Assessments of hydraulic responses to climate variability of dominant trees in successional forests in Khao Yai National Park for environmental management

Abstract

In Southeast Asia, tropical forests have been increasingly threatened by human disturbances and climate change-induced water stress. However, uncertainty remains as to how these forests would respond to environmental changes under water deficit conditions. Here, we conducted a study of seasonal variation in leaf water status and hydraulic properties of dominant tree species across forest successional stages in Khao Yai National Park, Thailand. Specifically, we measured midday leaf water potential (Ψmd) during the wet and dry season, and assessed branch xylem vulnerability at which 50% is lost (P50). The associations between P50 and maximum xylem hydraulic conductivity (Ksmax) and maximum vessel length (MVL) were examined. We also calculated percentage loss of hydraulic conductivity corresponding to Ψmd during the dry season (PLCdry). Our results showed that, lower Ψmd during the wet and dry season and greater variation in Ψmd resulted from seasonal changes were observed in drier sites. This could be an adaptive strategy to optimize photosynthesis and growth, but at higher risk of hydraulic failure. Substantial within-site variation in MVL, Ksmax, and P50 were observed in this study, leading to unclear variations across the successions. P50 was correlated with Ksmax, supporting the hydraulic efficiency-safety tradeoff, but not with MVL. Additionally, all studied tree species from each succession experienced changes in Ψmd during the dry season that could lead to PLCdry in different degrees. Species with more resistant to xylem cavitation exhibited lower PLCdry than species with higher xylem vulnerability. This observation highlights the significance of cavitation resistance for determining species’ risk of hydraulic dysfunction during low water availability. Therefore, knowing responses to water stress of different tree species and different forest successions would be beneficial for selecting ones that could be well adapted to specific environments, thus improving the strategies for managing forests in different successions under a warmer future.