Influence of fire and drainage on evapotranspiration in a degraded peat swamp forest in Central Kalimantan, Indonesia

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 a degraded 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 years between 2004 and 2016 by using the eddy covariance technique. The site burned in 2009 and 2014, and was drained in 2014. Monthly ET and net radiation (R-n) fluctuated in synchrony and thus they decreased considerably under fire-induced dense haze during the El Nino drought. Troughs of ET, groundwater level (GWL) and R-n and crests of vapor pressure deficit (VPD) coincided in their time-series variations. In the case of ET > precipitation (P), ET decreased when the GWL was deeper than -0.5 m. Half-hourly ET had a strong positive correlation with R-n (R = 0.89, p < 0.01), and partial positive relationship with VPD when VPD < 20 hPa and with GWL when GWL < -0.5 m. ET had no correlation with the Enhanced Vegetation Index (EVI), which represents above ground biomass 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 affected 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 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 excessively deepened. 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 Nina event. Drainage undoubtedly deepened the GWL, and ET severely decreased in cases with extremely deep GWL.

Automated summary

Tropical peat swamp forests play a significant role in water circulation, but their evapotranspiration is affected by fire and drainage. Environmental factors such as vapor pressure deficit and groundwater level can control ET differently depending on conditions, with VPD negatively affecting ET due to stomatal regulation functions under dry atmospheric conditions. Drainage deepens the groundwater level and severely decreases ET in cases with extremely deep groundwater levels.