Climatic water stress, such as droughts and warmer temperatures, may accelerate forest mortality. Increasesof frequency and intensity of drought events are predicted to increase in the monsoon Southeast Asia. Suchincreases could drive rapid and large-scale shifts in forest structure and species composition. Moreover, thedrought-induced mortality events may cause dramatic decreases in carbon stored by tropical forests whichare hotspots of biodiversity and a persistent carbon sink in the global carbon cycle.Secondary forests represent the majority of forested areas in the tropics and have higher carbonaccumulation rate than mature forests. However, compared to mature forests, we know relatively little abouttree functioning and ecophysiology of secondary forest ecosystem. Many forests in Southeast Asia consist ofvarious stages of ecological succession which are challenging to modelling climate-vegetation feedbacks inthis region. Differences in species composition of mature and secondary forests can further complicate ourunderstanding of how water will respond to climatic water stress in the future. Because changes in droughtfrequency and severity could have large consequences on forest structure and functioning, we need a betterunderstanding of the vulnerability of tropical forests to drought in order to more accurately predict globalcarbon and water cycling in light of climate change.Globally, canopy transpiration is the major component of total water transfer from forests to the atmosphere.Canopy transpiration is often used to estimate mean canopy stomatal conductance which is central tomodelling carbon uptake by forests. Differences in species compositions of mature and secondary forestsmay result in different canopy transpiration which can influence hydrologic and carbon cycles of theseforests. With limited understanding of water and carbon cycling in Southeast Asian secondary forests,especially under climatic water stress, uncertainty in climate-vegetation models may increase, leading toinaccurate forecast of future changes in the global water and carbon cycles.With these regards, we propose to estimate canopy transpiration and evaluate its variation with climaticconditions in a mature and a secondary forests in Thailand. We will also investigate species-specificresponses to water stress by assessing tree hydraulics and drought vulnerability of the dominant species ineach forest. Furthermore, we will explore the degree of soil water partitioning of species within each forest toenhance our understating of climate change impact on forest structure and function of both forest types.Additionally, this study will be the first, to our knowledge, that quantifies canopy transpiration in secondaryforests in Southeast Asia. The outcome can be used to plan restoration projects, as well as to promote thevalues of secondary forests worldwide.