SYDNEY - Nations round the world need to cut back on their water use if they want to save their precious woodlands and rivers, warned scientists at the Australia's National Centre for Groundwater Research and Training (NCGRT).
The work of PhD researcher Sepideh Zolfaghar reveals that even trees in areas with abundant rainfall are at risk from over-extraction of groundwater.
"Most groundwater research has focused on dry and semi-dry environments because there is much less water available and the trees are at risk of dying if too much groundwater is taken from these areas," says Professor Derek Eamus of NCGRT and University Technology of Sydney (UTS) who supervised the research.
"However, we have now shown that even trees growing in wet climates are far more sensitive to water stress than we originally thought, and may be less likely to survive if too much groundwater is extracted."
Currently, it is estimated, the world is extracting around 1000 cubic kilometres of groundwater every year - often far more than is naturally replaced from rainfall - and this is causing water shortages in many regions.
Mrs Zolfaghar explains that in regions with high rates of rainfall, if the water table is sufficiently shallow, forests will use this source of water. "But this is also why they can't cope with a sudden water shortage," she says.
"They've grown in an environment where water supply is continuous and plentiful, so they haven't evolved to cope with droughts. Trees in dry areas, in comparison, have developed drought resistance and are more resilient."
In the research, Ms Zolfaghar studied how groundwater depth affects eucalypts in a humid forest in New South Wales (NSW), Australia. Currently, the NSW government has reserved the groundwater beneath this forest for future use in Sydney, NSW, in case of drought.
"We found that once the groundwater depth exceeded 10 metres, the trees became less productive," says Mrs Zolfaghar.
"When groundwater depth increased from 5.5 metres to 9.8 metres, the trees were significantly shorter and tree density - the number of trees per hectare of land - was much lower than when groundwater depth was shallow.
"When water supplies are reduced, such as when too much groundwater is extracted, trees absorb less carbon dioxide. This means they can't grow as fast or as tall as when water supply is abundant."
Prof. Eamus says that, globally, drought is increasing in frequency and severity, putting further pressure on groundwater resources and hence even threatening forests growing in wet regions.
He says this is the first study to have tracked how groundwater depth affects trees across multiple scales, including small-scale (cellular, leaf), medium-scale (branches and whole tree) and larger scales (entire stands of trees).
"We found that the whole system is tightly linked - one change can affect everything else.
"Based on this study, we can also develop a mathematical model to help find out how much groundwater we can extract from woodlands growing in wet environments. This can potentially be adapted for use in forests worldwide."
The research indicates that despite their abundant rainfall, we should still be concerned about how groundwater extraction affects the humid areas of the world, Prof. Eamus adds.
"We're already facing a global water crisis - and we can't simply continue to extract groundwater from underneath forests without potentially damaging the forest.
"Not only that, since a lot of groundwater flows into rivers, we can also expect to see changes in river flow when groundwater extraction is excessive.
"Studies like these allow us to better understand how ecosystems use groundwater and how sensitive they are to changes in groundwater depth. For instance, we now know the limit of groundwater extraction from this forest in New South Wales, so we won't risk pumping out too much and killing the trees in times of drought."
The National Centre for Groundwater Research and Training is an Australian Government initiative, supported by the Australian Research Council and the National Water Commission.
For more information, go to www.groundwater.com.au