Simulating the carbon cycling of northern peatlands using a land surface scheme coupled to a wetland carbon model (CLASS3W-MWM)

Northern peatlands store approximately one-third of the terrestrial soil carbon (C), although they cover only 3% of the global land mass Northern peatlands can be subdivided into bogs and fens based on their hydrology and biogeochemistry Peatland hydrology and biogeochemistry are tightly coupled to climate and, therefore, may be very sensitive to climate variability and change To address the fate of the large peatland soil C storage under a future changed climate, a peatland C model, the McGill Wetland Model (MWM), was coupled to a land surface climate model (the wetland version of the Canadian Land Surface Scheme, CLASS3W), referred as CLASS3W-MWM We evaluated the CLASS3W-MWM for a bog (Mer Bleue, located at 45 41°N, 75 48°W, in eastern Canada) and a poor fen (Degerö Stormyr, located at 64°11′N, 19°33′E, in northern Sweden) CLASS3W-MWM captured the magnitude and direction of the present day C cycling very well for both bogs and fens Moreover, the seasonal and interannual variability were reproduced reasonably well Root mean square errors (RMSE) were 0 8 for the components of net ecosystem production (NEP) for both the Mer Bleue bog and the Degerö Stormyr fen The performance of the coupled model for both bog and fen is similar to that of the stand-alone MWM driven by observed weather rather than simulated surface and soil climate This modelling study suggests that northern peatlands are hydrologically and thermally con

Net ecosystem CO2 exchange in a temperate cattail marsh in relation to biophysical properties

Net ecosystem exchange (NEE) of carbon dioxide (CO2) was measured at a temperate cattail marsh using the eddy covariance technique in order to examine the relationships between NEE, weather, and vegetation properties. Analyses of CO2 fluxes for a complete year (May 9, 2005 to May 30, 2006) showed that the marsh wetland was a net CO2 sink for each month from June to September (monthly 24-h averages of -0.1, -5.1, -4.8, and -2.2 g C m-2 day-1, respectively) and a source of CO2 to the atmosphere for the remaining fall and winter months. Wintertime (November to April) average NEE was 0.5 g C m-2 day-1. The annual cumulative CO2 balance shows a net uptake of 264 g C m-2 year-1 by the marsh. Ecosystem respiration (ER) and gross ecosystem production (GEP) were estimated to be 567 and 831 g C m-2 year-1, respectively. Respiration rates were associated with a Q10 value of 2.8. Peak aboveground biomass (average of 1156 g m-2) and peak green LAI (average of 3.63) were reached in mid-August. Variations in growing season NEE were well correlated with variations in live biomass (r = 0.92) and green LAI (r = 0.94)

Dealing with microtopography of an ombrotrophic bog for simulating ecosystem-level CO2 exchanges

Peatlands contain approximately 25% of the global soil carbon (C), despite covering only 3% of the earth's land surface. In order to evaluate the role of peatlands in global C cycling, models of ecosystem biogeochemistry are required, but peatland ecosystems present a number of unique challenges, particularly how to deal with the large variability that occurs at scales of one to several metres. In models, spatial variability is considered either explicitly for each individual unit and the outputs averaged, referred to as flux upscaling, or implicitly by weighting model parameters by the fractional occurrence of the individual units, referred to as parameter upscaling. The advantage of parameter upscaling is that it is much more computationally efficient: a requirement for hemispheric scale simulations. In this study we determined the differences between modelling a raised bog peatland with hummock-hollow microtopography using flux and parameter upscaling. We used the McGill Wetland Model (MWM), a process-based ecosystem C model for peatlands, configured for hummocks and hollows separately and then a weighted mixture of both. The simulated output base

Contemporary carbon balance and late Holocene carbon accumulation in a northern peatland

Northern peatlands contain up to 25% of the world's soil carbon (C) and have an estimated annual exchange of CO2-C with the atmosphere of 0.1-0.5 Pg yr-1 and of CH4-C of 10-25 Tg yr-1. Despite this overall importance to the global C cycle, there have been few, if any, complete multiyear annual C balances for these ecosystems. We report a 6-year balance computed from continuous net ecosystem CO2 exchange (NEE), regular instantaneous measurements of methane (CH4) emissions, and export of dissolved organic C (DOC) from a northern ombrotrophic bog. From these observations, we have constructed complete seasonal and annual C balances, examined their seasonal and interannual variability, and compared the mean 6-year contemporary C exchange with the apparent C accumulation for the last 3000 years obtained from C density and age-depth profiles from two peat cores. The 6-year mean NEE-C and CH4-C exchange, and net DOC loss are -40.2±40.5 (±1SD), 3.7±0.5, and 14.9±3.1 g m-2yr-1, giving a 6-year mean balance of -21.5±39.0 g m-2yr-1 (where positive exchange is a loss of C from the ecosystem). NEE had the largest magnitude and variability of the components of the C balance, but DOC and CH4 had similar proportional variabilities and their inclusion is essential to resolve the C balance. There are large interseasonal and interannual ranges to the exchanges due to variations in climatic conditions. We estimate from the largest and smallest seasonal exchanges, quasi-maximum limits of the annual C balance between 50 and -105 g m-2yr-1. The net C accumulation rate obtained from the two peatland cores for the interval 400-3000 bp (samples from the anoxic layer only) were 21.9±2.8 and 14.0±37.6 g m-2yr-1, which are not significantly different from the 6-year mean contemporary exchange

A Multi-Year Record of Methane Flux at the Mer Bleue Bog, Southern Canada

The Mer Bleue peatland is a large ombrotrophic bog with hummock-lawn microtopography, poor fen sections and beaver ponds at the margin. Average growing-season (May-October) fluxes of methane (CH4) measured in 2002-2003 across the bog ranged from less than 5 mg m-2 d-1 in hummocks, to greater than 100 mg m-2 d-1 in lawns and ponds. The average position of the water table explained about half of the variation in the season average CH4 fluxes, similar to that observed in many other peatlands in Canada and elsewhere. The flux varied most when the water table position ranged between -15 and -40 cm. To better establish the factors that influence this variability, we measured CH4 flux at approximate

Estimating Peatland water table depth and net ecosystem exchange: A comparison between satellite and airborne imagery

Peatlands play a fundamental role in climate regulation through their long-term accumulation of atmospheric carbon. Despite their resilience, peatlands are vulnerable to climate change. Remote sensing offers the opportunity to better understand these ecosystems at large spatial scales through time. In this study, we estimated water table depth from a 6-year time sequence of airborne shortwave infrared (SWIR) hyperspectral imagery. We found that the narrowband index NDWI1240 is a strong predictor of water table position. However, we illustrate the importance of considering peatland anisotropy on SWIR imagery from the summer months when the vascular plants are in full foliage, as not all illumination conditions are suitable for retrieving water table position. We also model net ecosystem exchange (NEE) from 10 years of Landsat TM5 imagery and from 4 years of Landsat OLI 8 imagery. Our results show the transferability of the model between imagery from sensors with similar spectral and radiometric properties such as Landsat 8 and Sentinel-2. NEE modeled from airborne hyperspectral imagery more closely correlated to eddy covariance tower measurements than did models based on satellite images. With fine spectral, spatial and radiometric resolutions, new generation satellite imagery and airborne hyperspectral imagery allow for monitoring the response of peatlands to both allogenic and autogenic factors

Variability in exchange of CO2 across 12 northern peatland and tundra sites

Many wetland ecosystems such as peatlands and wet tundra hold large amounts of organic carbon (C) in their soils, and are thus important in the terrestrial C cycle. We have synthesized data on the carbon dioxide (CO2) exchange obtained from eddy covariance measurements from 12 wetland sites, covering 1-7 years at each site, across Europe and North America, ranging from ombrotrophic and minerotrophic peatlands to wet tundra ecosystems, spanning temperate to arctic climate zones. The average summertime net ecosystem exchange of CO2 (NEE) was highly variable between sites. However, all sites with complete annual datasets, seven in total, acted as annual net sinks for atmospheric CO2. To evaluate the influence of gross primary production (GPP) and ecosystem respiration (Reco) on NEE, we first removed the artificial correlation emanating from the method of partitioning NEE into GPP and Reco. After this correction neither Reco (P = 0.162) nor GPP (P = 0.110) correlated significantly with NEE on an annual basis. Spatial variation in annual and summertime Reco was associated with growing season period, air temperature, growing degree days, normalized difference vegetation index and vapour pressure deficit. GPP showed weaker correlations with environmental variables as compared with Reco, the exception being leaf area index (LAI), which correlated with both GPP and NEE, but not with Reco. Length of growing season period was found to be the most important variable describing the spatial variation in summertime GPP and Reco; global warming will thus cause these components to increase. Annual GPP and NEE correlated significantly with LAI and pH, thus, in order to predict wetland C exchange, differences in ecosystem structure such as leaf area and biomass as well as nutritional status must be taken into account

Exchange of carbon dioxide across twelve northern peatland and tundra sites

Many wetland ecosystems such as peatlands and wet tundra hold large amounts of organic carbon (C) in their soils, and are thus important in the terrestrial C cycle. We have synthesized eddy covariance data of the carbon dioxide (CO2) exchange from twelve wetland sites across Europe and North America, ranging from ombrotrophic and minerotrophic peatlands to wet tundra ecosystems, in temperate to arctic climates. The average summertime net ecosystem exchange of CO2 (NEE) was highly variable between sites. However, all sites with complete annual datasets, seven in total, acted as annual net sinks for atmospheric CO2. To evaluate the influence of gross primary production (GPP) and ecosystem respiration (Reco) on NEE, we first removed the artificial correlation emanating from the method of partitioning NEE into GPP and Reco, After this correction the level of significance for the previously significant relationship between annual NEE and GPP increased to p=0.110. The most important variables controlling the between-site variation in annual and summer-time Reco were growing season period, air temperature, growing degree days, normalized difference vegetation index and vapour pressure deficit. Summer-time GPP showed weaker correlations with environmental variables as compared to Reco, the exception being leaf area index (LAT), which correlated with both GPP and NEE, but not with Reco, Annual GPP and NEE correlated significantly with LAI and pH, indicating that various peatland and tundra types with different vegetation properties and nutritional status do not respond in the same way to changes in environmental conditions. This means that models of wetland C exchange and its response to climate change should address the issue of ecosystem structure as well as ecosystem function

The uncertain climate footprint of wetlands under human pressure

Significant climate risks are associated with a positive carbon-temperature feedback in northern latitude carbon-rich ecosystems, making an accurate analysis of human impacts on the net greenhouse gas balance of wetlands a priority. Here, we provide a coherent assessment of the climate footprint of a network of wetland sites based on simultaneous and quasi-continuous ecosystem observations of CO2 and CH4 fluxes. Experimental areas are located both in natural and in managed wetlands and cover a wide range of climatic regions, ecosystem types, and management practices. Based on direct observations we predict that sustained CH4 emissions in natural ecosystems are in the long term (i.e., several centuries) typically offset by CO2 uptake, although with large spatiotemporal variability. Using a space-for-time analogy across ecological and climatic gradients, we represent the chronosequence fro