Wired woodlands signal stress as climate dries
In 2015, the normal ‘breathing’ pattern of the Great Western Woodlands in south-western Australia became erratic. In response to lack of rain, the old-growth woodland started to ‘breathe in’ oxygen and ‘breathe out’ carbon dioxide – the opposite of what occurs in normal plant photosynthesis, and a sign the trees were ailing.
Lift your eyes above the orange- and gold-hued gimlets and salmon gums that characterise these woodlands and you can see the key to understanding this reversal of nature; a 36m tower equipped with highly sensitive instrumentation.
It’s the technology in that tower that has been tracking the trees’ struggle for survival; collecting data on the activity of the woodland by the second and updating daily CSIRO researchers based in Perth.
The tower, run by CSIRO researchers Dr Suzanne Prober and Dr Craig Macfarlane, is a part of Australia’s Terrestrial Ecosystem Research Network (TERN) OzFlux Facility. A series of 24 OzFlux towers and 10 associated Supersites have been established as sentinels of change across the country. Toward the TERN goal of creating a national ecosystem observatory, each monitors changes in carbon, water and biodiversity in a nationally significant landscape.
The Great Western Woodlands is the world’s most extensive area of temperate woodland, covering three times the area of Tasmania. It nevertheless took Dr Prober’s team some time to find enough uniform old growth salmon gum woodland in the Great Western Woodland where the tower could be constructed.
They found the site at the Credo Proposed Conservation Reserve, managed by the Department of Parks and Wildlife, 110 km north-west of Kalgoorlie. The 36m TERN OzFlux tower was built at twice the average height of the vegetation, from which instruments measure the activity of the woodlands over a 3.6 km diameter area – about 100 times the tower height.
“The Credo woodlands are literally ‘wired-up’,” says Dr Prober. “Networks of sensitive instruments continuously monitor the millimetre-by-millimetre growth in girth of the salmon gum and gimlet trees. Other instruments inserted just beneath the trees’ bark measure the flow of sap between the canopy and roots.
“Dawn and dusk bird choruses are monitored daily by automatic recording, photographs from high above the canopy capture a birds-eye view of the woodlands every hour, and moisture sensors capture the fluctuations in soil moisture to 1.2 m deep in the soil.”
Dr Macfarlane regularly climbs the tower to check instrument functioning.
“The instruments measure the ‘breathing’ of the woodlands – the exchange of carbon dioxide and water between the woodland’s trees and shrubs and the atmosphere,” explains Dr McFarlane.
“Data from the instruments is sampled ten times per second and streamed to the CSIRO’s offices in Perth, allowing researchers there to keep a daily eye on the health, growth and activity of the Credo woodland through good years and bad.”
Data tells the story of a woodland on the edge of survival
It was in July last year, six months after the last significant rain, that the OzFlux tower’s readings showed the Credo woodland had suddenly run out of water, following a dry summer in 2014-15.
“Rather than taking in carbon, as we would normally expect for a healthy growing woodland, the woodlands began to emit carbon back into the atmosphere, a sign of plummeting growth rates and shedding of leaves to survive the drought,” says Dr Macfarlane.
“Although we are now, one year later, seeing early signs of recovery, these data provide an important indication of how vulnerable these woodlands are to a warming or drying climate.
“With an average annual rainfall at Credo of only 260 mm, these trees live life on the edge – indeed, nowhere else in the world supports trees to 20 m tall at such low rainfall.”
Through the national network, TERN’s OzFlux sites together help researchers monitor and model Australia’s natural carbon budget, and paint a picture of how climate is changing the vegetation and landscape. Data from OzFlux towers – themselves part of a global FluxNet network of more than 650 towers – have also contributed to over 100 international ecosystem research projects in the past six months.
For its ECOSTRESS mission, NASA is linking OzFlux data with thermal infrared measurements from the International Space Station to gain an understanding of how climate change will affect water use by plants from different biomes across the planet.
Scientific and traditional knowledge track the changing ecology
Both scientists and the local community are concerned that increasing temperatures and declining winter rains are threatening the survival of these iconic trees, and that the woodlands are burning too often.
CSIRO researchers have worked previously with a local GWW Indigenous community, the Ngadju people, to document their traditional and contemporary knowledge and aspirations for managing fire in these landscapes.
“The Ngadju people say their old people usually wouldn’t start a fire in this country, because it takes hundreds of years to grow back. Indeed, unlike many eucalypt species that resprout after fire, the gimlets and salmon gums are easily killed by fire and need to begin again from tiny seeds,” says Dr Prober.
“These trees are exceptionally slow-growing. For example, gimlets on average grow only 1 mm per year, making a tree with 60 cm girth about 600 years old.”
Back at the Great Western Woodlands SuperSite, researchers have been intensively monitoring plants and animals to help them distinguish between natural fluctuations in species abundances due to annual variations in weather, and long-term trends associated with a changing climate which present a fire risk.
“It’s especially important for us to watch for new invading species that could completely change the ecology of the woodlands, a task also being taken seriously by rangers from Ngadju Conservation in woodlands south of Credo,” says Dr Prober.
“We are concerned that climate change could increase summer rains coming down from tropical cyclones in the north, resulting in invasion by summer-growing grasses that promote fire.
“Controlling such invaders before they establish in the woodlands could be one way to limit future fires, and help ensure these magnificent woodlands persist for future generations.”
Read back about work with the Nadju community on fire management.