Patterns of tree and stand water use in the eucalypt open-forests of northern Australia
Creators: name: O'Grady, Anthony
Publisher: name: Charles Darwin University
Year: 2000

description: The savannas of northern Australia occupy over 25% of the Australian mainland extending from Western Australia to far north Queensland and are restricted to the tropical and sub-tropical regions of the continent with an annual rainfall more than 500 mm. Climate is dominated by summer monsoons during which over 95% of the annual precipitation occurs, followed by a six month dry season, during which very little or no rainfall occurs. Annual rainfall decreases with increasing latitude, as does the predictability of an annual wet season, resulting in large-scale gradients in atmospheric and soil moisture within these savannas. Moisture availability is a key variable determining savanna structure and function in Australia and worldwide. The highly seasonal climate of northern Australia has been shown to affect many of the leaf scale processes and phenological patterns of trees. Increasing pressure on savanna resources, in particular groundwater, has led to a need for a greater understanding of the role of vegetation in the water balance of northern Australia. This thesis examines the temporal and spatial patterns of water use in these savannas at tree and stand scales in eucalypt-dominated communities. Detailed studies of tree water use and the spatial and temporal patterns of LAI were conducted in the Eucalyptus miniata/Eucalyptus tetrodonta open-forests, the dominant forest type of the savannas about the 1200 mm rainfall isohyet. Relationships between tree water use and tree size and seasonal patterns of tree water use were also examined at a high rainfall, intermediate rainfall and at a low rainfall site across a 700 km rainfall gradient. Leaf area index within E. miniata/E. tetrodonta open-forests was highly seasonal, reflecting the highly seasonal climate. Canopy LAI declined from approximately 1.0 during the wet season to less than 0.6 by the end of the dry season. Whilst up to 75% of the species that occur in these forests are deciduous, semi-deciduous or brevi-deciduous, the standing biomass of these forests was dominated by the two evergreen species, E. miniata and E. tetrodonta. Thus, a large proportion of the decline in canopy LAI in these forests was attributable to the dry season decline in leaf area of these two species. Leaf area index was strongly correlated to basal area during the wet season, although the relationships were more variable during the transition periods between the wet and dry seasons and during the dry season itself This was attributed to spatial variability in leaf phenology and to frequent and widespread fires during the dry seasons. Heat pulse techniques were used to examine whole tree water use in the dominant tree species in a number of savanna communities in northern Australia. In contrast to the seasonal patterns of observed at the leaf scale, whole tree transpiration rates (ie. water use expressed on a leaf area basis) were higher during the dry season than in the wet season. However, the total volume of water used in each season was similar. Higher transpiration rates during the the dry season resulted because of a decline in leaf area per tree and an increase in the atmospheric vapour pressure deficit, such that a similar volume of water was transpired through fewer leaves. Daily transpiration rates also exhibited a marked hysteresis in relation to vapour pressure deficit. Morning transpiration rates at any given vapour pressure deficit were higher than afternoon transpiration rates. The degree of hysteresis was also more evident in the dry season than in the wet season. Similar patterns were observed at three sites across a broad rainfall gradient. Tree water use and transpiration rates were lower in both seasons at the most and site, Newcastle Waters. It is proposed that the hydraulic adaptations to the annual dry season that limit susceptibility to xylem embolisms when water availability is lowest, limits wet season water use in the eucalypts of northern Australia. Tree water use was strongly correlated to a number of tree size parameters including diameter, cross sectional basal area and leaf area. These relationships were independent of species and formed the basis for scaling tree water use to stand water use in these forests. Aseasonal patterns of tree water use resulted in aseasonal patterns of stand water use. This was in contrast to total evapotranspiration measured using eddy co-variance techniques. Evapotranspiration varied from more than 3 mm day-1 during the wet season to approximately 1.2 mm day-1 in the dry season in the E. miniata/E. tetrodonta open-forests. Understorey grass and soil evapotranspiration contributed significantly to total evapotranspiration in the savannas of northern Australia. Further, tree basal area declined with increasing aridity in northern Australia, therefore the contribution of understorey evapotranspiration to total evapotranspiration would be expected to increase.