Ga. Duff et al., SEASONAL PATTERNS IN SOIL-MOISTURE, VAPOR-PRESSURE DEFICIT, TREE CANOPY COVER AND PRE-DAWN WATER POTENTIAL IN A NORTHERN AUSTRALIAN SAVANNA, Australian Journal of Botany, 45(2), 1997, pp. 211-224
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.