5:15 PM - 6:45 PM
[AHW18-P12] Leaf phenology of overstory trees governs evapotranspiration in a dry deciduous forest in Cambodia
Keywords:tropical dry deciduous forest, leaf phenology, evapotranspiration
Tropical dry deciduous forests have some unique characteristics, that is, the early leaf flush of overstory trees in the middle of dry season, and the co-occurrence of dense understory vegetation. The former may seem counterintuitive because it is disadvantageous for trees when soil water conditions are severely dry due to months without rain. Despite a number of studies detailing leaf phenology in this biome, very few studies have examined the effects of understory vegetation on the forest water balance in tropical regions. To understand the effects of tree phenology and understory vegetation on the forest water balance, we measured evapotranspiration and its components, specifically transpiration and evapotranspiration from understory vegetation, separately in a dry deciduous forest in Cambodia.
Evapotranspiration from the whole ecosystem (ETW) and understory vegetation (ETU) were measured with the band-pass eddy covariance method applied above and below the forest canopy, respectively. Transpiration from overstory trees was estimated with sap flux density measurements using the thermal dissipation method. The leaf area index (LAI) of overstory trees was estimated based on the ratio of the photosynthetic photon flux density above and beneath the forest canopy.
Measured sap flux densities clearly decreased in the middle of dry season, showing leaf fall of overstory trees. Then, soon after that, before the occurrence of rainfall, sap flux densities increased under severe dry soil water conditions. Thus, the early leaf flush was confirmed in this ecosystem. Seasonality of transpiration of overstory trees exhibited a linear correlation with LAI in the dry season. During the dry season, although ETW linearly decreased with decline of LAI, responses of ETU to LAI were small. On the other hand, in the wet season, variations of ETW were independent of LAI, but ETU decreased clearly with the increase in LAI. As a result, the contribution of ETU to ETW (ETU/ETW) was governed by LAI throughout the year. The shallow soil water condition (< 60 cm depth) was minimum at the early leaf flush, suggesting the utilization of water from deeper soil layers. Our tentative results of stable oxygen isotopes of tree branches and soil water showed the importance of water resources from deeper part (> 60 cm depth). Around the Indochina Peninsula, severe droughts are expected under the effects of El Nino Southern Oscillation, and, thus, further observations and investigations are required to conserve this unique forest ecosystem under climate change.
This presentation is partially based on: Iida et al. (2016) Ecohydrol., 9, 472-486; Iida et al. (2020) Agr. For. Meteorol., 295, 108170.
Evapotranspiration from the whole ecosystem (ETW) and understory vegetation (ETU) were measured with the band-pass eddy covariance method applied above and below the forest canopy, respectively. Transpiration from overstory trees was estimated with sap flux density measurements using the thermal dissipation method. The leaf area index (LAI) of overstory trees was estimated based on the ratio of the photosynthetic photon flux density above and beneath the forest canopy.
Measured sap flux densities clearly decreased in the middle of dry season, showing leaf fall of overstory trees. Then, soon after that, before the occurrence of rainfall, sap flux densities increased under severe dry soil water conditions. Thus, the early leaf flush was confirmed in this ecosystem. Seasonality of transpiration of overstory trees exhibited a linear correlation with LAI in the dry season. During the dry season, although ETW linearly decreased with decline of LAI, responses of ETU to LAI were small. On the other hand, in the wet season, variations of ETW were independent of LAI, but ETU decreased clearly with the increase in LAI. As a result, the contribution of ETU to ETW (ETU/ETW) was governed by LAI throughout the year. The shallow soil water condition (< 60 cm depth) was minimum at the early leaf flush, suggesting the utilization of water from deeper soil layers. Our tentative results of stable oxygen isotopes of tree branches and soil water showed the importance of water resources from deeper part (> 60 cm depth). Around the Indochina Peninsula, severe droughts are expected under the effects of El Nino Southern Oscillation, and, thus, further observations and investigations are required to conserve this unique forest ecosystem under climate change.
This presentation is partially based on: Iida et al. (2016) Ecohydrol., 9, 472-486; Iida et al. (2020) Agr. For. Meteorol., 295, 108170.