Seasonal Patterns in Soil Moisture, Vapour Pressure Deficit, Tree Canopy Cover and Pre-dawn Water Potential in a Northern Australian Savanna

Abstract

The wet–dry tropics of northern Australia are characterised by extreme seasonal variation in rainfall and atmospheric vapour pressure deficit, although temperatures are relatively constant throughout the year.This seasonal variation is associated with marked changes in tree canopy cover, although the exact determinants of these changes are complex. This paper reports variation in microclimate (temperature, vapour pressure deficit (VPD)), rainfall, soil moisture, understorey light environment (total daily irradiance), and pre-dawn leaf water potential of eight dominant tree species in an area of savanna near Darwin, Northern Territory, Australia. Patterns of canopy cover are strongly influenced by both soil moisture and VPD. Increases in canopy cover coincide with decreases in VPD, and occur prior to increases in soil moisture that occur with the onset of wet season rains. Decreases in canopy cover 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 phenological 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 both decreasing 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 relatively 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-dawn water potential, which are demonstrably independent of rainfall. This finding emphasises the importance of VPD as a determinant of physiological and phenological processes in Australian savannas.

Long-term temporal variability of the radon-222 exhalation flux from a landform covered by low uranium grade waste rock

“…Some irrigation of the TLF occurred during the dry season 2009 to assist with plant development, but after that the VWC follows closely the rainfall pattern with higher VWC (20e25%) during the wet season and lower VWC (10%) during the dry. The VWC reported here is similar to soil water content reported in Duff et al (1997) for natural soils in a northern Australian savanna of the wet-dry tropics, which shows the same seasonal variability and range.…”

Convergence in hydraulic architecture, water relations and primary productivity amongst habitats and across seasons in Sydney

“…Stomata of evergreen species may be more sensitive to LAVPD than deciduous species (Duff et al 1997). Evergreen species generally have lower conductivities than deciduous species (Eamus and Prior 2001).…”

Environmental influence on canopy phenology in the dry tropics

“…However, in the tropics, several irrigation experiments in the field have shown little impact of soil water availability on leaf phenology (Wright and Cornejo, 1990;Myers et al, 1998). Several authors have suggested that evaporative demand may also be an important environmental variable controlling leaf phenology (Wright and Cornejo, 1990;Duff et al, 1997;Myers et al, 1998).…”