Eucalypts are deep drinkers near Quirindi

Peter Walsh exposes Ironbark roots
Peter Walsh, of NSW State Forests, exposes the rooting habits of Ironbarks at “Paringa”, on the Liverpool Plains. The trees have been drawing almost 80 per cent of their water from a perched groundwater system, probably charged during earlier decades of annual cropping.

Eucalypts planted a decade ago in one metre deep soil on an outcropping sandstone ridge on the Liverpool Plains have survived, but where were they getting their water?

Researchers are confident they have answered the question and concluded in doing so that the trees help to reduce salinity. Flooding, soil erosion, dryland salinity and the run-down in soil organic matter fertility are historically the most important natural resource issues on the Liverpool Plains, one of the most productive farming areas in Australia.

However, the plant available water holding capacity of these soils is low and the sandstone ridges that cover 20 to 30 per cent of this part of the Upper Liverpool Plains catchment can contribute significant volumes of water during periods of high rainfall.

In 1996 David, Wendy and Sandy Cudmore put in a plantation of Ironbark on their property “Paringa”, about 20 kilometres west of Quirindi, to revegetate and create habitat on an area regularly cropped since the early 1900s. The site was ideal for a recent trial, which aimed to compare the water balance of a native pasture, a plantation and remnant native vegetation on an outcropping sandstone ridge.

Where ironbarks draw water:
A perched water table is a body of water under the soil surface that occurs above the main water table. It occurs when there is an impermeable layer of rock or clay above the main water table but below the surface. Water percolating down to the main aquifer gets trapped above this second impermeable layer.

Collaborating researchers wanted to determine the most appropriate form of land management for two reasons. “The first was to reduce water flow from sandstone ridges onto the highly productive alluvial plains, to mitigate flooding, water logging and the mobilisation of clay-bound salt into shallow alluvial water tables,” NSW Department of Primary Industries hydrologist at Queanbeyan, Tony Bernardi, said.

The second was to maintain a level of useful productivity on the sandstone ridges for grazing, wood production and amenity. According to another team member, Dawit Berhane, hydrogeologist with the newly created Department of Water and Energy (formerly Natural Resources), at a sampling in June 2006, the trees in the plantation were drawing only about eight per cent of their water from the shallow soil.

“Trees had been able to set their roots deep and got 15pc of their water from the fractured rock beneath the soil and 77pc from the perched groundwater system, probably charged during earlier decades of annual cropping,” Mr Berhane said.

Isotopic signatures:
The abundance of stable isotopes of water, hydrogen and oxygen – Deuterium and Oxygen 18 – can be used to characterise natural waters and thus determine water sources in hydrologic processes. ‘Isotopic signatures’ of plant water sources (i.e. ground, surface, and soil water at various depths) can be used to characterise seasonal plant water sources and demonstrate how plants use water and set roots.

Identifying their water sources is based on a comparison of the isotopic signature of the plant material to that of the potential source waters. If a plant obtained all water from one source, the isotopic composition of the plant water should be the same as that of the source water. In the presence of several sources, concentrations of Deuterium and Oxygen 18 can be used to quantify the contribution of each.

To work this out, they collected rainwater during various falls, and sampled ground water and twigs from trees in June 2005 after a six month dry spell, and again in July 2005 after 130 millimetres of rain. In June 2006 they collected rainfall, twig and soil samples for stable isotope analysis. They found that the isotopic signature of the twigs from June 2005 was similar to samples of the perched ground water isotopic signature sampled from observation bores.

After the 130mm rainfall, the twig signature was similar to the rainfall signature but different enough to indicate that the trees were still accessing water from the fractured rock and perched groundwater.

“Changes in soil water under all three vegetation types – the plantation, nearby remnant vegetation and native pastures, mainly Slender Bamboo Grass one kilometre away – were very similar at all times, suggesting similar rates of evapo-transpiration before all soil water was depleted,” DPI Research Agronomist Rick Young said. “However, once the soil was dry, plantation trees continued to transpire.”

Measurable perched groundwater 20 metres below the plantation has fallen steadily over the past three years, suggesting sucking by the plantation. In contrast, groundwater 52m or more below remnant native vegetation and 600m downslope under grassland has fluctuated by approximately one metre, apparently in response to wet periods that occurred six months prior.

The researchers concluded that tree plantations with groundcover or strategically placed tree belts with native pastures would significantly reduce water coming from the sandstone hills, and water logging and salt mobilisation on the Liverpool Plains. Should the top 20m of the regolith wet up to some extent as a result of an extended wet period, this water would be accessed by trees but not by crops and pastures, Mr Young said.

The research is part of the Key Sites project, a joint effort between the DPI, the new Department of Water and Energy, the CRC for Plant Based Management of Dryland Salinity and the universities of Sydney, and NSW. Visit the Key Sites webpage for more information about this project.