The water relations of Allosyncarpia ternata (Myrtaceae) at contrasting sites in the monsoonal tropics of northern Australia

Allosyncarpia ternata S.T.Blake (Myrtaceae) is an evergreen tree, restricted largely to rocky habitats on the Arnhem Land Plateau in the wet-dry tropics of northern Australia. Allosyncarpia ternata grows in a wide range of habitats, including sites near permanent springs, where it forms a distinctive closed-canopy forest with an understorey of rainforest plants, and sites on exposed cliffs and hilltops, where it occurs in open forest and woodland. Leaf water relations differ markedly between these contrasting sites. During the dry season, trees at open sites show strong diurnal hysteresis in stomatal conductance (g(s)); afternoon depressions in g(s) coincide with regular afternoon increases in vapour pressure deficit. Pressure-volume analyses indicate that A. ternata maintains turgor down to leaf water potential values of about -2.8 MPa, close to the minimum experienced by hilltop leaves late in the dry season. By contrast, trees on the ravine floor, with year-round access to water, exhibit much smaller diurnal and seasonal variation in stomatal conductance and little seasonal variation in leaf water potential. It is concluded that this flexible response in leaf water relations to seasonally dry conditions is partly responsible for the ability of A. ternata to occupy and dominate the vegetation in such a wide variety of habitats. The near confinement of the species to the Arnhem Land Plateau is in part due to the water-holding capacity of the bedrock.