[A-CG37] Biogeochemical cycles in Land Ecosystem

[ACG37-07] Influence of fire and drainage on evapotranspiration in a tropical ex-peat swamp forest

*Shinjiro Ohkubo¹, Takashi Hirano¹, Kitso Kusin² (1.National University Corporation, Hokkaido University, 2.University of Palangkaraya)

Keywords:Groundwater level, Haze, Latent heat flux, Net radiation, Precipitation, Vapor pressure deficit

Tropical peat swamp forests (PSFs) play a significant role in the exchange of water between land and the atmosphere. However, fire and drainage have been expanding in PSFs in recent decades. Although there is concern on the influence of fire and drainage on water circulation, their influence on evapotranspiration (ET) is insufficiently understood. Furthermore, repeated fire occurrences and their corresponding influence on the ET by recurrent burning and smoldering is unexplored. To elucidate these influences, we examined long-term variation of ET in an ex-peat swamp forest in Central Kalimantan, Indonesia that was affected by drainage and repeated fire. The continuous observation of energy fluxes was conducted for approximately 13 y between 2004 and 2016 by using the eddy covariance technique. The site burned in 2009 and 2014, and it was drained in 2014. Monthly ET was synchronized with net radiation (R_n) and thus decreased considerably under fire-induced dense haze during the El Niño drought. Troughs of ET were simultaneous with crests of vapor pressure deficit (VPD), and troughs of ground water level (GWL) and R_n. In the case of ET > precipitation (P), ET decreased when the GWL was lower than -0.5 m. Monthly ET had a strong correlation with R_n (R=0.87, p <0.01), and moderate correlation with VPD (R=−0.51, p <0.01), GWL (R=0.68, p <0.01), and P (R=0.38, p <0.01). ET had almost no correlation with the Enhanced Vegetation Index, which represents above ground biomass (R=0.01) for the entire observation period. Alternately, the results of path analysis showed that some environmental factors controlled ET differently depending on environmental conditions. Generally, VPD negatively affects ET due to stomatal regulation functions under dry atmospheric conditions. However, the effect was negligible during the water-logged periods. This is because atmospheric dryness facilitated evaporation from exposed water on the ground surface, which canceled out the negative effect of transpiration due to stomatal closure. After drainage by canal excavation, fluctuation of GWL did not significantly influence ET. Although, the absolute quantity of ET decreased. This may be due to the hydrophobic dried peat soil, which prevents rainwater infiltration, disconnection of capillary force, or disabled root water uptake as GWL is excessively lowered. Fire potentially decreased ET due to decreased transpiration by burning of vegetation. However, the decreasing effect was cancelled by increased evaporation from the waterlogged ground surface during the subsequent La Niña event. Drainage undoubtedly lowered the GWL, and ET severely decreased in cases with extremely low GWL.