Hydraulic safety margin of 17 co-occurring woody plants in a seasonal rain forest in Guangxi's Southwest karst landscape, China

Abstract

Aims Tropical seasonal rain forest of the karst landscape in southwestern Guangxi is one of the global biodiversity hotspots, with its habitats characterized with large exposed boulders, shallow and patchy soils, and cascading streams. As a result, plants in the karst landscapes experience frequent drought due to limited soil water holding capacity. In this study, we investigate the hydraulic characteristics of woody species in this drought-prone environment for sound conservation of plants in the fragile forests as well as through restoration actions. Methods We measured xylem vulnerability curves, minimum xylem water potential (Ψstem-min), sapwood density and leaf turgor loss point of 17 typical woody plant species that co-occur in a tropical seasonal rain forest. We compared cavitation-resistance and hydraulic safety margins among the species and/or by plant groups. In addition, we analyzed the relationships between hydraulic safety margins and other hydraulic characteristics. Important findings We found large inter-species variations in xylem water potential at 50% of hydraulic conductivity (P50 ranged from -0.51 to -2.51 MPa), with evergreen species showing more negative P50 than deciduous species. The hydraulic safety margin (HSM), calculated as the difference between Ψstem-min and P50 (Ψstem-min -P50), has been widely used to quantify the degree of hydraulic risk under droughts. The average HSM of the 17 woody species was 0.36 MPa, indicating a high hydraulic risk in this forest during the dry season. However, there existed significant differences in HSM among species. Interestingly, there was no significant correlation between HSM and other key hydraulic characteristics, likely because of different hydraulic strategies under drought stress. This study provides evidences of different hydraulic risks of the common species in response to droughts and highlights the importance of long-term monitoring of water potential for conservation of the plants, especially in extreme drought events.