People using TERN: Rachael Nolan

New research on two of Australia’s most iconic and widespread trees—the river red gum and mulga—is helping solve the puzzle of how they’re able to coexist in some of the nation’s hottest and driest environments. The results have major implications for how we manage our arid ecosystems and their groundwater resources and also for helping balance Australia’s carbon budget.

Appearances aren’t everything when it comes to two of Australia’s most beloved trees: the river red gum (Eucalyptus camaldulensis) and mulga (Acacia aptaneura). The majestic river red gum is a cultural and ecological icon in Australia, but when it comes to balancing the nation’s carbon budget its less striking cousin, the mulga, is king.

Mulga ecosystems cover about 20% of the Australian continent and absorb huge amounts of carbon from the atmosphere (photo courtesy of Rachael Nolan)

“There’s a hidden story behind Australia’s prolific and somewhat underappreciated mulga,” says Dr Rachael Nolan of University of Technology Sydney (UTS). “Just add water and these somewhat dull, shrub-like trees, flick the switch and start absorbing huge amounts of carbon from the atmosphere.”

Rachael and her colleagues from the Terrestrial Ecohydrology Research Group at UTS are investigating how this transformation works and why rates of photosynthesis in mulga species increase when it rains. They’re also studying the anatomy and physiology of trees to better understand how different species can co-exist in environments where water is such a limited and variable resource.

Rachael conducted her field research in the Northern Territory’s Ti-Tree Basin making use of the open access research infrastructure provided by TERN’s Alice Mulga SuperSite (above) (photo courtesy of Rachael Nolan)

“By studying the ecosystems at TERN’s central Australian observatories, we now know that mulga and river red gums use completely different strategies to survive in these semi-arid environments,” says Rachael.

“Mulga predominantly relies on rainfall and has a range of traits that allow it to survive seasonal dry periods. That’s why we see such big pulses in photosynthetic productivity and carbon uptake after major rainfall events. On the other hand, river red gums survive by accessing groundwater.”

Rachael’s work, which is part of an Australian Research Council (ARC) project, also shows that following repeated drought, mulga is able to adjust physiologically and continue photosynthesising. River red gums can’t do this and, instead, adjust by dropping leaves.

Multi-scale, long-term data collected by flux monitoring towers at TERN’s nation-wide ecosystem observatories, including the Alice Mulga SuperSite (above), are discoverable via the TERN Data Discovery Portal (photo courtesy of Rachael Nolan)

These findings on the functioning of groundwater-dependent ecosystems have major implications for how arid ecosystems and their water resources are managed.

“River red gum ecosystems rely on groundwater for survival in arid lands, but they’re faced with increased competition for groundwater resources by agriculture and mining,” says Rachael. “Understanding exactly what controls plant water-use and carbon uptake is the first step in achieving sustainable management of groundwater.”

It is also a vital step in balancing regional, continental and global terrestrial carbon budgets, considering the ecosystem services mulga provide in terms of carbon sequestration.

“Mulga ecosystems cover about 20% of the continent, which is more than the whole of the Northern Territory. They can absorb huge amounts of carbon from the atmosphere, especially during wet periods, and can significantly contribute to national and global CO₂ reduction efforts.”

“For many, mulga may be just a rather uninspiring tree, but it’s actually pretty special and has a vital role to play in global climate science.”

Rachael conducted her field research in the Northern Territory’s Ti-Tree Basin making use of site facilities and data from TERN. She utilised the open access research infrastructure provided by TERN’s Alice Mulga SuperSite, which includes two flux monitoring towers, six 1ha ecosystem surveillance plots, sapflux sensors, acoustic monitors, phenocams, and remote sensing data delivered by TERN’s Landscape Assessment capability. Also co-located at the site is the groundwater monitoring infrastructure provided by the NCRIS Groundwater project and the Northern Territory Government, which includes groundwater bores, electrical conductivity sensors, ephemeral flow sensors, cameras, automatic rain gauges, and soil moisture sensors.
Multi-scale, long-term data collected by TERN’s nation-wide ecosystem observatory are discoverable via the TERN Data Discovery Portal.
NCRIS Groundwater data are available here.
For more information on this research you can read a paper just published in the Journal of Arid Environments or contact Rachael Nolan.

Mulga ecosystem in the Northern Territory’s Ti-Tree Basin (photo courtesy of Rachael Nolan)

Rachael's research shows that river red gums (above) survive dry periods by accessing groundwater, a finding that has major implications for how arid ecosystems and their water resources are managed (photo courtesy of Rachael Nolan)

Published in TERN newsletter February 2018