Hideki Takizawa

Transpiration peak over a hill evergreen forest in northern Thailand in the late dry season: Assessing the seasonal changes in evapotranspiration using a multilayer model

[1]xa0The seasonal changes in evapotranspiration over a hill evergreen forest in northern Thailand (18°48′N, 98°54′E), in the Asian monsoon region, were simulated using a multilayer model and the boundary conditions above the canopy. The simulation considered the uncertainty in the leaf area index (LAI) and physiological parameters for both photosynthesis and stomatal conductance models. The parameters were based on the estimated LAI and determined by referring to the measured net photosynthesis rate and stomatal conductance for a single leaf. The simulated cumulative evapotranspiration and rainfall interception agreed with the values obtained from the water budget within these uncertainties. The sensitivity of these limits to both evaporation and transpiration was also investigated. The simulated transpiration peaked in the late dry season. The latent heat flux obtained with the eddy correlation technique showed that the forest continued to transpire in the late dry season. The heat pulse velocities also showed a peak in water use by individual trees in the late dry season. These results counter the view that evapotranspiration declines in the dry season, as has been reported previously for an evergreen forest and other vegetation in Thailand. The transpiration peak was thought to depend on the reduced wetness of the canopy, and the consequent lack of evaporation from it, and on the fact that there was little decline in stomatal conductance, even in the driest conditions.

Soil respiration and soil CO2 concentration in a tropical forest, Thailand

Soil respiration and soil carbon dioxide (CO2) concentration were investigated in a tropical monsoon forest in northern Thailand, from 1998 to 2000. Soil respiration was relatively high during the rainy season and low during the dry season, although interannual fluctuations were large. Soil moisture was widely different between the dry and wet seasons, while soil temperature changed little throughout the year. As a result, the rate of soil respiration is determined predominantly by soil moisture, not by soil temperature. The roughly estimated annual soil respiration rate was 2560u2009gu2009Cu2009m−2u2009year−1. The soil CO2 concentration also increased in the rainy season and decreased in the dry season, and showed clearer seasonality than soil respiration did.

Impact of rooting depth and soil hydraulic properties on the transpiration peak of an evergreen forest in northern Thailand in the late dry season

[1]xa0Previous research showed that transpiration in an evergreen broad-leaved forest in northern Thailand (18°48′N, 98°54′E) peaked at the end of the dry season. However, rooting depth limitations on soil water use were not investigated. This study examined the impact of rooting depth and soil hydraulic properties on transpiration using a newly developed soil plant air continuum model. The soil texture at the site was classified as silty sand from the measured relationship between the volumetric soil water content and soil water potential at 0.1-m depths. To effectively simulate heat pulse velocity variation corresponding to dry season transpiration and annual discharge, a rooting depth of 4–5 m was needed, assuming a silty sand soil texture under unsaturated conditions. This value is less than the reported maximum rooting depth of trees and is considered reasonable. A penetration test showed that soil became harder at depths of 4–5 m. The model results suggest that a shallower rooting depth is sufficient to maintain the late dry season transpiration peak if other soil textures with lower hydraulic conductivity are assumed. By contrast, the late dry season transpiration peak could not be maintained under unsaturated conditions if a sandy soil texture with high hydraulic conductivity was assumed, even with a rooting depth of 12 m.

Severe Drought Resulting from Seasonal and Interannual Variability in Rainfall and Its Impact on Transpiration in a Hill Evergreen Forest in Northern Thailand

Our previous study revealed that a hill evergreen forest in the Kog-Ma experimental watershed in northern Thailand, which is influenced by Asian monsoon cycles, transpired actively even in the late dry season. In this study, the impact of severe drought on the transpiration of this forest was investigated using data measured at the site over 8 years that showed seasonal and interannual variation in rainfall. To this aim, the impacts of soil drought on sap flow and water potential were examined during severe drought conditions. This site showed large interannual variation in the total amount of annual rainfall and in the length of the dry period. An unusually severe drought occurred in the late dry seasons of 1998 and 2004 as a result of the small amount of annual rainfall and a prolonged dry period coinciding with El Nino. Under the detected severe drought conditions in the late dry season of 2004, noticeable symptoms of water stress were apparent only in the smallest study tree. Decreases in sap flow velocity and water potential caused by soil drought were not apparent in larger trees. Deeper root systems of larger trees may explain the lower impact of severe drought on transpiration in larger trees. Transpiration in this forest could be maintained actively even under unusually severe drought conditions.