Species-Specific Water-Use Characteristics of Trees in Old-Growth and Secondary Tropical Forests of Thailand

Tree water use is a critical component of the forest water cycle and is influenced by global climate changes, such as shifts in precipitation patterns. These changes may disproportionately affect forest runoff depending on how sensitive tree water use is to environmental conditions. Therefore, understanding the water-use strategies of different tree species is essential for predicting how forests will respond to environmental change. This study investigated how daily sap flux density (J_s), which represents water flow per unit of sapwood area, varies with environmental factors in common tree species in successional tropical forests in Thailand. Using thermal dissipation probes, we measured J_s in both an old-growth forest (OF) and a young forest (YF). Results indicated that trees in the OF were highly sensitive to rising vapor pressure deficit (VPD) under low soil moisture, indicating a response to atmospheric demand, while trees in the YF could maintain their water use rate regardless of changes in VPD. In addition, species-specific patterns were observed across varying soil moisture conditions at both sites. In OF, Syzygium syzygoides and Cinnamomum subavenium exhibited conservative water use under low soil moisture, which might protect them from negative effects of droughts. In YF, Adinandra integerrima saturated its J_s earlier than other species under low soil moisture, likely to save water, indicating greater drought tolerance compared to others at this site. These findings provide valuable insights into species-specific water-use patterns across different successional stages, helping to predict how tropical forests may respond to environmental changes.