Stimulation of both photosynthesis and respiration in response to warmer and drier conditions in a boreal peatland ecosystem

Abstract

Peatland ecosystems have been consistent carbon (C) sinks for millennia, but it has been predicted that exposure to warmer temperatures and drier conditions associated with climate change will shift the balance between ecosystem photosynthesis and respiration providing a positive feedback to atmospheric CO 2 concentration. Our main objective was to determine the sensitivity of ecosystem photosynthesis, respiration and net ecosystem production (NEP) measured by eddy covariance, to variation in temperature and water table depth associated with interannual shifts in weather during 2004-2009. Our study was conducted in a moderately rich treed fen, the most abundant peatland type in western Canada, in a region (northern Alberta) where peatland ecosystems are a significant landscape component. During the study, the average growing season (May-October) water depth declined approximately 38 cm, and temperature [expressed as cumulative growing degree days (GDD, March-October)] varied approximately 370 GDD. Contrary to previous predictions, both ecosystem photosynthesis and respiration showed similar increases in response to warmer and drier conditions. The ecosystem remained a strong net sink for CO 2 with an average NEP ( AE SD) of 189 AE 47 g C m À2 yr À1 . The current net CO 2 uptake rates were much higher than C accumulation in peat determined from analyses of the relationship between peat age and cumulative C stock. The balance between C addition to, and total loss from, the top 0-30 cm depth (peat age range 0-70 years) of shallow peat cores averaged 43 AE 12 g C m À2 yr À1 . The apparent long-term average rate of net C accumulation in basal peat samples was 19-24 g C m À2 yr À1 . The difference between current rates of net C uptake and historical rates of peat accumulation is likely a result of vegetation succession and recent increases in tree establishment and productivity.

Modeling Soil and Biomass Carbon Responses to Declining Water Table in a Wetland-Rich Landscape

“…However, the steady state of total carbon for the first 100 years following 40-cm water table decline was consistent with Sulman and others (2009) and Flanagan and Syed (2011), who observed no change in NEE over short time scales following drainage of that magnitude in peatlands. In simulations that included only soil effects, the model did predict substantial losses of carbon for both depths of water table decline, indicating that the major difference between our simulations and short-term, soil-focused studies was the inclusion of plant community changes.…”

“…Increased plant growth resulting from declining water table has been observed in field studies over inter-annual time scales (Sulman and others 2009;Flanagan and Syed 2011), although Strack and others (2006) observed variations in the response between hummock, hollow, and lawn microforms. Wetland succession studies over decadal time scales have observed large increases in biomass as sedge, shrub, or moss-dominated wetland landscapes are converted to forest (Laine and others 1995;Laiho and others 2003).…”

“…These regions also include significant areas of forested wetlands without thick peat layers, which contribute significantly to the regional carbon cycle. Because peat accumulation and preservation depends on anaerobic soil conditions resulting from high water tables, changes in water table affect CO 2 emissions, and peat decomposition rates (Moore and Knowles 1989;Freeman and others 1992;Junkunst and Fiedler 2007;Yurova and others 2007;Sulman and others 2009;Olivas and others 2010;Flanagan and Syed 2011). Some peatland modeling studies have identified significant positive feedbacks to climate warming due to increased peat decomposition (for example, Tarnocai 2006;Ise and others 2008).…”

“…Some peatland modeling studies have identified significant positive feedbacks to climate warming due to increased peat decomposition (for example, Tarnocai 2006;Ise and others 2008). However, field and laboratory studies suggest that hydrological change also leads to changes in plant communities (Laine and others 1995;Weltzin and others 2003;Strack and others 2006;Talbot and others 2010), which can significantly affect the net CO 2 budget over the short term (Sulman and others 2009;Flanagan and Syed 2011). Studies of long-term responses to drainage have highlighted the importance of aboveground carbon accumulation, which in some cases can outweigh the loss of carbon from increased peat decomposition (Minkkinen and Laine 1998;Lohila and others 2011).…”

Substrate potential of Eemian to Holocene permafrost organic matter for future microbial greenhouse gas production

“…Schuur et al (2008)) and negative (e.g. Flanagan and Syed (2011)) feedback loops between climate warming and the carbon cycle in the Arctic leads to considerable uncertainties in how far the substrate potential in permafrost deposits plays a key role for future greenhouse gas production concerning shifts in the microbial 25 community composition, vegetation, hydrogeology and soil thermal regime. This study shows that OM especially deposited during the interstadial and glacial periods appear to contain a larger substrate potential than the interglacial deposits.…”