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Modeling Environmental Controls on
Net Ecosystem CO2 Exchange of a
Tropical Bog
Symon Mezbahuddin1, Robert Grant1 and Takashi Hirano2
1Department of Renewable Resources, University of Alberta, Canada
2Research Faculty of Agriculture, Hokkaido University, Japan
Introduction
Bogs are the peatland ecosystems that get water and
nutrient inputs only through precipitation
Drier weather resultant of increased climatic events such
as El-Niño could alter balance between tropical peatland
ecosystem photosynthesis and respiration and contribute to
increasing atmospheric CO2
Eddy covariance (EC) net ecosystem productivity (NEP)
estimates of natural tropical peatlands range from -296 to
-594g C m-2y-1 [Hirano et al. 2009]
Ecosystem processes X multiple environmental factors
cause large variations and uncertainties in predictions of
peatland ecosystem response to climate change
Objectives
To examine how wetter vs. drier weather
conditions cause diurnal, seasonal and interannual
variations in net ecosystem CO2 exchange of a
tropical bog [Sebangau River, Palangka Raya,
Central Kalimantan, Indonesia; MAT: 26.3C and
MAP: 2600 mm] by using hourly time step process
based ecosystem model ecosys (Grant et al. 2001) tested
against valid eddy covariance data.
*Eddy covariance and micrometeorological data (2002-2005) were
collected from flux station over the site (23042S; 114211E )
through AsiaFlux Network.
Objectives contd.
70
2003 (Total precipitation = 2291 mm) 70
2005 (Total precipitation = 2620 mm)
Precipitation (mmh )
-1
Precipitation (mmh )
-1
60 Dry year 60 Wet year
50 50
40 40
30 30
20 20
10 10
0 0
0.5 0.5
0.4 0.4
(m3m-3)
(m3m-3)
(m m )
-3
(m m )
-3
3
0.3
3
0.3
0.2 0.2
0 30 60 90 120 150 180 210 240 270 300 330 360
0 30 60 90 120 150 180 210 240 270 300 330 360
Day of year Day of year
Figure: Hourly measured precipitation and soil water content at 0-20cm depth during 2003
and 2005 over Palangka Raya peat swamp forest, Indonesia
Conceptual Hypotheses
Hypothesis - I
NEP = GPP - RE
Increased atmospheric Reduced precipitation
dryness
Reduced soil water content
Increased vapour
pressure deficit Decreased soil, root and canopy
water potentials
Higher potential Increased stomatal resistance and
transpiration consequent stomatal closure
Less actual transpiration at a cost
of CO2 fixation and hence reduced
GPP
Conceptual Hypotheses contd.
Hypothesis - II
Oxic
NEP = GPP - RE WT
Reduced soil water Water table drawdown
WT
content
Anoxic
Increased peat aeration
Reduced heat capacity
of soil
Rapid aerobic peat decomposition
Increased soil
temperature
Higher heterotrophic respiration
Faster biogeochemical
reaction Increased ecosystem respiration
Hypotheses contd.
Hypothesis - III
However, Desiccation
Surface peat desiccation
Inhibition of heterotrophic
respiration at surface layers WT
Partial or complete offset of any
increment in heterotrophic
respiration stimulated by deeper
peat aeration
Reduction or elimination of
drier and warmer weather
effects on NEP through its
impact on RE
Study Site Tower fetches: 0.4 km N; >5 km S; 0.5 km E; 1.5 km W
⦿ Woody peat
⦿ >10,000 years BP
⦿ Natural swamp
forest
⦿ ~ 4m thick
⦿ Drainage,
partial thinning
~8 years before
Figure: Map of flux tower
the study site installation
Picture courtesy:
(Hirano et al. 2007)
Modeling Experiment
(a)
⦿ A modeled tropical peat swamp
ecosystem of 100 years spin up run using
repeated hourly weather cycles (2002-
2005)
⦿ The spinning period allowed the modeled
ecosystem to grow, regenerate and attain
dynamic C equilibrium
⦿ Soil and vegetation characteristics are (b)
(b)
from site measured data reported in
different studies Figure: Modeling experiment layout
(a) horizontal and (b) vertical
Preliminary Results
2003 (dry year)
Precip'n (mmh )
-1
70
60
50
40
30
20
10
0
0.50
0.45
(m m )
(m3m-3)
3 -3
0.40
0.35
0.30
0.25
0.20
WT depth (m)
0.0
-0.5
-1.0
-1.5
-2.0
0 30 60 90 120 150 180 210 240 270 300 330 360
Day of year
Figure: Hourly measured (dots) and simulated (lines) precipitation, soil water content at
0-20cm depth and daily water table depths during 2003 over Palangka Raya peat swamp
forest, Indonesia
Preliminary Results contd.
2005 (wet year)
Precip'n(mmh )
-1
70
60
50
40
30
20
10
0
0.50
0.45
(m3m) )
(m3m-3 -3
0.40
0.35
0.30
0.25
0.20
WT depth (m)
0.0
-0.5
-1.0
-1.5
-2.0
0 30 60 90 120 150 180 210 240 270 300 330 360
Day of year
Figure: Hourly measured (dots) and simulated (lines) precipitation, soil water content
at 0-20cm depth and daily water table depths during 2005 over Palangka
Raya peat swamp forest, Indonesia
Preliminary Results contd.
3.5
3.0
2.5
2005
(wet year)
Vapour pressure deifict (KPa)
2.0
1.5
1.0
0.5
0.0
250 251 252 253 254 255
3.5
3.0
2.5
2.0
1.5
2003
1.0 (dry year)
0.5
0.0
239 240 241 242 243 244
Day of year
Figure: Hourly measured vapour pressure deficit during 2005 and 2003 over
Palangka Raya peat swamp forest, Indonesia
Preliminary Results contd.
Rnobs LEobs LEgap Hobs Hgap
Rnsim LEsim Hsim
900
600
H/LE=0.36
2005
300 (Rn >700 Wm-2)
(wet year)
Ecosystem energy fluxes (Wm )
-2
0
-300
-600
+ influxes
-900 - effluxes
250 251 252 253 254 255
900
600
300
0 H/LE=0.62 2003
(dry year)
(Rn >700 Wm-2)
-300
-600
-900
239 240 241 242 243 244
Day of year
Figure: Hourly measured (dots) and simulated (lines) ecosystem net radiation (Rn), latent
heat (LE) and sensible heat (H) fluxes during 2005 and 2003 over Palangka Raya peat
swamp forest, Indonesia
Preliminary Results contd.
25
20
2005
Ecosystem CO2 fluxes (mol m s )
15
-1
10
(wet year)
-2
5
0
-5
-10
-15 + influxes
-20 - effluxes
250 251 252 253 254 255
25
20
15
10
5 0.25 mm 2003
0
-5
(dry year)
-10
-15
-20
239 240 241 242 243 244
Day of year
Figure: Hourly measured (closed symbol), gap filled (open symbol) and simulated (lines)
ecosystem CO2 fluxes during 2005 and 2003 over Palangka Raya peat swamp forest,
Indonesia
Preliminary Results contd.
Table: Regression results of simulated vs measured hourly parameters (2003-2005)
Parameters Units n 1a 2b R2 3RMSD 4RMSE
CO2 flux μmolm-2s-1 12187 0.79 0.90 0.85 4.5 4.5
Rn Wm-2 26304 16 0.93 0.99 10 -
LE flux Wm-2 13924 -31 1.07 0.76 70 57
H flux Wm-2 14282 -01 0.94 0.56 44 32
(0-20cm) m3m-3 26304 0.11 0.70 0.52 0.02 -
Ts (5cm) C 26304 2.90 0.80 0.61 0.47 -
Rn = Net radiation; LE = Latent heat flux; H= Sensible heat; = Soil water content; Ts = Soil temperature
Table: Regression results of simulated vs measured daily water table depths (m)
(2004-2005)
n 1a 2b R2 3RMSD
444 -0.12 0.87 0.77 0.10
1intercept and 2slope from regression of simulated over measured data; 3root mean square for differences
from regression of measured over simulated data; 4root mean square of random measurement errors
calculated for the measured data based on algorithms described in Richardson et al. (2006)
Preliminary Results contd.
2003
0.45 (dry year)
(mm-3-3)
(m3 3m ) 0.40
0.35
0.30
0.25
0.20
6
ET (mmd )
-1
5
4
3
2
+NEP = C sink
-2 -1
4
NEP gCm d
2 - NEP = C source
0
-2 Open symbols
-4 have
-6 >50% gap
0 30 60 90 120 150 180 210 240 270 300 330 360 filled data
Day of year
Figure: Hourly measured (dots) and simulated (lines) soil water content; three day moving
averages of measured (dots) and simulated (lines) evapotranspiration and net ecosystem
productivity during 2003 over Palangka Raya peat swamp forest, Indonesia
Preliminary Results contd.
2005
0.45 (wet year)
(m3m-3) 0.40
-3
0.35
3
0.30
(m
0.25
0.20
6
5
ET (mmd )
-1
4
3
2
4 +NEP = C sink
-2 -1
2 - NEP = C source
NEP gCm d
0
-2 Open symbols
-4 have
-6 >50% gap
0 30 60 90 120 150 180 210 240 270 300 330 360 filled data
Day of year
Figure: Hourly measured (dots) and simulated (lines) soil water content; three day moving
averages of measured (dots) and simulated (lines) evapotranspiration and net ecosystem
productivity during 2005 over Palangka Raya peat swamp forest, Indonesia
Preliminary Results contd.
Table: Annual ecosystem carbon balance for Palangka Raya Drained forest, Indonesia
during 2003 and 2005
Components 2003 (drier year) 2005 (wetter year) Values from
of ecosystem other studies
Ecosys EC Ecosys EC
carbon
budget
-------------------------------------------------g C m-2-----------------------------------------------------------------
GPP 3398 34611 3578 - 33705
Ra 1570 - 1431 - -
Rroot 795 - 954 - -
Rh 1450 - 1324 - -
Rsoil 2245 - 2278 - 21004
21305
RE 3815 38381 3709 - -
NEP -217 -3781 -131 -4411 -5422
5223
1Hirano et al. 2007; 2Henson 2005; 3Suzuki et al. 1999; 4Melling et al. 2005; 5 Kosugi et al. 2008; 5Jauhianen et
al. 2005
Summary
There was not a complete shift in ecosystem energy
balance due to water stress resultant of prolonged dry
season.
Still a reduction in Bowen ratio (H/LE) due to drier
weather condition resulted lower GPP contributing to lower
NEP as evident in both EC-gap filled and simulated CO2
fluxes.
Summary contd.
Ecosys couldn’t up to this point fully capture the large
negative NEPs at the onset of the dry season. So, we remain
inconclusive about our hypothesis of a possible increase in
ecosystem respiration stimulated by water table drawdown
in drier years. We are currently working on that.
Peatlands widely vary in their physicochemical
characteristics. Rigorous testing of process based models
based on our current fundamental understanding across
contrasting peatlands would be interesting to improve our
insights on peat carbon cycling.
Acknowledgements
1. Mary Louise Imrie Travel Grant
2. AsiaFlux Network
3. NSERC Discovery Grant
4. Faculty of Graduate Studies and Research, University of
Alberta
5. University of Alberta Computing Facilities
6. All of my lab group members.
Thank You
