Relationship between ecosystem productivity and photosynthetically active radiation for northern peatlands

We analyzed the relationship between new ecosystem exchange of carbon dioxide (NEE) and irradiance (as photosynthetic photon flux density of PPFD), using published and unpublished data that have been collected during midgrowing season for carbon balance studies at seven peatlands in North America and Europe.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 set for all 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 = PPFD P max (PPFD + PMAX) + R) better described the NEE-PPFD relationships ,while bogs had lower respiration rates (R = -2.0 umol m-2 s-1 for bogs and -2.7 umol m-2 s-1 for fens) and lower NEE at moderate and high light levels (Pmax = 5.2 umol m-2 s-1) than the upland exosystems (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

Tundra shrub effects on growing season energy and carbon dioxide exchange

Increased shrub cover on the Arctic tundra is expected to impact ecosystem-atmosphere exchanges of carbon and energy resulting in feedbacks to the climate system, yet few direct measurements of shrub tundra-atmosphere exchanges are available to corroborate expectations. Here we present energy and carbon dioxide (CO2) fluxes measured using the eddy covariance technique over six growing seasons at three closely located tundra sites in Canada's Low Arctic. The sites are dominated by the tundra shrub Betula glandulosa, but percent cover varies from 17%-60% and average shrub height ranges from 18-59 cm among sites. The site with greatest percent cover and height had greater snow accumulation, but contrary to some expectations, it had similar late-winter albedo and snow melt dates compared to the other two sites. Immediately after snowmelt latent heat fluxes increased more slowly at this site compared to the others. Yet by the end of the growing season there was little difference in cumulative latent heat flux among the sites, suggesting evapotranspiration was not increased with greater shrub cover. In contrast, lower albedo and less soil thaw contributed to greater summer sensible heat flux at the site with greatest shrub cover, resulting in greater total atmospheric heating. Net ecosystem exchange of CO2 revealed the potential for enhanced carbon cycling rates under greater shrub cover. Spring CO2 emissions were greatest at the site with greatest percent cover of shrubs, as was summer net uptake of CO2. The seasonal net sink for CO2 was ∼2 times larger at the site with the greatest shrub cover compared to the site with the least shrub cover. These results largely agree with expectations that the growing season feedback to the atmosphere arising from shrub expansion in the Arctic has the potential to be negative for CO2 fluxes but positive for turbulent energy fluxes

Spring warming and carbon dioxide exchange over low Arctic tundra in central Canada

Tundra-atmosphere exchanges of carbon dioxide (CO2) and water vapour were measured near Daring Lake, Northwest Territories in the Canadian Low Arctic for 3 years, 2004-2006. The measurement period spanned late-winter until the end of the growing period. Mean temperatures during the measurement period varied from about 2 °C less than historical average in 2004 and 2005 to 2 °C greater in 2006. Much of the added warmth in 2006 occurred at the beginning of the study, when snow melt occurred 3 weeks earlier than in the other years. Total precipitation in 2006 (163 mm) was more than double that of the driest year, 2004 (71 mm). The tundra was a net sink for CO2 carbon in all years. Mid-summer net ecosystem exchange of CO2 (NEE) achieved maximum values of -1.3 g Cm-2 day-1 (2004) to -1.8 g Cm-2 day-1 (2006). Accumulated NEE values over the 109-day period were -32,-51 and -61 g Cm-2 in 2004, 2005 and 2006, respectively. The larger CO2 uptake in 2006 was attributed to the early spring coupled with warmer air and soil conditions. In 2004, CO2 uptake was limited by the shorter growing season and mid-summer dryness, which likely reduced ecosystem productivity. Seasonal total evapotranspiration (ET) ranged from 130 mm (2004) to 181 mm (2006) and varied in accordance with the precipitation received and with the timing of snow melt. Maximum daily ET rates ranged from 2.3 to 2.7 mm day-1, occurring in mid July. Ecosystem water use efficiency (WUEeco) varied slightly between years, ranging from 2.2 in the driest year to 2.5 in the year with intermediate rainfall amounts. In the wettest year, increased soil evaporation may have contributed to a lower WUEeco (2.3). We speculate that most, if not all, of the modest growing season CO2 sink measured at this site could be lost due to fall and winter respiration leading to the tundra being a net CO2 source or CO2 neutral on an annual basis. However, this hypothesis is untested as yet

Respiration from soil and ground cover vegetation under tundra shrubs

Atmospheric warming is expected to cause shifts in arctic tundra vegetation composition, especially in the abundance and distribution of shrub species. Greater shrub abundance will impact the carbon exchanges between tundra ecosystems and the atmosphere, including ecosystem respiration. Here, total respiration under the shrub canopy (RT) and its components soil respiration (RS) and respiration from the ground cover vegetation (RG) were investigated at three tundra sites in the Canadian Low Arctic with varying shrub coverage. Seasonal RT and RS mean values were significantly greater (P < 0.05) at the site with greatest shrub abundance; mean values were 3.70 and 3.22 μmol m-2 s-1, respectively. Mean RG did not differ among sites; mean values ranged from 0.45 to 0.52 μmol m-2 s-1. Soil temperature exerted a stronger control on RT and RS compared to soil moisture. Differences in RT and RS among sites were attributed to differences in soil properties, such as soil total N content and bulk density. These findings suggest that belowground sources of respired carbon dioxide in Low Arctic tundra may vary with long-term shrub expansion as soil microclimate conditions and physiochemical properties adjust to changes in shrub coverage

Landscape-scale variability in soil organic carbon storage in the central Canadian Arctic

Arctic soils constitute a vast, but poorly quantified, pool of soil organic carbon (SOC). The uncertainty associated with pan-Arctic SOC storage estimates - a result of limited SOC and land cover data - needs to be reduced if we are to better predict the impact of future changes to Arctic carbon stocks resulting from climate warming. In this study landscape-scale variability in SOC at a Southern Arctic Ecozone site in the central Canadian Arctic was investigated with the ultimate goal of up-scaling SOC estimates with a land cover classification system. Total SOC was estimated to depths of 30 cm and 50 cm for 76 soil pits, together representing eight different vegetation communities in seven different broad landscape units. Soil organic carbon to 50 cm was lowest for the xerophytic herb community in the esker complex landscape unit (7.2±2.2 SD kg m-2) and highest in the birch hummock terrain in the lowland tundra landscape unit (36.4±2.8 kg m-2), followed by wet sedge and dry sedge communities in the wetland complex (29.8±9.9 and 22.0±2.0 kg m-2, respectively). The up-scaled estimates of mean SOC for the study area (excluding water) were 15.8 kg m-2 (to 50 cm) and 11.6 kg m-2 (to 30 cm). On a landscape scale, soil moisture content was found to have an important influence on SOC variability. Overall, this study highlights the importance of SOC variability at fine scales and its impact on up-scaling SOC in Arctic landscapes

Modeling peat thermal regime of an ombrotrophic peatland with hummock-hollow microtopography

The theory of conductive heat transfer cannot explain different attenuations of the daily amplitude of peat temperatures (Ts) in hummocks (detectable below the 20-cm depth) and hollows (disappearing above the 10cm depth). Large readily drained macropores in the upper fibric peat determine a large air permeability and hence may enhance heat transfer by air convection in porous media, driven by temperature gradients between hummock sides and interiors. In this study, the ecosys model was used to simulate a peat thermal regime at Mer Bleue peadand, Ontario, Canada. It was hypothesized that adding the air-convective heat transfer to conductive plus water-convective heat transfers would improve simulations of Ts. The results for T s ground heat fluxes, G, and sensible heat fluxes, H, modeled with and without air-convective heat transfer were tested with continuous hourly measurements from 2000 to 2004 using thermocouples, heat flux plates, and eddy covariance. Simulated air-convective heat transfer caused an average increase in G and a corresponding decrease in H of -20 W m-2 from the simulated conductive plus water-convective heat transfer. Hastened soil warming in hummocks resulted in better agreement between measured and simulated hummock Ts values with (RMSD of 2.23°C) than without air-convective heat (RMSD of 2.54°C). Enhanced hummock Ts caused an indirect increase in hollow Ts in the model with (RMSD of 1.68°C) compared to without air-convective heat (RMSD of 1.82°C). Our results suggest that air convection is probably an important mechanism of heat transfer in peat hummocks and should be included in peatland biogeochemical models

Modeling the effects of hydrology on gross primary productivity and net ecosystem productivity at Mer Bleue bog

The ecosys model was applied to investigate the effects of water table and subsurface hydrology changes on carbon dioxide exchange at the ombrotrophic Mer Bleue peatland, Ontario, Canada. It was hypothesized that (1) water table drawdown would not affect vascular canopy water potential, hence vascular productivity, because roots would penetrate deeper to compensate for near-surface dryness, (2) moss canopy water potential and productivity would be severely reduced because rhizoids occupy the uppermost peat that is subject to desiccation with water table decline, and (3) given that in a previous study of Mer Bleue, ecosystem respiration showed little sensitivity to water table drawdown, gross primary productivity would mainly determine the net ecosystem productivity through these vegetation-subsurface hydrology linkages. Model output was compared with literature reports and hourly eddy-covariance measurements during 2000-2004. Our findings suggest that late-summer water table drawdown in 2001 had only a minor impact on vascular canopy water potential but greatly impacted hummock moss water potential, where midday values declined to -250 MPa on average in the model. As a result, simulated moss productivity was reduced by half, which largely explained a reduction of 2-3 μmol CO 2 m-2 s-1 in midday simulated and measurement-derived gross primary productivity and an equivalent reduction in simulated and measured net ecosystem productivity. The water content of the near-surface peat (top 5-10 cm) was found to be the most important driver of interannual variability of annual net ecosystem productivity through its effects on hummock moss productivity and on ecosystem respiration. Copyright 2011 by the American Geophysical Union

Modeling the subsurface hydrology of mer bleue bog

In this study, the ecosys model was used to simulate the hydrology of the Mer Bleue bog, Ontario, Canada, with seasonally varying water tables in the upper 1 m. The soil profile was divided into three zones of peat (fibric, hemic, and sapric). In the model, large, readily drained macropore fractions in the fibric peat caused low water-holding capacity and high infiltration rates, in contrast to hemic and sapric peat, with small macropore fractions, high water-holding capacities, and low infiltration rates. Model results for peat water contents, θ, and water table depths, Z, were tested with continuous hourly measurements from 2000 to 2004 using time domain reflectometry probes and piezometers. Within the zone of pronounced water table variation, the θ and Z modeled with the Hagen-Poiseuille equation for macropore flow and Richards' equation for peat matrix flow corresponded better to the measured θ and Z (regression slopes between 0.62 and 1.03, intercepts between -0.05 and 0.02 m3 m-3, and R2 between 0.40 and 0.56), than did the modeled θ and Z with Richards' equation alone (regression slopes between 0.33 and 1.43, intercepts between 0.11 and 0.22 m3 m-3, and R2 between 0.27 and 0.41). The Richards equation alone, even when parameterized with extremely high or low bulk saturated hydraulic conductivities of fibric peat, modeled slower infiltration, greater water retention, and lower Z than measured. The implications of macropore flow might be of great importance for peatland hydrology, therefore this experimental and modeling work should be extended to other wetlands as well

Modeling the effects of hydrology on ecosystem respiration at Mer Bleue bog

The ecosys model was applied to examine the effects of peatland hydrology on soil respiration and ecosystem respiration at Mer Bleue peatland, Ontario, Canada. It was hypothesized that a decrease in near-surface microbial respiration in peat hummocks resulting from water table (WT) drawdown and subsequent desiccation of the uppermost peat would offset an increase of soil respiration at depth with improved aeration (respiration offsetting mechanism). In contrast, shallower water table in hollows would not allow near-surface desiccation to offset increased soil respiration at depth during drying. However, increased hollow soil respiration with WT drawdown would be offset by decreased aboveground moss respiration with near-surface desiccation in hummocks. Model results for microbial respiration were tested against independent laboratory experiments and ecosystem respiration against hourly eddy-covariance measurements of bog CO2 exchange from 2000 to 2004. The respiration offsetting mechanism modeled in hummocks resulted in CO 2 production of 0.85 μmol CO2 m-2 s -1 with both low (67 cm) and intermediate (38 cm) water tables in the summers of 2001 and 2004, and of 0.81 μmol CO2 m-2 s-1 and 0.95 μmol CO2 m-2 s-1 with high (31 cm) and intermediate (41 cm) water tables in the summers of 2000 and 2001. Ecosystem respiration was 2.01 μmol CO2 m-2 s-1 and 2.23 μmol CO2 m-2 s-1, and 2.62 μmol CO2 m-

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