Tree height effects on vascular anatomy of upper-canopy twigs across a wide range of tropical rainforest species

Vessel diameter variation along the hydraulic pathway determines how much water can be moved against the force of gravity from roots to leaves. While it is well-documented that tree size scales with vessel diameter variation at the stem base due to the effect of basipetal vessel widening, much less is known whether this likewise applies to terminal sun-exposed twigs. To analyze the effect of tree height on twig xylem anatomy, we compiled data for 279 tropical rainforest tree species belonging to 56 families in the lowlands of Jambi Province, Indonesia. Terminal upper-canopy twigs of fully grown individuals were collected and used for wood anatomical analysis. We show that hydraulically weighted vessel diameter (Dh) and potential hydraulic conductivity (Kp) of upper canopy twigs increase with tree height across species although the relationship was weak. When averaged across given tree height classes irrespectively of species identity, however, a strong dependency of tree height on Dh and Kp was observed, but not on the lumen-to-sapwood area ratio (Al:Ax) or vessel density (VD). According to the comparison between actual tree height and the maximum tree height reported for a given species in the stand, we show that the vascular xylem anatomy of their terminal twigs reflects their canopy position and thus ecological niche (understory versus overstory) at maturity. We conclude that the capacity to move large quantities of water during the diurnal peak in evaporative demand is a prerequisite for growing tall in a humid tropical environment.