Northern Territory University, Darwin: Completed
Summary | Objectives | Background | Results | More information |
George Kelley gets a lift to her study site
This project was part of the first-round TS-CRC's research
project, Water and Carbon Exchange of Savannas and
concentrated on the way in which soil properties and plant
physiological characteristics affected fluxes of water in the
soil-plant-atmosphere continuum. Study sites were contained within
the Howard River East catchment Northern Territory, in conjunction
with other activities of the Centre's research project.
For more information on the PhD project, go to the the
first-round CRC project Water and Carbon Exchange of
The objectives of the project included:
- To monitor both soil water and groundwater levels in the
catchment on a regular basis and to determine seasonal changes in
both these measures. This work is conducted in collaboration with
Water Resources Division of Lands, Planning and Environment.
- The daily and seasonal monitoring of plant water status in
three major ecosystems of the savanna: eucalypt woodland, Melaleuca
swamp and monsoon vine forest.
- To monitor the storage of water in the stems of trees on a
daily and seasonal basis, in order to determine the extent to which
trees rely on water stored within their trunks for
- To quantify physical properties of the soils of the Howard
River East catchment, in order to determine how these properties
affect the availability and use of water by plants.
Savannas, a vegetation system comprising both trees and grasses
in varying proportions, cover more than 10 per cent of the global
land surface and approximately 25 per cent of the Australian
continent. Globally they are increasingly subject to rising levels
of human habitation and development. Australian savannas are
relatively pristine and consequently provide a rare opportunity to
investigate the structure and function of this ecosystem during
early stages of development.
Australian savannas are largely restricted to tropical and
sub-tropical regions and experience a highly seasonal rainfall of
between 500 and 1900 mm per year. Climate is dominated by summer
monsoons and a wet season of five or six months duration.
Development of groundwater resources in northern Australia is
increasing as population and horticultural requirements for water
increase. Within the extensive savanna of northern Australia,
smaller patches of wet monsoon forest and Melaleuca swamps
are found, typically in lower-lying areas with access to perennial
George’s work investigated aspects of the physiological
and hydrological properties (soil and plant) of trees in these
three contrasting vegetation assemblages. In addition, the PhD
investigated sources of water used by trees using stable isotope
analyses of xylem sap, soil water and groundwater.
The heat pulse technique was used to examine tree water use in
several dominant tree species of open-forest savanna,
Melaleuca swamp and monsoon forest in wet and dry
Transpiration (per unit leaf area) did not differ between
savanna and Melaleuca swamp. However, in the savanna, stand
water use was aseasonal (0.9 mm d -1 ) but in the
Melaleuca swamp forest stand water use was significantly
larger in the wet season (1.7 mm d -1 ), compared to the
dry season (1.0 mm d -1 ). Transpiration did not appear
to be reduced by waterlogging in the wet and early-dry seasons.
Water use was also seasonal in the monsoon forest (1.5 mm d
-1 in the wet season, 0.4 mm d -1 in the dry
season). Changes in leaf area and stomatal conductance may cause
seasonal patterns of water use in swamp and monsoon forests. Annual
water use was 315 mm in the savanna, 508 mm in the Melaleuca
swamp forest and 568 mm in the monsoon forest.
Whole tree hydraulic conductance ( K ) was measured for
savanna and Melaleuca swamp forest trees. There was no
significant difference in K (per unit sapwood area) between
seasons or communities. This suggests that trees in the two
communities exhibit similar hydraulic properties despite
significant differences in soil type and soil water
Groundwater use by Melaleuca trees was assessed by
comparing deuterium and oxygen isotope composition of xylem sap,
soil water and groundwater. Soil water was predominantly used in
the wet and early dry season (despite inundation during this time)
although groundwater was used towards the end of the dry season.
Dependence of Melaleuca swamp forest and possibly monsoon
forests (similarly located in low-lying areas) on groundwater has
important implications for management of groundwater resources.
Time domain reflectometers (TDR) were used to measure changes in
soil water content at several depths throughout over a 2 year
period in the savanna. During the wet season both overstorey (trees
) and understorey (predominantly grasses) utilised water in the
soil lying above a lateritic duricrust layer at 1.2 m depth.
Following cessation of rains, trees used water from above and below
the duricrust layer; during the later 1999 dry season water below
the duricrust was used exclusively. Water stored in the upper 5 m
of soil was sufficient to supply the entire volume of water
transpired by trees in the dry season.
Groundwater dependence by savanna vegetation appears unlikely.
However, Melaleuca swamp forests and monsoon forests are
more likely to be dependent on groundwater during the dry season.
This must be
an important consideration for future management of catchments and