The seasonal variation in leaf xylem pressure potential at dawn (ψdawn), leaf tissue water characteristics and daily maximum leaf conductance was measured in eight woody species in a wet–dry tropical savanna near Darwin, northern Australia, between October 1992 and October 1993. The species were Eucalyptus miniata, E. tetrodonta, E. clavigera, Xanthostemon paradoxus, Erythrophleum chlorostachys, Planchonia careya, Terminalia ferdinandiana and Cochlospermum fraseri. The species represented the major leaf phenological types, evergreen, semi-deciduous and fully deciduous. The climate of the region is characterised by annual drought during the winter months, when virtually no rain falls and vapour pressure deficit (VPD) in the afternoon reaches 3 kPa for 5 consecutive months each year. Despite this drought, ψdawn remained high (–1.3 to –1.5 MPa in evergreen species and –0.5 to –1.5 MPa in deciduous species) relative to those trees that experience summer drought in temperate and arid Australia. There was a tendency for evergreen and semi-deciduous species to maintain positive turgor to lower xylem pressure potentials (mean osmotic potential at incipient plasmolysis, π0 = –2.15 MPa) than the fully deciduous species (π0 = –2.03 MPa). For all species, the daily maximum leaf conductance (gmax) was maximal in the wet and decreased during the dry season. Diurnally, (gmax occurred near midday in the wet season, but at about 0800–1000 hours during the dry season and the ‘buildup’, the transitional period between the dry and wet seasons. There was substantial decrease in (gmax (from 650–1000 mmol m-2 s-1 in March to 200 mmol m-2 s-1 in May) early in the dry season in two of the three fully deciduous species (Planchonia careya and Cochlospermum fraseri). The dominant evergreen species Eucalyptus miniata, by contrast, had high gmax (> 400 mmol m-2 s-1) throughout the dry season, suggesting it had access to groundwater. For each species, gmax declined with decreasing dawn water potential in a log-linear manner; the slope of this relationship tended to increase with increasing degree of deciduousness.

Data from savannas of northern Australia are presented for net radiation, latent and sensible heat, ecosystem surface conductance (G ) and stand water use for sites covering a latitudinal range of 5° or 700 km. Measurements were made at three locations of increasing distance from the northern coastline and represent high- (1,750 mm), medium- (890 mm) and low- (520 mm) rainfall sites. This rainfall gradient arises from the weakened monsoonal influence with distance inland. Data were coupled to seasonal estimates of leaf area index (LAI) for the tree and understorey strata. All parameters were measured at the seasonal extremes of late wet and dry seasons. During the wet season, daily rates of evapotranspiration were 3.1-3.6 mm day and were similar for all sites along the rainfall gradient and did not reflect site differences in annual rainfall. During the dry season, site differences were very apparent with evapotranspiration 2-18 times lower than wet season rates, the seasonal differences increasing with distance from coast and reduced annual rainfall. Due to low overstorey LAI, more than 80% of water vapour flux was attributed to the understorey. Seasonal differences in evapotranspiration were mostly due to reductions in understorey leaf area during the dry season. Water use of individual trees did not differ between the wet and dry seasons at any of the sites and stand water use was a simple function of tree density. G declined markedly during the dry season at all sites, and we conclude that the savanna water (and carbon) balance is largely determined by G and its response to atmospheric and soil water content and by seasonal adjustments to canopy leaf area.

Daily and seasonal patterns of transpiration were measured in evergreen eucalypt trees growing at a wet (Darwin), intermediate (Katherine) and dry site (Newcastle Waters) along a steep rainfall gradient in a north Australian savanna. Relationships between tree size and tree water use were also determined. Diameter at breast height (DBH) was an excellent predictor of sapwood area in the five eucalypt species sampled along the rainfall gradient. A single relationship existed for all species at all sites. Mean daily water use was also correlated to DBH in both wet and dry seasons. There were no significant differences in the relationship between DBH and tree water use at Darwin or Katherine. Among the sites, tree water use was lowest at Newcastle Waters at all DBHs. The relationship between DBH and tree leaf area was similar between species and locations, but the slope of the relationship was less at the end of the dry season than at the end of the wet season at all locations. There was a strong relationship between sapwood area and leaf area that was similar at all sites along the gradient. Transpiration rates were significantly lower in trees at the driest site than at the other sites, but there were no significant differences in transpiration rates between trees growing at Darwin and Katherine. Transpiration rates did not vary significantly between seasons at any site. At all sites, there was only a 10% decline in water use per tree between the wet and dry seasons. A monthly aridity index (pan evaporation/rainfall) and predawn leaf water potential showed strong seasonal patterns. It is proposed that dry season conditions exert control on tree water use during the wet season, possibly through an effect on xylem structure.

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.