Hydraulic safety margins of co-occurring woody plants in a tropical karst forest experiencing frequent extreme droughts

Abstract

Hydraulic safety margins (HSM) have been widely studied to assess the hydraulic risks and/or strategies of plant species under drought. However, calculations of HSM tend to vary between studies, leading to conflicting conclusions. In this study, HSM of 16 co-occurring woody species (including evergreen trees, brevi-deciduous trees, and lianas) in a tropical karst forest was investigated. They were expressed as: (1) the difference between minimum leaf water potential and water potential causing 50% loss of leaf hydraulic conductance (HSMleaf), (2) the difference between water potential at stomatal closure and that at 50% loss of branch hydraulic conductivity (HSMstomatal), and (3) the difference between water potential at 50% loss of leaf hydraulic conductance and that at 50% loss of branch hydraulic conductivity (HSMsegmentation). We asked the following questions: were HSMleaf in the normal and extreme dry seasons across plant groups different? Were there correlations between different HSM-calculations? Results showed that, on average, the three plant groups showed similar and positive HSMleaf in the normal dry season, but evergreen trees declined to a more negative value in the extreme dry season while the other two groups remained positive. The massive loss of leaf hydraulic conductance in several evergreen tree species was consistent with their extensive shoot dieback under extreme droughts. Across species, there were no significant relationships between HSMstomatal and HSMleaf. Most species (mainly lianas and brevi-deciduous trees) showed negative HSMsegmentation, which did not support the vulnerability segmentation hypothesis that branches are more resistant to cavitation than leaves. Moreover, more negative HSMsegmentation tended to have lower HSMstomatal and larger HSMleaf in the extreme dry season. This study indicates that karst evergreen trees are more likely to experience leaf hydraulic failure under extreme droughts, and reveals potential correlations between branch and leaf hydraulic safety strategies. Further studies on HSM-demographic rate relationship in the tropical karst forests are recommended.