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, rest ricted largely to rocky habitats on the Arnhem Land Plateau in the wet -dry tropics of northern Australia. Allosyncarpia ternata grows in a w ide range of habitats, including sites near permanent springs, where i t forms a distinctive closed-canopy forest with an understorey of rain forest plants, and sites on exposed cliffs and hilltops, where it occu rs in open forest and woodland. Leaf water relations differ markedly b etween these contrasting sites. During the dry season, trees at open s ites show strong diurnal hysteresis in stomatal conductance (g(s)); af ternoon 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 t he 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 po tential. It is concluded that this flexible response in leaf water rel ations to seasonally dry conditions is partly responsible for the abil ity of A. ternata to occupy and dominate the vegetation in such a wide variety of habitats. The near confinement of the species to the Arnhe m Land Plateau is in part due to the water-holding capacity of the bed rock.