Modeling Environmental Controls on Net Ecosystem CO 2 Exchange of a Tropical Bog Symon Mezbahuddin 1, Robert Grant 1 and Takashi Hirano 2 1 Department of Renewable Resources, University of Alberta, Canada 2 Research 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 CO 2  Eddy covariance (EC) net ecosystem productivity (NEP) estimates of natural tropical peatlands range from -296 to -594g C m -2 y -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 CO 2 exchange of a tropical bog [Sebangau River, Palangka Raya, Central Kalimantan, Indonesia; MAT: 26.3  C 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 ( ) were collected from flux station over the site (2  3042  S; 114  211  E ) through AsiaFlux Network.

Objectives contd (Total precipitation = 2291 mm) 2005 (Total precipitation = 2620 mm)  (m 3 m -3 ) Figure: Hourly measured precipitation and soil water content at 0-20cm depth during 2003 and 2005 over Palangka Raya peat swamp forest, Indonesia Dry year Wet year

Conceptual Hypotheses NEP = GPP - RE Increased atmospheric dryness Increased vapour pressure deficit Higher potential transpiration Reduced precipitation Reduced soil water content Decreased soil, root and canopy water potentials Increased stomatal resistance and consequent stomatal closure Less actual transpiration at a cost of CO 2 fixation and hence reduced GPP Hypothesis - I

Conceptual Hypotheses contd. NEP = GPP - RE Reduced soil water content Reduced heat capacity of soil Increased soil temperature Water table drawdown Increased peat aeration Rapid aerobic peat decomposition Higher heterotrophic respiration Increased ecosystem respiration Faster biogeochemical reaction Hypothesis - II Oxic Anoxic WT

Hypotheses contd. Surface peat desiccation Inhibition of heterotrophic respiration at surface layers 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 However, Hypothesis - III WT Desiccation

Study Site Figure: Map of the study site Picture courtesy: (Hirano et al. 2007) 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 flux tower installation

Modeling Experiment Figure: Modeling experiment layout (a) horizontal and (b) vertical (b) A modeled tropical peat swamp ecosystem of 100 years spin up run using repeated hourly weather cycles ( ) The spinning period allowed the modeled ecosystem to grow, regenerate and attain dynamic C equilibrium Soil and vegetation characteristics are from site measured data reported in different studies (b) (a)

Preliminary Results 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 2003 (dry year)  (m 3 m -3 )

Preliminary Results contd.  (m 3 m -3 ) 2005 (wet 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. Figure: Hourly measured vapour pressure deficit during 2005 and 2003 over Palangka Raya peat swamp forest, Indonesia 2005 (wet year) 2003 (dry year)

Preliminary Results contd (wet year) Figure: Hourly measured (dots) and simulated (lines) ecosystem net radiation (R n ), latent heat (LE) and sensible heat (H) fluxes during 2005 and 2003 over Palangka Raya peat swamp forest, Indonesia 2003 (dry year) + influxes - effluxes H/LE=0.36 (R n >700 Wm -2 ) H/LE=0.62 (R n >700 Wm -2 )

Preliminary Results contd (wet year) Figure: Hourly measured (closed symbol), gap filled (open symbol) and simulated (lines) ecosystem CO 2 fluxes during 2005 and 2003 over Palangka Raya peat swamp forest, Indonesia 2003 (dry year) + influxes - effluxes 0.25 mm

Preliminary Results contd. ParametersUnitsn 1a1a 2b2bR2R23 RMSD 4 RMSE CO 2 flux μmolm -2 s RnRn Wm LE flux Wm H flux Wm  (0-20cm) m 3 m T s (5cm) CC Table: Regression results of simulated vs measured hourly parameters ( ) n 1a1a 2b2bR2R23 RMSD intercept and 2 slope from regression of simulated over measured data; 3 root mean square for differences from regression of measured over simulated data; 4 root mean square of random measurement errors calculated for the measured data based on algorithms described in Richardson et al. (2006) Table: Regression results of simulated vs measured daily water table depths (m) ( ) R n = Net radiation; LE = Latent heat flux; H= Sensible heat;  = Soil water content; T s = Soil temperature

Preliminary Results contd. 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  (m 3 m -3 ) Open symbols have >50% gap filled data +NEP = C sink - NEP = C source 2003 (dry year)

Preliminary Results contd. 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  (m 3 m -3 ) Open symbols have >50% gap filled data +NEP = C sink - NEP = C source 2005 (wet year)

Preliminary Results contd. Components of ecosystem carbon budget 2003 (drier year)2005 (wetter year)Values from other studies EcosysECEcosysEC g C m GPP RaRa R root RhRh R soil RE NEP Table: Annual ecosystem carbon balance for Palangka Raya Drained forest, Indonesia during 2003 and Hirano et al. 2007; 2 Henson 2005; 3 Suzuki et al. 1999; 4 Melling et al. 2005; 5 Kosugi et al. 2008; 5 Jauhianen 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 CO 2 fluxes.

 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. Summary contd.

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