Carbon dioxide fluxes in a northern fen during a hot, dry summer

Abstract

Atmospheric gradient techniques were used to measure the net ecosystem exchange of CO2 for a subarctic sedge fen near Churchill, Manitoba, during the summer of 1994. This was the second driest and wannest summer since 1943. The mean daily temperature was 2°C above average, and the rainfall was 55% below normal. More than half of the rain fell after the main growth period. The fen landscape comprises hummocks and hollows. Equilibrium retention storage occurs at an average standing water depth of 80 mm above the hollow bottoms (water table reference is O). During the summer of 1994 the average water table position at −117 mm resided well below the zero equilibrium retention depth. Periodically this decreased to −265 mm, well below a 30‐year average depth of −70 mm. During the full summer period, measurements indicate that the fen was a source of CO2. Only during a relatively short period of most active photosynthesis in midseason was there a small net CO2 uptake. A deep and warm soil aerobic layer promoted a large respiration flux, and this exceeded the photosynthetic CO2 uptake of the stressed sedge community. Diurnally, changes in surface temperature and incident solar radiation can explain most of the changes in the net CO2 exchange. It is hypothesized that in 1994 photosynthesis was significantly decreased and the respiration loss enhanced by the hot, dry conditions. If this hypothesis is correct, by analogy, climate warming would need to be accompanied by a substantial rainfall increase to maintain a condition of net CO2 gain to this peatland.

Contrasting carbon dioxide fluxes between a drying shrub wetland in Northern Wisconsin, USA, and nearby forests

“…found that soil respiration increased in response to lowering water table, and several other studies have observed changes in wetland carbon balance from a net sink to a source during unusually warm, dry seasons (Shurpali et al, 1995;Schreader et al, 1998;Alm et al, 1999;Joiner et al, 1999;Bubier et al, 2003;Arneth et al, 2002). These studies reported short-term reductions in GEP attributed to water stress and early senescence, as well as temporary rises in ER or soil respiration.…”

Carbon exchange over four growing seasons for a subarctic sedge fen in northern Manitoba, Canada

“…At the same fen in 1993, Burton et al (1996) found that ER was nearly three times higher during a dry period (18-29 July) than during a wet period (30 July -24 August) but the water level differences might have been more drastic than those that we observed. Similarly, a drought period in 1994 at the fen at Churchill was 2°C warmer and rainfall was 55% lower than normal so that the water table dropped 25-30 cm below the mean sedge-peat surface; this decreased NEE to only −0.05 g C m −2 day −1 during the peak growing period of 22 June -8 August (Schreader et al 1998). Rouse et al (2002) reported on five years of NEE for the fen at Churchill in the late 1990s and found that the years with the largest CO 2 uptake by the ecosystem had warmer-than-normal air temperature and normal to wetter-than-normal precipitation, which supports our observations during the 2011 study period.…”

“…At a subarctic fen at Churchill, Manitoba, underlain by continuous permafrost, Burton et al (1996) reported that GPP and ER increased with warmer and drier conditions in late July compared to cooler and wetter conditions in early August of 1993. Schreader et al (1998) reported a net loss of 22 g C m −2 under unprecedented warm and dry conditions at the Churchill fen in the summer of 1994 (hottest and driest growing season in 50 years). They suggested that GPP was unaffected but ER increased during this year.…”

“…In addition to these sites, extensive flux studies have taken place in permafrost-dominated landscapes in the Northwest Territories (Lafleur and Humphreys 2008;Humphreys and Lafleur 2011), in the Hudson Bay Lowlands (Humphreys et al 2014), and at a subarctic fen at Churchill. The Churchill studies began in the 1990s (Burton et al 1996;Schreader et al 1998;Griffis et al 2000aGriffis et al , 2000bLafleur et al 2001;Rouse et al 2002) and have periodic continuity to recent years, although the methods have changed from gradient techniques to eddy covariance. At this same subarctic fen near Churchill, we previously reported the seasonal dynamics of methane fluxes for 2008-2011 measured using eddy covariance (Hanis et al 2013).…”

Arctic Ecosystem Responses to Changes in Water Availability and Warming: Short and Long-Term Responses

“…Warming is expected to increase evapotranspiration resulting in lower water tables that, in turn, will increase soil oxygen availability, microbial activity, and organic matter decomposition, reducing the Arctic's carbon sink potential (Schreader et al 1998). However, as a result of high spatial heterogeneity of the arctic landscape, differences in microtopography are likely to interact with temperature to control near the surface moisture changes (Engstrom et al 2005).…”