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Challenges for Wetland Carbon Cycle Modeling Benjamin N. Sulman Ankur R. Desai Nicole M. Schroeder Nicanor Z. Saliendra Peter M. Lafleur Larry B. Flanagan.

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Presentation on theme: "Challenges for Wetland Carbon Cycle Modeling Benjamin N. Sulman Ankur R. Desai Nicole M. Schroeder Nicanor Z. Saliendra Peter M. Lafleur Larry B. Flanagan."— Presentation transcript:

1 Challenges for Wetland Carbon Cycle Modeling Benjamin N. Sulman Ankur R. Desai Nicole M. Schroeder Nicanor Z. Saliendra Peter M. Lafleur Larry B. Flanagan Oliver Sonnentag D. Scott Mackay Alan Barr Lisa Murphy Bill Riley NACP site synthesis participants

2 Outline What are wetlands? How are they important to the carbon cycle? How can we model them better? – Water table dynamics – Different peatland types – Complex topography – Non-CO 2 carbon losses Conclusions

3 What are wetlands? Swamp Mire Marsh Fen Bog Wet meadow Wet prairie Slough Vernal pool

4 Official definitions Canadian official definition: Land that is saturated with water long enough to promote wetland or aquatic processes… (National Wetlands Working Group, 1988) International treaty definition: Areas of marsh, fen, peatland, or water… permanent or temporary, with water that is static or flowing, fresh brackish, or salt… the depth of which at low tide does not exceed 6 meters (Ramsar Convention) U.S. Clean Water Act definition: Areas that are inundated or saturated by surface or ground water… sufficient to support … vegetation typically adapted for saturated soil conditions (U.S. Army Corps of Engineers)

5 What is a wetland? Land that is wet! “Wetland” is not a very useful ecological classification For carbon cycling, “Peatland” may be more useful: an ecosystem with substantial accumulated, poorly decomposed soil organic matter

6 Northern peatlands Fen Groundwater and surface water fed Usually shrubs or sedges dominate Peat results from anaerobic soil Fen Groundwater and surface water fed Usually shrubs or sedges dominate Peat results from anaerobic soil Tundra Permafrost soils Seasonal thawing leads to flooding of low areas Peat results from cold temperatures Tundra Permafrost soils Seasonal thawing leads to flooding of low areas Peat results from cold temperatures Lost Creek (WI) Barrow (AK) (Photo from specnet.info) Bog Rain-fed Nutrient-poor Often dominated by mosses Peat results from anaerobic soil Bog Rain-fed Nutrient-poor Often dominated by mosses Peat results from anaerobic soil Mer Bleue (ON) Western Peatland (AB)

7 Of 144 Ameriflux and CCP sites: 6 tundra 4 northern fen 1 bog 4 southern marsh and mangrove 2 fen, 1 bog, and 3 tundra sites were included in NACP site synthesis Only one site has a “wetland” IGBP classification NameLocationTypeIGBP class Eastern PeatlandOntarioBog Saskatchewan peatlandSaskatchewanFen Western PeatlandAlbertaFen AtqasukAlaskaTundraGrassland BarrowAlaskaTundraOpen Shrubland Shark River SloughFloridaLong hydroperiod marshPermanent wetlands Shark River SloughFloridaMangrove forestWater Taylor SloughFloridaShort hydroperiod marshWoody savanna Happy ValleyAlaskaTundraOpen shrubland Imnaviat CreekAlaskaTundra (3 towers) IvotukAlaskaTundraOpen Shrubland Lost CreekWisconsinShrub fenDeciduous broad-leaf forest San JoaquinCaliforniaFreshwater marsh (2 towers)Urban and built-up UCI 1964-wetManitobaTreed fenEvergreen needle-leaf forest UpadAlaskaTundraOpen shrubland

8 NACP sites: example timeseries Lost Creek Shrub fen Western Peatland Sedge fen Mer Bleue (Eastern Peatland) Bog NEE (gC/m2/ day) Water table (cm) NEE (gC/m2/ day) Water table (cm) NEE (gC/m2/ day) Water table (cm)

9 Outline What are wetlands? How are they important to the carbon cycle? How can we model them better? – Water table dynamics – Different peatland types – Complex topography – Non-CO 2 carbon losses Conclusions

10 The global peatland carbon pool is large Mitra et al, 2005, Curr. Sci. Boreal and subarctic wetlands contain an estimated 455 Pg soil carbon. This is up to 1/3 of total global soil carbon pool (Gorham, 1991)

11 Wetlands in northern landscapes contain a large fraction of total C WI: Buffam et al., GCB (2011); MN: Weishampel et al., For. Ecol. Man. (2009) Fractions exclude lake area and carbon storage in lake sediments

12 Outline What are wetlands? How are they important to the carbon cycle? How can we model them better? – Water table dynamics – Different peatland types – Complex topography – Non-CO 2 carbon losses Conclusions

13 Effects of water table change Saturated, anoxic Unsaturated, oxygenated CH 4 CO 2 CH 4 CO 2

14 Measured effects Eddy-covariance summer carbon flux anomaly vs. water table anomaly for four northern fen sites Both ER and GEP increase with deeper water tables (long time scales) Drying over short time scale can lead to reduction in GEP and net CO 2 emission NEE has no significant correlation with water table Sulman et al., GRL, 2010

15 Do NACP models capture this variability?

16 Monthly NACP model residuals (model – observed) June-July-August monthly model residuals for three peatland sites Schroeder et al. (in prep) TECO SiBCASA SiB ORCHIDEE LPJ DLEM Positive correlation with observed water table at fens GPP and ER overestimated at sedge and bog sites

17 Including water table improves modeled CO 2 fluxes TREES model: Upland version Wetland version (Plant hydraulics and photosynthesis coupled to water table) Upland version underestimates C uptake compared to wetland version, but only at deep water table Mackay et al. (in prep)

18 Implications for modeling Fen carbon fluxes have a clear dependence on water table It may be possible to capture wetland-specific processes at fen sites by adding water table to models Predicting carbon fluxes in these cases would not require a separate model ecosystem type

19 Outline What are wetlands? How are they important to the carbon cycle? How can we model them better? – Water table dynamics – Different peatland types – Complex topography – Non-CO 2 carbon losses Conclusions

20 Measured differences: Bog C fluxes (white symbols) have lower magnitude and opposite sign correlation with water table “Wetland” is not a sufficiently descriptive biome type Sulman et al., GRL, 2010

21 Peatland types can change over time Ombrotrophication (fen to bog) – Buildup of peat raises surface until it is isolated from runoff and becomes primarily rain-fed – Example of Sphagnum “engineering” its environment – Transitions can occur on decadal time scales (Belyea, 2009) Drying wetlands – Dry periods can increase vascular and woody plant growth – Increased transpiration is a positive feedback to drying Thawing tundra – Thawing areas sink and flood during warm periods – Frozen areas support trees that shade soil and preserve permafrost

22 Implications for modeling Different wetland types have different responses to hydrological change Successfully predicting carbon fluxes at bogs may require a separate model ecosystem type Capturing long-term responses to pressures may require dynamic land-cover and succession Tundra succession is an important issue: there is a tundra talk Thursday afternoon, and several posters featuring tundra research

23 Outline What are wetlands? How are they important to the carbon cycle? How can we model them better? – Water table dynamics – Different peatland types – Complex topography – Non-CO 2 carbon losses Conclusions

24 Peatland topography J. S. Aber, Accessed from 1/13/2011. See Aber et al., Suo, 2002 Männikjärve bog, Estonia

25 Peatland topography Sonnentag, unpublished PhD thesis (2008)

26 Microtopography in wet peatlands What does water table depth mean, really?

27 Microtopography in wet peatlands Water table can vary by tens of cm at small scales Mean water table at a peatland does not capture the real range of variability Topographical variations lead to micro-ecosystems within the peatland

28 Measured effects Strack and Waddington, Glob. Biogeochem. Cy., 2007 Waddington and Roulet, Glob. Biogeochem. Cy., 1996 CH 4 and CO 2 fluxesEffects of lowered water table

29 Implications for modeling Topography at larger scales (> 100 m) determines wetland location and type Ignoring topography at small scales (1-100 m) may skew results, especially for methane fluxes Fractional areas or a topography distribution function could be a good approach, but spatial interactions can cause further issues (see Baird et al. 2009)

30 Outline What are wetlands? How are they important to the carbon cycle? How can we model them better? – Water table dynamics – Different peatland types – Complex topography – Non-CO 2 carbon losses Conclusions

31 An example: Mer Bleue bog NEE was larger than other factors, but ignoring DOC and CH 4 would lead to overestimate of net carbon uptake High inter-annual variability leads to high uncertainty Roulet et al., Glob. Change Biol., 2007

32 Northern Wisconsin landscape Results for northern Wisconsin Wetland litter + wetland runoff = 17.7% of wetland NEE Litter + runoff + methane = 28% of wetland NEE Forest litter + runoff = 2.6% of forest NEE Buffam et al., Glob. Change Biol., 2011

33 Implications for modeling Methane fluxes should not be ignored, as methane is an important greenhouse gas and can represent a significant fraction of wetland carbon fluxes Since dissolved carbon flows are significant wetland carbon fluxes, wetlands must be understood as part of a regional hydrological system

34 Outline What are wetlands? How are they important to the carbon cycle? How can we model them better? – Water table dynamics – Different peatland types – Complex topography – Non-CO 2 carbon losses Conclusions

35 How might wetlands surprise us? Slow and fast hydrological changes can have opposite effects on carbon fluxes Different types of wetlands can have opposite responses to similar forcings Multiple micro-ecosystems within a peatland due to topography could lead to higher resilience than expected Tundra, northern wetlands, coastal wetlands, and tropical wetlands could have different responses to similar forcings

36 Considerations for peatland modeling: “Wetland” is a broad classification, and probably not useful for carbon cycle modeling Water tables are important. Just adding water table can significantly improve modeling of some wetlands Bogs behave differently from fens, and may require a separate modeled ecosystem type Microtopography is important, especially for methane fluxes – Use a distribution function for water table rather than a mean value Methane and dissolved carbon fluxes should be included in modeled wetland carbon budgets

37 Acknowledgements Thanks for funding from the BART IGERT fellowship program Natural Sciences and Engineering Research Council of Canada (NSERC), the Canadian Foundation for Climate and Atmospheric Sciences (CFCAS), and BIOCAP Canada North American Carbon Program (NACP) and NASA Terrestrial Ecology Program U.S. Department of Energy (DOE) Office of Biological and Environmental Research (BER) National Institute for Climatic Change Research (NICCR) Midwestern Region Subagreement Z19 Thanks to my coauthors and all the contributors to the NACP site synthesis AGU Geophysical Monograph Series An excellent resource

38 References Aber et al. Patterns in Estonian bogs as depicted in color kite aerial photographs. Suo (2002) vol. 53 pp Baird et al. Upscaling of peatland-atmosphere fluxes of methane: small-scale heterogeneity in process rates and the pitfalls of “bucket-and-slab” models. In Carbon Cycling in Northern Peatlands, A. J. Baird, L. R. Belyea, X. Comas, A. S. Reeve, and L. D. Slater, eds. American Geophysical Union, Washington, D.C., Belyea, L. R. Nonlinear dynamics of peatlands and potential feedbacks on the climate system. In Carbon Cycling in Northern Peatlands, A. J. Baird, L. R. Belyea, X. Comas, A. S. Reeve, and L. D. Slater, eds. American Geophysical Union, Washington, D.C., Buffam et al. Integrating aquatic and terrestrial components to construct a complete carbon budget for a north temperate lake district. Global Change Biology (2011) vol. 17 (2) pp Mitra et al. An appraisal of global wetland area and its organic carbon stock. Current Science (2005) vol. 88 (1) pp Roulet et al. Contemporary carbon balance and late Holocene carbon accumulation in a northern peatland. Global Change Biology (2007) vol. 13 pp Strack and Waddington. Response of peatland carbon dioxide and methane fluxes to a water table drawdown experiment. Global Biogeochem. Cycles (2007) vol. 21 (1) pp. GB1007 Sulman et al. CO 2 fluxes at northern fens and bogs have opposite responses to inter-annual fluctuations in water table. Geophys Res Lett (2010) vol. 37 (19) L19702 Waddington and Roulet. Atmosphere-wetland carbon exchanges: Scale dependency of CO 2 and CH 4 exchange on the developmental topography of a peatland. Global Biogeochem. Cycles (1996) vol. 10 (2) pp Waddington et al. Water table control of CH 4 emission enhancement by vascular plants in boreal peatlands. J. Geophys. Res (1996) vol. 101 (D17) pp Weishampel et al. Carbon pools and productivity in a 1-km 2 heterogeneous forest and peatland mosaic in Minnesota, USA. Forest Ecology and Management (2009) vol. 257 (2) pp


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