Modeling Environmental Controls on Net Ecosystem CO2 Exchange of a Tropical Bog

Warmer and drier weather resulting from climate change could alter the balance between peatland ecosystem photosynthesis and respiration and contribute to increasing atmospheric CO2. Most of the studies about the impact of weather variables like precipitation and air temperature and soil physical properties like water table depth and soil temperature on peatland net ecosystem productivity are focused on northern temperate/boreal peatlands developed under low temperature climate regimes. In order to model how warmer and drier weather condition might lead to seasonal and interannual variations in net ecosystem CO2 exchange under a warmer climate, we deployed the hourly time step ecosystem model ecosys to test whether it can simulate CO2 and energy exchange patterns at hourly and seasonal time scales over a drained tropical peat swamp forest at Central Kalimantan, Indonesia. This test was further extended to explore how fundamental ecosystem processes are affected by warmer and drier weather thus causing seasonal (dry vs wet season) and interannual (dry vs wet year) variations to occur in ecosystem net carbon balance. We used hourly meteorological data measured over the site during 2002-2005 together with site microtopography (hummocks vs hollows), soil physicochemical properties, and physiological ecology of plant community to simulate an 89 years old peat swamp forest ecosystem representative of the real ecosystem out there. Model outcomes were then calibrated against valid eddy covariance (EC) measurements (excluding the gap-filled data) at the flux station on study site. Ecosys simulated EC CO2 fluxes (R2=0.85), latent heat fluxes (LE) (R2=0.85) and sensible heat fluxes (H) (R2=0.80) to an acceptable limit (RMSD < 2RMSE) during the study period. No seasonal or interannual systematic modeling bias was found. Net ecosystem productivity (NEP) was found to decrease during dry seasons and drier years due mostly to increased soil heterotrophic respiration stimulated by warmer and drier weather. NEP was further decreased by reduced CO2 fixation due to soil dryness stimulated by increased temperature and reduced precipitation in dry seasons and drier years. This study allowed us to develop valuable micro-scale insights about how peatland carbon storage will behave under future climate change scenarios.