Can continental bogs with stand the pressure due to climate change?

Not all peatlands are alike. Theoretical and process based models suggest that ombrogenic, oligotrophic peatlands can withstand the pressures due to climate change because of the feedbacks among ecosystem production, decomposition and water storage. Although there have been many inductive explanations inferring from paleo-records, there is a lack of deductive empirical tests of the models predictions of these systems’ stability and there are few records of the changes in the net ecosystem carbon balance (NECB) of peatlands that are long enough to examine the dynamics of the NECB in relation to climate variability. Continuous measurements of all the components of the NECB and the associated general climatic and environmental conditions have been made at the Mer Bleue (MB) peatland, a large, 28 km2, 5 m deep, raised ombro-oligotrophic, shrub and Sphagnum covered bog, near Ottawa, Canada from May 1, 1998 until the present. The sixteen-year daily CO2, CH4, and DOC flux and NECB covers a wide range of variability in peatland water storage from very dry to very wet growing seasons. We used the MB data to test the extent of MB peatland’s stability and the strength of the underlying key feedback between the NECB and changes in water storage projected by the models. In 2007 we published a six-year (1999-2004) net ecosystem carbon balance (NECB) for MB of ∼22 ± 40 g C m-2 yr-1, but we have since recalculated the 1998-2004 NECB to be 32 ± 40 g C m-2 yr-1 based on a reanalyzed average NEP of 51 ± 41 g C m-2 yr-1. Over the same period the net loss of C via the CH4 and DOC fluxes were -4 ± 1 and -15 ± 3 g C m-2 yr-1. The 1998-2004 six-year MB average NECB is similar to the long-term C accumulation rate, estimated from MB peat cores, for the last 3,000 years. The post 2004 MB NEP has increased to an average of ∼96 ± 32 g C m-2 yr-1 largely to there being generally wetter growing seasons. The losses of C via DOC (18 ± 1 g C m-2 yr-1) and CH4 (7 ± 4 g C m-2 yr-1) while showing considerable year-to-year variability are not significantly different post 2004. Hence, the proportional loss of C as DOC and CH4 in the MB NECB is slightly less post-2004 than it was before 2004 though the cumulative errors preclude statistically differences. As a result the MB NECB has increased to 79 ± 29 g C m-2 yr-1 post 2004 yielding a 14 year contemporary NECB of 56 ± 36 g C m-2 yr-1, which is double the long-term accumulation rate of C. The variability in the annual NECB and growing season mean NEP for the MB bog can be explained (r2 = 0.35, p \u3c 0.01) by the variability in growing season water table depth. These results suggest the carbon balance – water table feedback is sufficient enough to create stability in continental bogs so they will withstand a considerable amount of climate change

Burn severity influences postfire CO2 exchange in Arctic tundra

Author Posting. © Ecological Society of America, 2011. This article is posted here by permission of Ecological Society of America for personal use, not for redistribution. The definitive version was published in Ecological Applications 21 (2011): 477–489, doi:10.1890/10-0255.1.Burned landscapes present several challenges to quantifying landscape carbon balance. Fire scars are composed of a mosaic of patches that differ in burn severity, which may influence postfire carbon budgets through damage to vegetation and carbon stocks. We deployed three eddy covariance towers along a burn severity gradient (i.e., severely burned, moderately burned, and unburned tundra) to monitor postfire net ecosystem exchange of CO2 (NEE) within the large 2007 Anaktuvuk River fire scar in Alaska, USA, during the summer of 2008. Remote sensing data from the MODerate resolution Imaging Spectroradiometer (MODIS) was used to assess the spatial representativeness of the tower sites and parameterize a NEE model that was used to scale tower measurements to the landscape. The tower sites had similar vegetation and reflectance properties prior to the Anaktuvuk River fire and represented the range of surface conditions observed within the fire scar during the 2008 summer. Burn severity influenced a variety of surface properties, including residual organic matter, plant mortality, and vegetation recovery, which in turn determined postfire NEE. Carbon sequestration decreased with increased burn severity and was largely controlled by decreases in canopy photosynthesis. The MODIS two-band enhanced vegetation index (EVI2) monitored the seasonal course of surface greenness and explained 86% of the variability in NEE across the burn severity gradient. We demonstrate that understanding the relationship between burn severity, surface reflectance, and NEE is critical for estimating the overall postfire carbon balance of the Anaktuvuk River fire scar.This work was supported by NSF grants #0632139 (OPP-AON), #0808789 (OPP-ARCSS SGER), #0829285 (DEB-NEON SGER), and #0423385 (DEBLTER) to the Marine Biological Laboratory

Insights into the Kinetics of Supramolecular Comonomer Incorporation in Water

Multicomponent supramolecular polymers are a versatile platform to prepare functional architectures, but a few studies have been devoted to investigate their noncovalent synthesis. Here, we study supramolecular copolymerizations by examining the mechanism and time scales associated with the incorporation of new monomers in benzene-1,3,5-tricarboxamide (BTA)-based supramolecular polymers. The BTA molecules in this study all contain three tetra(ethylene glycol) chains at the periphery for water solubility but differ in their alkyl chains that feature either 10, 12 or 13 methylene units. C(10)BTA does not form ordered supramolecular assemblies, whereas C(12)BTA and C(13)BTA both form high aspect ratio supramolecular polymers. First, we illustrate that C(10)BTA can mix into the supramolecular polymers based on either C(12)BTA or C(13)BTA by comparing the temperature response of the equilibrated mixtures to the temperature response of the individual components in water. Subsequently, we mix C(10)BTA with the polymers and follow the copolymerization over time with UV spectroscopy and hydrogen/deuterium exchange mass spectrometry experiments. Interestingly, the time scales obtained in both experiments reveal significant differences in the rates of copolymerization. Coarse-grained simulations are used to study the incorporation pathway and kinetics of the C(10)BTA monomers into the different polymers. The results demonstrate that the kinetic stability of the host supramolecular polymer controls the rate at which new monomers can enter the existing supramolecular polymers

McGill wetland model: evaluation of a peatland carbon simulator developed for global assessments

We developed the McGill Wetland Model (MWM) based on the general structure of the Peatland Carbon Simulator (PCARS) and the Canadian Terrestrial Ecosystem Model. Three major changes were made to PCARS: (1) the light use efficiency model of photosynthesis was replaced with a biogeochemical description of photosynthesis; (2) the description of autotrophic respiration was changed to be consistent with the formulation of photosynthesis; and (3) the cohort, multilayer soil respiration model was changed to a simple one box peat decomposition model divided into an oxic and anoxic zones by an effective water table, and a one-year residence time litter pool. MWM was then evaluated by comparing its output to the estimates of net ecosystem production (NEP), gross primary production (GPP) and ecosystem respiration (ER) from 8 years of continuous measurements at the Mer Bleue peatland, a raised ombrotrophic bog located in southern Ontario, Canada (index of agreement [dimensionless]: NEP = 0.80, GPP = 0.97, ER = 0.97; systematic RMSE [g C m<sup>−2</sup> d<sup>−1</sup>]: NEP = 0.12, GPP = 0.07, ER = 0.14; unsystematic RMSE: NEP = 0.15, GPP = 0.27, ER = 0.23). Simulated moss NPP approximates what would be expected for a bog peatland, but shrub NPP appears to be underestimated. Sensitivity analysis revealed that the model output did not change greatly due to variations in water table because of offsetting responses in production and respiration, but that even a modest temperature increase could lead to converting the bog from a sink to a source of CO<sub>2</sub>. General weaknesses and further developments of MWM are discussed

McGill Wetland Model: evaluation of a peatland carbon simulator developed for global assessments

International audienceWe developed the McGill Wetland Model (MWM) based on the general structure of the Peatland Carbon Simulator (PCARS) and the Canadian Terrestrial Ecosystem Model. Three major changes were made to PCARS: 1. the light use efficiency model of photosynthesis was replaced with a biogeochemical description of photosynthesis; 2. the description of autotrophic respiration was changed to be consistent with the formulation of photosynthesis; and 3. the cohort, multilayer soil respiration model was changed to a simple one box peat decomposition model divided into an oxic and anoxic zones by an effective water table, and a one-year residence time litter pool. MWM was then evaluated by comparing its output to the estimates of net ecosystem production (NEP), gross primary production (GPP) and ecosystem respiration (ER) from 8 years of continuous measurements at the Mer Bleue peatland, a raised ombrotrophic bog located in southern Ontario, Canada (index of agreement [dimensionless]: NEP=0.80, GPP=0.97, ER=0.97; systematic RMSE [g C m?2 d?1]: NEP=0.12, GPP=0.07, ER=0.14; unsystematic RMSE [g C m?2 d?1]: NEP=0.15, GPP=0.27, ER=0.23). Simulated moss NPP approximates what would be expected for a bog peatland, but shrub NPP appears to be underestimated. Sensitivity analysis revealed that the model output did not change greatly due to variations in water table because of offsetting responses in production and respiration, but that even modest temperature increases could lead to converting the bog from a sink to a source of CO2. General weaknesses and further developments of MWM are discussed

Using MODIS derived <i>f</i>PAR with ground based flux tower measurements to derive the light use efficiency for two Canadian peatlands

International audienceWe used satellite remote sensing data; fraction of photosynthetically active radiation absorbed by vegetation (fPAR) from the Moderate Resolution Imaging Spectroradiometer (MODIS) in combination with tower eddy covariance and meteorological measurements to characterise the light use efficiency parameter (?) variability and the maximum ? (?max) for two contrasting Canadian peatlands. Eight-day MODIS fPAR data were acquired for the Mer Bleue (2000 to 2003) and Western Peatland (2004). Flux tower eddy covariance and meteorological measurements were integrated to the same eight-day time stamps as the MODIS fPAR data. A light use efficiency model: GPP=? * APAR (where GPP is Gross Primary Productivity and APAR is absorbed photosynthetically active radiation) was used to calculated ?. The ?max value for each year (2000 to 2003) at the Mer Bleue bog ranged from 0.58 g C MJ?1 to 0.78 g C MJ?1 and was 0.91 g C MJ?1 in 2004, for the Western Peatland. The average growing season ? for the Mer Bleue bog for the four year period was 0.35 g C MJ?1 and for the Western Peatland in 2004 was 0.57 g C MJ?1. The average snow free period ? for the Mer Bleue bog over the four year period was 0.27 g C MJ?1 and for the Western Peatland in 2004 was 0.39 g C MJ?1. Using the light use efficiency method we calculated the ?max and the annual variability in ? for two Canadian peatlands. We determined that temperature was a growth-limiting factor at both sites Vapour Pressure Deficit (VPD) however was not. MODIS fPAR is a useful tool for the characterization of ? at flux tower sites

Direct thrust measurement of a permanent magnet helicon double layer thruster

Direct thrust measurements of a permanent magnet helicon double layer thruster have been made using a pendulum thrust balance and a high sensitivity laser displacement sensor. At the low pressures used (0.08 Pa) an ion beam is detected downstream of the thruster exit, and a maximum thrust force of about 3 mN is measured for argon with an rf input power of about 700 W. The measured thrust is proportional to the upstream plasma density and is in good agreement with the theoretical thrust based on the maximum upstream electron pressure