SEASONAL PATTERNS IN SOIL-MOISTURE, VAPOR-PRESSURE DEFICIT, TREE CANOPY COVER AND PRE-DAWN WATER POTENTIAL IN A NORTHERN AUSTRALIAN SAVANNA

The wet-dry tropics of northern Australia are characterised by extreme seasonal variation in rainfall and atmospheric vapour:pressure defici t, although temperatures are relatively constant throughout the year. This seasonal variation is associated with marked changes in tree cano py cover, although the exact determinants of these changes are complex . This paper reports variation in microclimate (temperature, vapour pr essure deficit (VPD)), rainfall, soil moisture, understorey light envi ronment (total daily irradiance), and pre-dawn leaf water potential of eight dominant tree species in an area of savanna near Darwin, Northe rn Territory, Australia. Patterns of canopy cover are strongly influen ced by both soil moisture and VPD. Increases in canopy cover coincide with decreases in VPD, and occur prior to increases in soil moisture t hat occur with the onset of wet season rains. Decreases in canopy cove r coincide with decreases in soil moisture following the cessation of wet season rains and associated increases in VPD. Patterns of pre-dawn water potential vary significantly between species and between leaf p henological guilds. Pre-dawn water potential increases with decreasing VPD towards the end of the dry season prior to any increases in soil moisture. Decline in pre-dawn water potential coincides with bath decr easing soil moisture and increasing VPD at the end of the dry season. This study emphasises the importance of the annual transition between the dry season and the wet season, a period of 1-2 months of relativel y low VPD but little or no effective rainfall, preceded by a 4-6 month dry season of no rainfall and high VPD. This period is accompanied by markedly increased canopy cover, and significant increases in pre-daw n water potential, which are demonstrably independent of rainfall. Thi s finding emphasises the importance of VPD as a determinant of physiol ogical and phenological processes in Australian savannas.