Identifying environmental drivers of carbon and water fluxes and the origin of atmospheric water over a tropical mountain rainforest in Sulawesi, Indonesia
Final Report Abstract
One of the primary focuses of the BaririFlux project was to quantify the carbon uptake of a montane rainforest in central Sulawesi, Indonesia. As previous measurements with the eddy-covariance (EC) method at this site led to an estimated sink exceeding by far observations from other rainforests, we aimed to eliminate possible sources of overestimation and to substantiate new findings with independent measurements. To this end, we investigated possible storage of CO2 below the EC-sensor, correlations of carbon uptake with environmental drivers (radiation, diffuse radiation, air temperature, vapour pressure deficit, wind turbulence), soil respiration rates and their variability as well as changes in aboveground biomass. Addressing limited feasibility of accurately measuring night-time CO2-fluxes at our site due to ubiquitous nonturbulent conditions we applied a variety of gap-filling approaches, including a multi-layer ecosystem model (Mixfor-SVAT). Besides radiation as expected, diffuse radiation was the second most important factor for variability in NEE explaining 9% of the variability, other drivers had little impact. Including canopy storage lead to a small decrease in estimated carbon uptake compared to EC-only estimates, but the problem of underestimated night-time respiration persisted. The soil respiration measurements lead to a confirmation of the suspected night-time loss, enabling us to calibrate our model and to validate gap-filling approaches. Comparing direct biomass measurements and cumulative carbon flux sums using the ecosystem model to substitute night-time respiration measurements indicate that the forest at our site has been a moderate, but persistent carbon sink, yet uncertainty remains high. Furthermore, the BaririFlux project focused on the determination of the isotopic composition of the water fluxes exchanged between the atmospheric boundary layer and the ecosystem using a high frequency laser spectrometer. The project experienced, however, several unexpected challenges such as an overly long laser spectrometer repair and inaccessibility of the research site. Consequently, the research program had to be adjusted. We initially installed a series of pluviometer along a N-S transect in the Central Sulawesi region. The results showed the influence exerted by the local topography on the “amount effect”. We then moved to a research site in Jambi (East Sumatra) in the middle of an oil palm plantation, where our group is operating also an eddy covariance flux site. Here we measured continuously the isotopic composition of the atmospheric water vapor (δDv and δ18Ov) using a vertical profile system. We investigated and characterized, combining observations and models, the influence exerted by a mesoscale atmospheric perturbation, the Madden Julian Oscillation (MJO), on the local water vapor isotopic composition. Results show that the local isotopic composition is affected by the interaction between the mesoscale phenomena and the topography of the island. Within 2 days, as the MJO’s convective cluster approached Western Sumatra, the local δDv decreased by ≈ 60 ‰ (≈ 10 ‰ the δ18Ov) due to the progressive convective rainout and horizontal advection to our site. A 1-D isotopic distillation model showed that up to 30% of the isotopic depletion resulted from the rain reevaporation that took place during the intensification of the western Sumatra rainfall. We further quantified the isotopic composition of evapotranspiration (δET). δET showed a diurnal cycle with maximum values in the early afternoon. The last part of the research project focused on the determination of a specie-specific physiological trait of oil palms. Oil palm leaves and soil samples were collected and their water content cryogenically extracted. The isotopic compositions of bulk leaf water (δDBL and δ18OBL) and soil water (δDS and δ18OS) samples were used in a leaf water enrichment model in order to determine the leaves “effective path length” (Leff). Overall, our isotope based measurement campaign provided unique datasets of various aspects of the hydrological cycle in Indonesia.
Publications
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(2015): Response of CO2 and H2O fluxes in a mountainous tropical rainforest in equatorial Indonesia to El Nino events. Biogeosciences, 12, 6655-6667
Olchev A, Ibrom A, Panferov O, Gushchina D, Kreilein H, Popov V, Propastin P, June T, Rauf A, Gravenhorst G, Knohl A
