Vessel diameter and xylem hydraulic conductivity increase with tree height in tropical rainforest trees in Sulawesi, Indonesia

In humid environments, where trees rarely experience severe soil water limitation, the hydraulic system of trees requires a functional architecture for effectively transporting of water to the crown despite a comparably low atmospheric evaporative demand for most of the year. Strategies of tropical trees to adapt their hydraulic properties to a perhumid climate are not well studied, as is the impact of tree height on the hydraulic conductivity and vessel anatomy of tropical canopy trees. We analyzed the dependence of hydraulic architecture on tree height in several phylogenetically different canopy tree species growing under the non-seasonal wet climate of a lower montane rainforest in Sulawesi, Indonesia. We determined leaf-specific conductivity (LSC), sapwood-area specific hydraulic conductivity (kS), and wood anatomy (vessel diameter and density) of sun-exposed twigs and of the trunk of 51 trees of eight abundant species ranging in tree height between 6.5 and 44 m. LSC and kS significantly increased with tree height (radj2=0.50 and 0.46, respectively) as did mean vessel diameter. We found this trend consistent for both, trunk (radj2=0.61 ) and twig (radj2=0.47) xylem vessel diameters. In contrast, vessel density and tree height were significantly negative correlated in twigs but not in the trunks. We assume that in a tropical perhumid climate with prevailing high atmospheric humidity, it seems more advantageous for tall trees to promote a high hydraulic conductivity in the conducting tissue of both the trunk and the upper crown, rather than to minimize the risk of drought-induced xylem embolism. However, the tree size-effect as observed in our study has to be validated in a larger sample of tropical tree species before universal rules could be deduced.