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.