Abstract: Forests in Southeast Asia have been disrupted by widespread deforestation and land use change. Most countries in this region attempt to return the forested areas through reforestation. However, planting trees efficiently is difficult because changing environmental conditions could affect the water consumption of different tree-species under climate change. Hence, the information regarding water use characteristics of different tree-species is important to the selection tree species for reforestation. Nonetheless, available information on species-specific water-use characteristics is still limited, especially in secondary tropical forests. To gain the information on species-specific water-use characteristics in secondary forests, we estimated tree water use (T) of dominant tree species including Syzygium antisepticum and Adinandra integerrima, hereafter Sa and Ai respectively, in a secondary tropical forest at Khao Yai National Park Thailand using data from custom-made thermal dissipation probes. Specifically, we evaluated the responses of T of both species in large and small sizes to environmental factors including soil moisture and vapor pressure deficit (VPD) which represent soil and atmospheric humidity, respectively. Results of different soil moisture conditions in both sizes showed consistently higher T in Sa compared to Ai at across VPD ranges. Overall, both species of all sizes were more responsive to rising VPD when soil moisture were higher with high degrees of sensitivity in Sa. Under low water availability, small Ai could maintain their water use at low and high VPD levels while small Sa consumed more water at low VPD before decreasing the consumption rate at high VPD to save water. This result implies that small Ai was more tolerance than small Sa and strongly controlled their water use under low water availability, which may prevent it from potentially negative effects from droughts. Moreover, large Ai could still maintain their water use regardless change in VPD under low water availability. Our results suggest that Ai in small size may be suitable for reforestation through its conservative water-use behavior and may benefit downstream ecosystems with increasing runoff from the forest during drought. This study highlights that the response of T depends on species and tree size. Such information would benefit the selection of tree species for reforestation that could adapt well to environments and support policy design on the management of tropical forests and natural resources.
