WA's South West is facing severe environmental consequences as a result of the region "drying out", according to a new report.
An interim federal Parliamentary report on Australia's biodiversity in a changing climate found the drying trend – caused by changing rainfall patterns – was leading to tree decline and falling groundwater levels.
During site visits and consultation across the South West, committee members were told that while overall rainfall reductions had been moderate, changes in the seasonality and intensity of the rain had led to "dramatically reduced amounts of run-off."
"The total annual inflow into Perth dams has reduced from an average of 338 gigalitres prior to 1975 down to an average of only 74.5 gigalitres over the past six years," the report found.
"This was illustrated in Margaret River, where the committee heard that although rainfall has reduced by only 9 per cent in the past 10 years, this has resulted in a 46 per cent reduction in stream flow compared to previous decades."
The committee was also told ground water levels in the region had fallen by up to 10 to 20 metres in some parts.
"The impact of a drying climate on water supplies is already being observed in WA, and this appears to be having a correspondingly severe impact on the region's biodiversity," the report said.
The committee examined widespread declines in a range of woodland tree species, including tuart, wandoo, WA peppermint, jarrah and marri.
It found while the exact mechanisms leading to the declines were uncertain, "they are at least partially due to the hotter and drier conditions attributed to climate change in the area."
In 2010–11 alone, there was a mass collapse of 18,000 hectares of northern jarrah forest, coinciding with a period of high temperatures and the driest year on record.
The committee found this "decline and rapid collapse of tree communities" in response to climate change was a "concern".
It recommended long-term monitoring of the problem, and supported limited trials of management techniques which could help minimise future damage and help conserve the region's biodiversity.
It also called for more to be done to prevent the spread of phytophthora dieback, a deadly introduced plant disease which is worsening across the south of the state.
The disease was found to be the "single biggest threat to biodiversity in southwest WA".
"Of particular concern is that the pathogen is widespread in parts of South West WA with very high levels of biodiversity, such as the Stirling Range National Park.
"These areas are home to many unique and iconic plant species, which could be threatened with extinction if the threat from dieback is not contained," the committee said in its findings.
It recommended increased resourcing for efforts to monitor, contain, and treat affected areas, and to better educate the community about its threat.