Sponsor: International Peat Society
Chair:
Dr. Nigel Roulet
Geography Department
Centre for Climate and Global Change Research
McGill University
805 Sherbrooke Ouest
Montréal, Québec
Canada H3A 2K6
Phone: (514) 398-4945
Fax: (514) 398-7437
E-mail: roulet@geog.mcgill.ca
Overview of Symposium:
Northern peatlands contain between 200 and 450 Pg of carbon (1 Pg = 1 Gt = 1015 g). The contemporary sequestration of carbon in peatlands is believed to be between 20 and 30 g C/m2 /yr. The sink of CO2 results from net primary production (NPP) persistently exceeding the decomposition of peat and plant material. Decomposition is greatly reduced in peatlands by the presence of water and the cool temperatures common to the latitudes where most peatlands are located. Accumulation of peat continues until and equilibrium is reached. The maximum growth of a peatland is controlled by a combination of internal feedbacks and external forcings such as climate, topography, and geographic setting.
Most general circulation models (GCMs) suggest the northern regions that contain most of the world's peatlands may become significantly warmer over the next century. GCMs also indicate, but with greater uncertainty, that the mid-continental areas may be much drier, while coastal climate region may become wetter. Since both NPP and decomposition in peatlands is closely related to the moisture and thermal regime, if the predicted changes in climate occur, we would expect to see significant alterations in the carbon dynamics of peatlands. However, there are many uncertainties in magnitude and the direction of potential changes. Over the last few years there has been a consider increase in research that is attempting to better understand the carbon dynamics of northern peatlands. In this special symposium, results of some this recent work will be presented. Proposed topics for six or twelve presentation are listed below.
First Session
Jukka Laine, Department of Forest Ecology, Helsinki University, Helsinki, Finland - Carbon balance in northern peatlands and global change.
It is presently not possible to give quantitative estimates of the response of carbon cycle in boreal and subarctic peatlands to predict climatic warming. It has been suggested that the impact will be largely indirect through alterations in the water table level. The predicted changes in temperature and hydrology might increase carbon accumulation in northern peatlands but might create a greater source of carbon dioxide in the more southern peatlands. Recent findings from southern and middle boreal peatlands indicate that the response is connected to present vegetation types and moderate drying might create an enhanced carbon sink in nutrient poor peatlands of these regions. The knowledge of the decrease in the amount of carbon lost as methane from peatland ecosystems after drying is well-established.
Peter Lafleur, Department of Geography, Trent University, Peterborough, Ontario, Canada -Annual net ecosystem exchange of CO2 at a boreal bog.
Peatlands contain about one third of the global soil carbon stocks, which is also about 60% of the atmospheric carbon pool. Therefore, their annual exchange of carbon dioxide is of potential significance for the global carbon cycle and climatic change. Although northern peatlands are considered to have been persistent sinks of carbon over the last 6,000 to 8,000 years, recent studies of growing season carbon balance indicate a variety of results, ranging from net sinks to sources. This paper discusses the first measurements of the full annual CO2 balance for a northern peatland. Eddy covariance measurements of net ecosystem exchange have been conducted for two years at the Mer Bleue bog near Ottawa, Ontario. Mean daily growing season uptake of CO2 (1.9-3.6 g /m2 /d) was comparable with other studies. Despite consistent loses of CO2 during the winter (about 0.8 g /m2 /d), the accumulated sink for the seven month growing period produced annual uptakes between 200-300 g /m2. The interannual variability in the net ecosystem exchange is discussed in relation to water balance in the soil and the winter loss. Since the measured carbon sink at Mer Bleue is significantly larger than historical estimates of carbon accumulation, the current results have important implication for the role of peatlands in the global carbon cycle.
Patrick Crill, Complex Systems Research Center, University of New Hampshire, Durham, New Hampshire, USA - A decade of carbon gas exchange between the atmosphere and a temperate poor fen.
Northern peatlands above 40o N have accumulated roughly 450 Gt C or about 30% of the total world pool of soil C even though aboveground net primary productivity is lower than that of other northern ecosystems. This near surface carbon storage could be significantly sensitive to, as well as directly affect, feedbacks driven by changing climate that determine the distribution of C between tropospheric and terrestrial reservoirs.
Development of long term datasets are important in evaluating the variable responses of natural systems to climatic forcing. Aspects of C exchange have been measured in Sallie's Fen, NH, U.S.A. since 1989. The objectives are to demonstrate relationships between photosynthetically active radiation, temperature and net C exchange and to utilize these relationships to interpolate exchange rates between infrequent direct chamber measurements and to extrapolate the observations between years. The techniques are important to quantify the magnitude and variability in C balance over annual and multi-year timescales.
Results of the C balance model suggest that Sallie's Fen has been a sink of CO2 from the atmosphere with most (52 - 91 gC-CO2 /m2) fixed in the summer and a consistent loss of 12-19 gC-CO2 /m2 in the winter. However, when CH4 emissions (48-122 gC-CH4 /m2) and DOC export (3 gC /m2) are added to the total, Sallie's Fen lost C in 1994 and was a weak sink of C in other years.
Steve Frolking, University of New Hampshire, Durham, New Hampshire, USA; Nigel T. Roulet , Tim R. Moore, and B. Ouyang, McGill University, Montreal, Québec, Canada; J.L. Bubier, Université de Québec, Montreal, Québec, Canada; P.J.H. Richard, Université de Montréal, Montréal, Québec, Canada; and P.M. Crill, Trent University, Peterborough, Ontario, Canada - Modeling the carbon balance of northern peatlands.
We present a new, process-oriented model of the contemporary carbon balance of northern peatlands. Components of the ecosystem model are: (1) vascular and non-vascular plant photosynthesis and respiration, net above- and below-ground production and litterfall; (2) aerobic and anaerobic decomposition of organic matter in the peat profile; (3) production, oxidation, and net flux of methane; and (4) dissolved organic carbon (DOC) loss with drainage water. The model operates on an hourly/daily time step, and is driven by air and soil temperatures, soil moisture profile and drainage. In typical runs these driving variables are generated by a peatland parameterization of the Canadian Land Surface Scheme (CLASS) coupled to a Local Climate Model (LCM). Simulations predict a complete peatland C balance for one season to several years. Model evaluation uses eddy covariance tower (CO2) and/or surface chamber flux (CO2, CH4) measurements from Mer Bleue Peatland near Ottawa, Ontario, the BOREAS Northern Fen site near Thompson, Manitoba, and Sallies Fen in Barrington, NH, USA.
Torben R. Christensen, Department of Ecology, Lund University, Lund, Sweden - Biogenic controls on carbon cycling and methane emissions in northern wetland ecosystems.
Northern wetlands and wet tundra regions contain large amounts of organic carbon stored in the form of peat and their potential for exchange of greenhouse gases (CO2, CH4) with the atmosphere are therefore great. Many unknowns remain in relation to the pool sizes and turnover times in the transformation of carbon from fixation as inorganic CO2 through plant and microbial systems to the ultimate respiratory release as CO2 and CH4. In particular, an intimate linkage between primary production of vascular plants and CH4 formation have been shown in recent studies but the dynamics of this linkage is poorly understood. This presentation provides an overview of results from an EC project called CONGAS concerned with these issues. CONGAS is a multiscale project operating from monolith studies in the laboratory, over plot scale environmental manipulations at five northern wetland sites through to eddy correlation measurements of CO2 and CH4 flux in a central part of the West Siberian Lowlands. In the laboratory studies we document a closed budget for the carbon flow in a boreal bog over a period of more than four months. The results are obtained from a series of 14CO2 labeling experiments on intact "whole-ecosystem" monoliths kept under controlled conditions. A synthesis of the measurements at the different scales will be presented.
Lia Chasar, J. Chanton, P. Glaser, D. Siegel, and J. Rivers, Department of Oceanography, Florida State University, Tallahassee, Florida, USA - Carbon dynamics of large northern peatlands: radiocarbon and stable isotope analyses.
This study compares two systems (a fen and bog in Northern Minnesota) which are representative of the different hydrologic regimes and floral assemblages found across most northern peatlands. Radiocarbon and stable carbon isotope values were obtained for porewater DOC, DIC, CH4 and solid-phase peat. At both sites, DOC radiocarbon content is enriched relative to solid-phase peat by 150-300 per mil, indicating that DOC of recent origin is transported down into the peat column. Fen radiocarbon values for DIC and CH4 closely track the radiocarbon content of the DOC pool, indicating that recent plant production is the dominant substrate for microbial respiration. Bog DIC and CH4 radiocarbon values are similar and intermediate between DOC and solid-phase peat values, suggesting microbial utilization of a mixture of both modern and older substrates. The differences in radiocarbon results between systems, in conjunction with stable isotope analyses, highlight the influence of groundwater flow and vegetation on below ground carbon cycling.
B. Isernhagen, T.R. Moore, and N.T. Roulet, Geography Department, McGill University, Montreal, Québec, Canada - Effects of beaver pond drainage on CO2 and CH4 fluxes in a Canadian temperate peatland.
Beaver ponds are important parts of peatland landscapes, and have high fluxes of CO2 and CH4. This study was undertaken to determine the response of a beaver pond to drainage (lowering by 25 cm) as a sink or source of carbon. Plants redistributed themselves along a new water table gradient. Each vegetation community and the remaining beaver pond were sampled for fluxes of CO2 and CH4 from May-November, 1999. Variations in CO2 flux amongst the sites along the gradient were related to species composition, PAR and peat temperature and water table. A seasonal estimate of net ecosystem exchange of CO2 was made. CH4 fluxes ranged from 0 to 1 g /m2 /d, increasing from the beaver pond margin to the open water surface. We integrated the measurements to an areal estimate of CO2 and CH4 flux for the drained beaver pond.
S. Gratzel, T. Moore, N. Roulet, M. Dalva, and D. Van Dyk, Department of Geography, McGill University, Montreal, Québec, Canada - The effect of restoration on carbon cycling in bog, eastern Québec.
We examined the effect of drainage/harvesting and varying types of restoration on the exchange between the peat surface and the atmosphere of carbon dioxide (CO2) and methane (CH4) and the concentration and chemistry of dissolved organic carbon (DOC). We used natural bog, actively harvested, abandoned, block-cut and pro-actively restored sites near Riviere du Loup, eastern Quebec. From May to October 1999, all sites were net sources of CO2. The unvegetated harvested site released the lowest and the abandoned site emitted the highest amount of CO2. Under bright sunshine, communities with Eriophorum spissum and wet Sphagnum spp. sequestered CO2. CH4 emission rates were small (<10 mg /m2 /d) except where flooded or where Eriophorum spissum dominated, resulting in rates of <2 g /m2 /d. The DOC concentration in the pore water of the different sites did not vary considerably (range 50-100 mg/l), except at 1 and 1.5 m depth of the block-cut site where substantially increased amounts of DOC were found (<500 mg/l).
Yu Zhang, McGill University, Montreal, Quebec, Canada; C. Li, University of New Hampshire, Durham, New Hampshire, USA; C.C. Trettin, USDA Forest Service, Charleston, South Carolina, USA, and G. Sun, North Carolina State University, Ralieigh, North Carolina, USA - Modelling soil carbon dynamics of forested wetlands.
Modeling soil carbon dynamics is important for assessing biogeochemical cycling of nutrients, and the carbon balance with respect to global change processes or land management practices. Although wetlands occupy a small proportion of the terrestrial surface of the earth (< 3%), they are an important terrestrial carbon pool (15-30%) and a major atmospheric methane source (21%). To fill a void in soil carbon modeling capabilities, we have developed an integrated model designed specifically for forested wetlands. The model uses DNDC, a denitrification-decomposition model, as the foundation and incorporates wetland hydrology, and aerobic and anaerobic forest soil processes for carbon and nitrogen. Above and below ground biomass production are simulated by a generalized forest ecosystem model (PnET). The model is designed for both mineral and organic soils, multiple spatial scales, and assessing ecological linkages with upland ecosystems.
N. Roulet, T.R. Moore, and D. Hilbert, Departmnt of Geography, McGill University, Montreal, Québec, Canada - Sensitivity of the carbon stored in peatlands to climate variability and change.
The accumulation of carbon in peatlands occurs because net primary production (NPP) exceeds decomposition. Hilbert, Roulet & Moore (1999) developed the Peatland Accumulation Model (PAM) to examine of the relationship between peatland hydrology and net carbon accumulation. PAM contains explicit functions relating NPP and decomposition to the position of the water table. We varied precipitation over time, before and after the peatland had reached a steady state, and compared the results with base runs where precipitation was held constant. Small, random changes in precipitation have little effect on carbon storage. In contrast, larger persistent changes result in substantial changes in carbon store compared to the base run. Carbon loss occurs with both increased and decreased precipitation. Using realistic changes in precipitation based on the past reconstruction or future predictions no modeling scenario resulted in a runaway loss of carbon.
Carl C. Trettin, USDA Forest Service, Charleston, South Carolina, USA - Soil carbon in forested wetlands: how much is really there?
Wetlands are recognized to contain a significant proportion of the terrestrial carbon pool (18-30%). However, the estimate of soil C storage in wetlands is sensitive to estimates of depth, bulk density, carbon density, and area. Changes and some improvements in these parameters reduce the current estimate of total soil C storage. Previous estimates of soil C storage did not include subsoil C below peatlands, or account for storage in microtopography; these components may add 5-15% to the total soil pool. Many wetlands are managed for agriculture and silviculture, with the presumption that soil C pools are reduced. However, increases in site productivity may enhance C accumulation in peatlands. Determination of soil gains and losses is conceptually easy, but difficult to ascertain in practice. Accordingly, if wetland restoration and management are going to be considered an option for enhancing C sequestration, expanded efforts are needed to address the considerable uncertainties about the factors regulating C accumulation and turnover. Studies using gas exchange methods and well designed long-term experiments are needed to determine the C balance in forested wetlands. Correspondingly, improved inventories are needed to facilitate application and extension of research data.
Second Session
Tim Moore, Department of Geography and the Centre for Climate and Global Change Research, McGill University, Montreal, Québec, Canada - Plant production and decomposition in carbon cycling.
Plant production and decomposition rates are primary controls on the rates of C accumulation in northern peatlands. Above-ground Net Primary Production ranges from 100 to 800 g /m2 /yr, influenced by environmental controls such as trophic status, water table position and temperature. Below-ground biomass and production are the major gaps in our knowledge and are likely to be several times larger than above-ground. Initial substrate composition plays a dominant role in controlling rates of plant tissue decomposition. Exponential decay constants (k) from 6-year litter bag studies in eastern Canada range from -0.22 to -0.68 for deciduous tree leaves and herbs, to -0.20 to -0.37 for sedges, -0.15 to -0.21 for shrub leaves and -0.02 for hummock and -0.08 for hollow/lawn Sphagnum. Initial placement conditions play a minor role in determining early decomposition rates of surface litter, but do affect subsurface decomposition rates. Peat decomposition is controlled by temperature, substrate characteristics and oxic/anoxic conditions. Oxic:anoxic decomposition ratios vary greatly in the literature (2:1 to 30:1) and need to be better defined. Evidence for the effect of N deposition on changing production and decomposition rates in peatlands is reviewed.
R. Kelman Wieder, Department of Biology, Villonova University, Villanova, Pennsyvania, USA - Empirical modeling of present and future carbon balance of Sphagnum peatlands.
Northern hemisphere boreal and subarctic peatlands store 455 Pg of C, are believed to be net sinks for atmospheric CO2 (23 Tg/yr), and are located in regions expected to experience considerable warming as atmospheric CO2 concentrations increase. Predicting how peatland C balance will respond to changing climate requires models. Recent past (150-200 yr) net C accumulation as peat can be quantified from , 210Pb-dating of cores. From such information, we developed a mass-balance, bookkeeping model that estimates both net primary production (NPP) and depth-dependent decomposition (exponential decay k values) for a particular location. NPP and k values were estimated for eight North American peatlands, ranging in mean annual temperature from 0.2 to 8.7 oC and in mean annual precipitation from 465 to 1202 mm. Correlations indicate that with increasing temperature and/or increasing precipitation: NPP increases, decomposition in the upper 30 cm of peat increases, turnover of photosynthetically fixed C increases, but net C accumulation as peat is unaffected. In addition, future C accumulation can be modeled under scenarios in which NPP and/or k values change in response to climatic conditions. Model predictions indicate that only small decreases in NPP and/or increases in decomposition rates (10 % over 100 yr) could be sufficient to switch boreal peatlands from net sinks of atmospheric C to net sources.
John Pastor, Department of Biology, University of Minnesota, Duluth, Minnesota, USA; Scott Bridgham, Natural Resources Institute, University of Minnesota, Duluth, Minnesota, USA; Karen Updegraff, Department of Forest Resources, University of Minnesota, St. Paul, Minnesota, USA; Jason Keller, Department of Biological Sciences, University of Notre Dame, Notre Dame, Illinois, Duluth, Minnesota, USA; Peter Weishhampel, Department of Natural resources, Cornell University, Ithaca, NewYork, USA; Calvin Harth and Brad Dewey, Natural Resources Institute, University of Minnesota, Duluth, Minnesota, USA; and Jake Weltzin, Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, Tennessee, USA - Response of peatland ecosystems to climate change: a manipulative experiment.
We constructed a large mesocosm facility in northern Minnesota to examine the effects of climate change on peatlands. Twenty-seven intact peat monoliths (2.1 m2, 60-cm depth) were removed each from a bog and intermediate fen and subjected to three infrared-loading treatments and three water-table treatments. Net ecosystem respiration was dominated by CO2, which responded only to soil temperature and did not differ between bog and fen plots, suggesting that respiratory flux of CO2 in northern peatlands will directly increase with increases in temperature with climate change. Increased water table depth and increased infrared loading increased shrub production at the expense of bryophyte production in the bogs and at the expense of graminoid production in the fens, and concurrent with these shifts in species composition there was also an increased allocation of production below ground into woody roots. Shifts in species composition in response to climate change could therefore have a major influence on the carbon balance of peatlands. These changes in respiration and net primary production and its allocation resulted in a net loss of carbon from the fens but a net gain in carbon in the bogs. Therefore, differential responses of peatlands at local, community-level scales could have major implications for carbon balances at regional scales in northern latitudes.
Sanna Saanio, Department of Biology, University of Joensuu, Joensuu, Finland; J. Juahainen, Department of Forest Ecology, University of Helsinki, Helsinki, Finland; T. Saarinen, Department of Ecology and Systematics, University of Helsinki, Helsinki, Finland; H. Vasander, Department of Forest Ecology, University of Helsinki, Helsinki, Finland; and J. Silvola, Department of Biology, University of Joensuu, Joensuu, Finland - Response of Sphagnum mosses to increased CO2 concentration and NH4NO3 availability
Effects of increased CO2 concentration and NH4NO3 availability on photosynthesis, growth (length, biomass), nutrient concentrations in tissues and uptake rates of NH4+ and NO3- ions in different Sphagnum species were studied in several laboratory experiments. Net CO2 uptake of Sphagnummosses increased in raised CO2 concentration so that the temperature/light optimum was shifted to a higher value. Raised CO2 concentration seemed to increase photosynthesis also in low irradiation levels causing decrease in the light compensation point. The typical decrease in the CO2 uptake rate at high water contents, found at lower CO2 concentrations, disappeared gradually under higher CO2 concentrations. Photosynthesis of mosses acclimated to high CO2 concentration under prolonged exposure to raised CO2. Increased availability of NH4+ and NO3- ions increased N concentration in tissues of Sphagnummosses. Concentrations of chlorophyll and amino acids such as arginine, asparagine and glutamic acid were also increased in tissues under increased N input. Accumulation of NH4+N in tissues lowered the ability to reduce nitrate. Depending on Sphagnumspecies, photosynthesis, plant density, dry mass, length increment or species vitality either increased or decreased under increased N input. Duration of the N treatment, added amount and form of the supplied nitrogen also affected species' response.
Richard Clymo, School of Biological Sciences, Queen Mary and Westfield College, London, United Kingdom - Modeling the long-term carbon dynamics of peatlands.
It is easy to make models of carbon accumulation over millennia. The main variables are rate of production and rate of decay. The simple models - ones that assume constant, or at least predictable, environment - have explicit solutions but even the most complex can be simulated with a computer. What USE are these models?
The simple ones answer the question 'what if?', sometimes in a surprising way, and thus enlarge understanding. If their results accord with Nature then our confidence increases, but the assumption of constant conditions over millennia is untrue in detail at least. We may hope for agreement with Nature but we should not EXPECT it.
To perfect the fit we need only simulate temporal variation of one or more parameters, or introduce more parameters or variables. Each simulation is specific and the understanding that comes from generalizing is vaguer than that from an explicit solution. This is the rapid route to frivolity.
I use the two-layer (acrotelm/catotelm) model with three assumptions about decay processes and two about productivity as illustrations.
Jukka Turunen, Department of Biology, University of Joensuu, Joensuu, Finland - Carbon accumulation in boreal and subarctic mires.
Stratigraphical analyses based on 1302 dated peat cores from Finland and 128 cores from Canada and Russia were used to infer the average carbon accumulation rates for natural mires in boreal and subarctic regions. Peat columns were also used to estimate rates of growth and decay of peat. The average long-term apparent rate of carbon accumulation (LORCA) for natural Finnish mire areas was 18.5 g /m2 /yr. The average LORCA of the bog sites in West Siberia was 17.1 g /m2 /yr. The average LORCA for boreal and subarctic mires of Canada was 28.7 g /m2 /yr. Mire fires can slow the progress of vertical peat accumulation, resulting in large C losses. The average rate of C loss in Patvinsuo mire, eastern Finland was 9.5 g /m2 /yr and the mean C loss in an individual fire was 2.5 kg /m2. Podzolic texture under mires is also a significant C sink, and may account for approximately 5% of the unaccounted C in the global carbon budget. Based on 273 soil profiles in Finland, the mean C density in the mineral subsoil of mires was 1.5-fold higher than in adjacent forest profiles. The average C input was 13.6 g /m2 /yr. The addition of dry mass to the catotelm and decay of peat material in the boreal mires of Canada were examined. There was a rather clear relationship between the rate of addition and the degree-days above 0C. There was a near-linear relationship on a logarithmic scale between the decay coefficient and mean annual temperature.
