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
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-CO2 carbon losses
• Conclusions
What are wetlands?
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Swamp
Mire
Marsh
Fen
Bog
Wet meadow
Wet prairie
Slough
Vernal pool
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)
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
Northern peatlands
Fen
• Groundwater and
surface water fed
• Usually shrubs or
sedges dominate
• Peat results from
anaerobic soil
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Bog
Rain-fed
Nutrient-poor
Often dominated by
mosses
Peat results from
anaerobic soil
Tundra
• Permafrost soils
• Seasonal thawing
leads to flooding
of low areas
• Peat results from
cold temperatures
Western
Peatland
(AB)
Barrow (AK)
(Photo from specnet.info)
Lost Creek
(WI)
Mer Bleue (ON)
Of 144 Ameriflux and CCP sites:
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6 tundra
4 northern fen
1 bog
4 southern marsh and
mangrove
Name
Eastern Peatland
Saskatchewan peatland
Western Peatland
Atqasuk
Barrow
Shark River Slough
Shark River Slough
Taylor Slough
Happy Valley
Imnaviat Creek
Ivotuk
Lost Creek
San Joaquin
UCI 1964-wet
Upad
Location
Ontario
Saskatchewan
Alberta
Alaska
Alaska
Florida
Florida
Florida
Alaska
Alaska
Alaska
Wisconsin
California
Manitoba
Alaska
• 2 fen, 1 bog, and 3 tundra
sites were included in NACP
site synthesis
• Only one site has a “wetland”
IGBP classification
Type
Bog
Fen
Fen
Tundra
Tundra
Long hydroperiod marsh
Mangrove forest
Short hydroperiod marsh
Tundra
Tundra (3 towers)
Tundra
Shrub fen
Freshwater marsh (2 towers)
Treed fen
Tundra
IGBP class
Grassland
Open Shrubland
Permanent wetlands
Water
Woody savanna
Open shrubland
Open Shrubland
Deciduous broad-leaf forest
Urban and built-up
Evergreen needle-leaf forest
Open shrubland
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)
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-CO2 carbon losses
• Conclusions
The global peatland carbon pool is large
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)
Mitra et al, 2005, Curr. Sci.
Wetlands in northern landscapes
contain a large fraction of total C
100%
90%
80%
70%
60%
50%
Upland %
40%
Wetland %
30%
20%
10%
0%
WI area
WI C
MN area
MN 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
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-CO2 carbon losses
• Conclusions
Effects of water table change
CH4
CO2
Saturated, anoxic
CO2
Unsaturated,
oxygenated
CH4
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 CO2 emission
• NEE has no significant
correlation with water table
Sulman et al., GRL, 2010
Do NACP models capture this
variability?
Monthly NACP model residuals
(model – observed)
• Positive correlation
with observed
water table at fens
• GPP and ER
overestimated at
sedge and bog sites
TECO
SiBCASA
SiB
ORCHIDEE
LPJ
DLEM
June-July-August
monthly model
residuals for three
peatland sites
Schroeder et al. (in prep)
Including water table improves
modeled CO2 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)
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
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-CO2 carbon losses
• Conclusions
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
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
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
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-CO2 carbon losses
• Conclusions
Peatland topography
Männikjärve bog,
Estonia
J. S. Aber, 2001.
Accessed from http://www.emporia.edu/earthsci/estonia/estonia.htm, 1/13/2011.
See Aber et al., Suo, 2002
Peatland topography
Sonnentag, unpublished PhD thesis (2008)
Microtopography in wet peatlands
What does water
table depth
mean, really?
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
Measured effects
CH4 and CO2 fluxes
Waddington and Roulet, Glob. Biogeochem.
Cy., 1996
Effects of lowered water table
Strack and Waddington, Glob. Biogeochem. Cy., 2007
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)
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-CO2 carbon losses
• Conclusions
An example: Mer Bleue bog
• NEE was larger than
other factors, but
ignoring DOC and CH4
would lead to
overestimate of net
carbon uptake
• High inter-annual
variability leads to high
uncertainty
Roulet et al., Glob. Change Biol., 2007
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
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
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-CO2 carbon losses
• Conclusions
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
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
Acknowledgements
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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
050516Z19
Thanks to my coauthors and all the
contributors to the NACP site synthesis
AGU Geophysical Monograph
Series
An excellent resource
References
• Aber et al. Patterns in Estonian bogs as depicted in color kite aerial photographs. Suo (2002) vol. 53 pp. 1-15
• 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., 2009.
• 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., 2009.
• 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. 1193-1211
• Mitra et al. An appraisal of global wetland area and its organic carbon stock. Current Science (2005) vol. 88 (1)
pp. 25-35
• Roulet et al. Contemporary carbon balance and late Holocene carbon accumulation in a northern peatland.
Global Change Biology (2007) vol. 13 pp. 397-411
• 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. CO2 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 CO2 and CH4 exchange
on the developmental topography of a peatland. Global Biogeochem. Cycles (1996) vol. 10 (2) pp. 233-245
• Waddington et al. Water table control of CH4 emission enhancement by vascular plants in boreal peatlands. J.
Geophys. Res (1996) vol. 101 (D17) pp. 22775
• Weishampel et al. Carbon pools and productivity in a 1-km2 heterogeneous forest and peatland mosaic in
Minnesota, USA. Forest Ecology and Management (2009) vol. 257 (2) pp. 747-754