Leaf physiology does not predict leaf habit; examples from tropical dry forest

Leaf structure and physiology are thought to be closely linked to leaf longevity and leaf habit. Here we compare the seasonal variation in leaf hydraulic conductance (k(leaf)) and water potential of two evergreen tree species with contrasting leaf life spans, and two species with similar leaf longevity but contrasting leaf habit, one being deciduous and the other evergreen. One of the evergreen species, Simarouba glauca, produced relatively short-lived leaves that maintained high hydraulic conductance year round by periodic flushing. The other evergreen species, Quercus oleoides, produced longer-lived leaves with lower k(leaf) and as a result minimum leaf water potential was much lower than in S. glauca (-2.8 MPa vs -1.6 MPa). Associated with exposure to lower water potentials, Q. oleoides leaves were harder, had a higher modulus of elasticity, and were less vulnerable to cavitation than S. glauca leaves. Both species operate at water potentials capable of inducing 20 (S. glauca) to 50% (Q. oleoides) loss of k(leaf) during the dry season although no evidence of cumulative losses in k(leaf) were observed in either species suggesting regular repair of embolisms. Leaf longevity in the deciduous species Rhedera trinervis is similar to that of S. glauca, although maximum k(leaf) was lower. Furthermore, a decline in leaf water potential at the onset of the dry season led to cumulative losses in k(leaf) in R. trinervis that culminated in leaf shedding.