8Њ-1.0ЊC 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.

Evaluation of Remotely Sensed Evapotranspiration Over the CEOP EOP-1 Reference Sites

“…2 and Table 4, it is shown that both the climate and the vegetation type have an important control over the daytime surface heat flux patterns. It is true that similar conclusion has been made by others before (Lafleur and Rouse 1995;Bridgham et al 1999), basically at a longer time scales, such as seasonal or annual. The finding here is still meaningful since it is confirmed again by a model (SEBS) which is driven by different input data (combination of surface meteorology and satellite data) to predict the terrestrial ET.…”

“…Both observations and theoretical simulation show that the climate condition and vegetation cover type have a major control on surface energy flux patterns (Lafleur and Rouse 1995;Bridgham et al 1999;MacKay et al 2002). At the same time, changes in these energy flux patterns could have a complex effect on the climate variability (Delworth and Manabe 1993) and may ''amplify or reduce the effects of potential climatic change'' (Eugster et al 2000).…”

Long‐term effects of climate change on vegetation and carbon dynamics in peat bogs

“…Some recent models (Frolking et al 2001(Frolking et al , 2002Nungesser 2003;Bauer 2004) acknowledge that Sphagnum decomposes more slowly than vascular plants but those models do not simulate changes in the species composition. However, interactions between species and feedbacks through species effects on nutrient and moisture availability (Berendse 1998(Berendse , 2005Bridgham et al 1999) may change the vegetation response to experimental treatments during a longer period Heijmans et al 2001b). Such feedbacks are particularly important in bogs, where the soil is entirely composed of plant remains and where both soil and vegetation have a large influence on hydrology.…”

“…Experimental studies have been conducted to study the response of Sphagnum bog plant communities to components of global change. In these experiments temperature (Bridgham et al 1999;Weltzin et al 2000Weltzin et al , 2003Gunnarsson et al 2004), atmospheric CO 2 Heijmans et al 2001b), water level (Bridgham et al 1999;Weltzin et al 2000Weltzin et al , 2003 and N deposition Heijmans et al 2001b;Gunnarsson et al 2004;Limpens et al 2004) have been manipulated during three or four growing seasons. All these studies showed that the main response was a shift in the relative abundance of species.…”

Environmental Impacts—Terrestrial Ecosystems

“…5). A long-term lowering of the water table due to increased evapotranspiration is expected to be modulated by changes in leaf area and the distribution of plant functional groups leading to increased surface resistance, reduced evapotranspiration and an attenuated fall in water tables (Bridgham et al 1999;Moore et al 2013). Desiccation of the moss layer during summer droughts can also lead to reduced evapotranspiration (Sottocornola and Kiely 2010).…”