Peter M. Lafleur

Interannual variability in the peatland-atmosphere carbon dioxide exchange at an ombrotrophic bog

This loss was equivalent to between 30 and 70% of the net CO2 uptake during the growing season. During the first 3 years of study, the bog was an annual sink for CO2 (260 g CO2 m 2 yr 1 ). In the fourth year, with the dry summer, however, annual NEE was only 34 g CO2 m 2 yr 1 , which is not significantly different from zero. We examined the performance of a peatland carbon simulator (PCARS) model against the tower measurements of NEE and derived ecosystem respiration (ER) and photosynthesis (PSN). PCARS ER and PSN were highly correlated with tower-derived fluxes, but the model consistently overestimated both ER and PSN, with slightly poorer comparisons in the dry year. As a result of both component fluxes being overestimated, PCARS simulated the tower NEE reasonably well. Simulated decomposition and autotrophic respiration contributed about equal proportions to ER. Shrubs accounted for the greatest proportion of PSN (85%); moss PSN declined to near zero during the summer period due to surface drying. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 1615 Global Change: Biogeochemical processes (4805); 1890 Hydrology: Wetlands; KEYWORDS: peatland, bog, net ecosystem exchange, eddy covariance, carbon dioxide

Spatial and temporal variability in growing-season net ecosystem carbon dioxide exchange at a large peatland in Ontario, Canada

We measured net ecosystem exchange of carbon dioxide (CO2) (NEE) during wet and dry summers (2000 and 2001) across a range of plant communities at Mer Bleue, a large peatland near Ottawa, southern Ontario, Canada. Wetland types included ombrotrophic bog hummocks and hollows, mineral-poor fen, and beaver pond margins. NEE was significantly different among the sites in both years, but rates of gross photosynthesis did not vary spatially even though species composition at the sites was variable. Soil respiration rates were very different across sites and dominated interannual variability in summer NEE within sites. During the dry summer of 2001, net CO2 uptake was significantly smaller, and most locations switched from a net sink to a source of CO2 under a range of levels of photosynthetically active radiation (PAR). The wetter areas—poor fen and beaver pond margin— had the largest rates of CO2 uptake and smallest rates of respiratory loss during the dry summer. Communities dominated by ericaceous shrubs (bog sites) maintained similar rates of gross photosynthesis between years; by contrast, the sedge-dominated areas (fen sites) showed signs of early senescence under drought conditions. Water table position was the strongest control on respiration in the drier summer, whereas surface peat temperature explained most of the variability in the wetter summer. Q 10 temperature-respiration quotients averaged 1.6 to 2.2. The ratio between maximum photosynthesis and respiration ranged from 3.7:1 in the poor fen to 1.2:1 at some bog sites; it declined at all sites in the drier summer owing to greater respiration rates relative to photosynthesis in evergreen shrub sites and a change in both processes in sedge sites. Our ability to predict ecosystem responses to changing climate depends on a more complete understanding of the factors that control NEE across a range of peatland plant communities.

Annual cycle of CO2 exchange at a bog peatland

Eddy covariance measurements of the carbon dioxide flux from an ombrotrophic bog near Ottawa, Canada, were made between June 1, 1998, and May 31, 1999. Net ecosystem exchange of CO2 (NEE) showed a distinct annual cycle, with net daily uptake increasing rapidly after snowmelt, peaking in midsummer and declining toward the fall. Summer (June to September) mean daily NEE flux was an uptake of −2.8±0.23 (standard error) g CO2 m−2 d−1, but daily values ranged considerably from a loss of 4.8 g CO2 m−2 d−1 to a maximum uptake of −8.3 g CO2 m−2 d−1. Daytime fluxes of CO2 were closely related to the photosynthetically active radiation flux, with derived relationships varying monthly. A curvilinear relationship developed between nighttime NEE and soil temperature produced a Q10 value of 3.0. Throughout the late fall and the snow-covered periods (November 5 to April 6), mean daily fluxes showed a fairly constant efflux of ∼1.1±0.003 g CO2 m−2 d−1. The integrated non-growing season CO2 loss was 183 g CO2 m−2. However, this was offset by gains during the long growing period resulting in an integrated annual NEE net uptake of 248±68 g CO2 m−2 yr−1 for this peatland. The growing season measurements of CO2 flux at this site were similar to those reported in the few previous studies on northern peatland ecosystems, but no previous annual estimates of NEE based on year-round measurements have been published for other peatlands. Compared to annual measurements at forest sites in North America, the net exchange at this site falls between that of a small annual loss recorded at a northern boreal spruce forest and the substantial uptakes measured at a temperate mixed forest and boreal aspen forest sites.

Relationship Between Ecosystem Productivity and Photosynthetically Active Radiation for Northern Peatlands

We analyzed the relationship between net ecosystem exchange of carbon dioxide (NEE) and irradiance (as photosynthetic photon flux density or PPFD), using published and unpublished data that have been collected during midgrowing season for carbon balance studies at seven peatlands in North America and Europe. NEE measurements included both eddy-correlation tower and clear, static chamber methods, which gave very similar results. Data were analyzed by site, as aggregated data sets by peatland type (bog, poor fen, rich fen, and all fens) and as a single aggregated data set for all peatlands. In all cases, a fit with a rectangular hyperbola (NEE = α PPFD Pmax/(α PPFD + Pmax) + R) better described the NEE-PPFD relationship than did a linear fit (NEE = β PPFD + R). Poor and rich fens generally had similar NEE-PPFD relationships, while bogs had lower respiration rates (R = −2.0μmol m−2s−1 for bogs and −2.7 μmol m−2s−1 for fens) and lower NEE at moderate and high light levels (Pmax = 5.2 μmol m−2s−1 for bogs and 10.8 μmol m−2s−1 for fens). As a single class, northern peatlands had much smaller ecosystem respiration (R = −2.4 μmol m−2s−1) and NEE rates (α = 0.020 and Pmax = 9.2μmol m−2s−1) than the upland ecosystems (closed canopy forest, grassland, and cropland) summarized by Ruimy et al. [1995]. Despite this low productivity, northern peatland soil carbon pools are generally 5–50 times larger than upland ecosystems because of slow rates of decomposition caused by litter quality and anaerobic, cold soils.

Seasonal trends in energy, water, and carbon dioxide fluxes at a northern boreal wetland

Micrometeorological measurements were made over a northern boreal fen near Thompson, Manitoba, Canada, as part of the Boreal Ecosystem-Atmosphere Study. The measurement period extended from the start of snowmelt until the early fall, at which time senescence was widespread throughout the fen. Data analysis concentrated on identifying seasonal trends in energy, water, and carbon dioxide fluxes and linking them to observed surface cover changes. Albedos (solar and photosynthetically active radiation (PAR)) showed large decreases over the melt period, reaching seasonal lows at the end of melt. Solar albedo increased in the summer in response to vegetation growth on the fen, while PAR albedo remained constant. Incoming and outgoing longwave flux seasonal trends were similar, so seasonal changes in net radiation were driven by the net solar flux. During the spring thaw, the melting of snow and ground ice was equal to about 28% of the daily total net radiation, while the soil heat flux accounted for about 5%. Bowen ratios at this time were above unity. Mean Bowen ratio decreased to 0.70 during the period between spring thaw and leaf-out. As the vascular vegetation cover developed, Bowen ratios decreased to seasonal lows of 0.10-0.20 near midsummer and then increased to above unity during senescence. The daily evaporative fraction (EF) was highest (≥0.80) during midsummer when the vascular vegetation was in full leaf and actively photosynthesizing, and EF decreased to a mean of 0.55 during senescence. Eddy correlation measurements of carbon dioxide flux showed the fen acting as a net sink for CO 2 only while the vascular vegetation was actively photosynthesizing with a daily mean flux of -0.81 g CO 2 -C m -2 d -1 (standard error = 0.16). Before leafing and during senescence the fen was a net source of CO 2 . Integrated over the study period of 124 days, the fen experienced a net loss of 30.4 g CO 2 m -2 to the atmosphere.

Modeling seasonal to annual carbon balance of Mer Bleue Bog, Ontario, Canada

[1] Northern peatlands contain enormous quantities of organic carbon within a few meters of the atmosphere and play a significant role in the planetary carbon balance. We have developed a new, process-oriented model of the contemporary carbon balance of northern peatlands, the Peatland Carbon Simulator (PCARS). Components of PCARS are (1) vascular and nonvascular plant photosynthesis and respiration, net aboveground and belowground production, and litterfall; (2) aerobic and anaerobic decomposition of peat; (3) production, oxidation, and emission of methane; and (4) dissolved organic carbon loss with drainage water. PCARS has an hourly time step and requires air and soil temperatures, incoming radiation, water table depth, and horizontal drainage as drivers. Simulations predict a complete peatland C balance for one season to several years. A 3-year simulation was conducted for Mer Bleue Bog, near Ottawa, Ontario, and results were compared with multiyear eddy covariance tower CO2 flux and ancillary measurements from the site. Seasonal patterns and the general magnitude of net ecosystem exchange of CO2 were similar for PCARS and the tower data, though PCARS was generally biased toward net ecosystem respiration (i.e., carbon loss). Gross photosynthesis rates (calculated directly in PCARS, empirically inferred from tower data) were in good accord, so the discrepancy between model and measurement was likely related to autotrophic and/or heterotrophic respiration. Modeled and measured methane emission rates were quite low. PCARS has been designed to link with the Canadian Land Surface Scheme (CLASS) land surface model and a global climate model (GCM) to examine climate-peatland carbon feedbacks at regional scales in future analyses. INDEX TERMS: 1615 Global Change: Biogeochemical processes (4805); 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 1890 Hydrology: Wetlands; 9350 Information Related to Geographic Region: North America; KEYWORDS: peatland, decomposition, NPP, NEE, carbon accumulation, model

Interannual variability in carbon dioxide exchanges at a boreal wetland in the BOREAS northern study area

Climatological measurements, including carbon dioxide flux density, were made from April to September in 1994 and from April to November in 1996 at a fen wetland near Thompson, Manitoba, Canada, as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). For both years, the study period was warmer and drier than the 24-year climate normals. The period of CO2 uptake was similar for both years, reaching maximum measured assimilation rates of −0.55 mg m−2 s−1 in midsummer. However, warmer air temperatures and an earlier snowmelt in the spring of 1994, which led to an earlier thaw for the fen surface, and warmer and drier conditions in the fall of 1994 promoted CO2 production at times when the vascular vegetation was not photosynthesizing. As a result, in 1994 over the study period of 124 days the fen was a net source of CO2-carbon to the atmosphere, losing 30.8 g C m−2; for the same period in 1996 the fen was a net sink of CO2-carbon, assimilating −91.6 g C m−2. Given the immense store of carbon in boreal peatlands and given a growing understanding of the relative importance of the soil carbon pool to net ecosystem exchange and of the sensitivity of this carbon storage to temperature and wetness, this boreal fens response to earlier spring warming and drier conditions extends our understanding of the impact of climate change on the carbon balance for northern ecosystems.

Image classification of a northern peatland complex using spectral and plant community data

Abstract Ordination and cluster analysis are two common methods used by plant ecologists to organize species abundance data into discrete “associations”. When applied together, they offer useful information about the relationships among species and the ecological processes occurring within a community. Remote sensing provides surrogate data for characterizing the spatial distribution of ecological classes based on the assumption of characteristic reflectance of species and species associations. Currently, there exists a need to establish and clarify the link between theories and practices of classification by ecologists and remote sensing scientists. In this study, high spatial resolution Compact Airborne Spectrographic Imager (CASI) reflectance data were examined and compared to plant community data for a peatland complex in northern Manitoba, Canada. The goal of this research was to explore the relationship between classification of species cover and community data and reflectance values. Ordination and cluster analysis techniques were used in conjunction with spectral separability measures to organize clusters of community-based data that were suitable for classification of CASI reflectance data, while still maintaining their ecological significance. Results demonstrated that two-way indicator species analysis (TWINSPAN) clusters did not correspond well to spectral reflectance and gave the lowest classification results of the methods investigated. The highest classification accuracies were achieved with ecological classes defined by combining the information obtained from a suite of analysis techniques (i.e., TWINSPAN, correspondence analysis (CA), and signature separability analysis), albeit not statistically superior to the classification obtained from the signature separability analysis alone.

Energy balance and evapotranspiration from a subarctic forest

Abstract Measurements of energy balance components were made over subarctic forest near Churchill, Manitoba, Canada, during summer 1989. For the study, soil heat storage comprised 9% of net radiation, and the sensible and latent heat fluxes were of similar magnitude. However, on a daily basis the Bowen ratio varied considerably around an overall mean of 0.94. Canopy transpiration was computed from measurements of leaf conductance and vapour pressure deficit. These results indicated that, on average, only 20% of the total evaporation flux originated from the tree canopy.

Evapotranspiration from sedge-dominated wetland surfaces

Abstract The evapotranspiration (ET) regimes of two sedge communities in a subarctic coastal wetland were investigated during non-vegetated and vegetated periods. Surface moisture availability strongly controlled ET during non-vegetated conditions, but was less important during the vegetated period. The results suggest that the evaporation efficiency of these sites changed in response to vegetation growth. Vegetation cover reduced the evaporating efficiency of the wet site and slightly increased evaporation efficiency of the dry site. Simple linear regression models based on Penmans open water evaporation formula were found to predict ET accurately at each site under all surface cover conditions. However, model coefficients for the two sites differed substantially. Dividing the data set into non-vegetated and vegetated periods improved the model performance only marginally.