Site of the Month: Wombat Stringybark Eucalypt SuperSite

This month, we’re in the Wombat State Forest located near Daylesford in Central Victoria, showcasing a decade of research infrastructure-enabled science on forest carbon cycles, soil, vegetation growth dynamics, and their responses to drought and other disturbances.

Like the OzFlux network to which it also belongs, TERN’s Wombat Stringybank Eucalypt SuperSite is celebrating a milestone birthday this year – reaching 10 years of continuous data collection. It is managed by Professor Stefan Arndt and Dr Nina Hinko-Najera from the University of Melbourne, with significant contributions from the Victoria State Government.

A decade of data collection facilitates vital ecosystem science

The site’s 35 m tall flux tower was installed in 2010 with the overarching aim of investigating the impacts of climate change and disturbances, such as fire and drought, on the dry sclerophyll eucalypt forests in southeast Australia.

To collect the data to enable such investigations, a core flux tower site and three satellite sites within a 1 km radius of the tower were installed.  The satellite sites have additional infrastructure that reduces rainfall at them by 40% to simulate drought conditions and allow comparative studies of ecosystem responses.

“We deliberately picked the location for our long-term research site in the Wombat Forest, because the forest type is very common in south eastern Australia. In fact, about 40% of the forest area of State Forests in Victoria are lowland or foothill mixed species forests, such as the Wombat Forest.”

Professor Stefan Arndt, the University of Melbourne

Across its 10-year operation, the site’s infrastructure and the data it collects have allowed researchers from across Australia to assess the net ecosystem carbon exchange of the forest, tease apart stem and canopy growth dynamics, understand the influence of climate on growth processes, quantify soil respiration and soil methane uptake from the forest floor, and investigate the responses of trees to drought and other disturbances.

TERN flux data from Wombat Forest have also been shared through OzFlux and Fluxnet and contribute to global scale synthesis research.

“The site is designed to not only study the carbon, water and energy fluxes between the forest and the atmosphere but get a more holistic understanding of how the forest as an ecosystem is shaped by and responds to its environment.

This is why we installed a whole range of additional instruments, such as dendrometers, sap flux sensors and VegNet sensors to study growth dynamics. We installed an automated chamber system to study the below ground greenhouse gas fluxes, i.e. soil respiration, methane and nitrous oxide fluxes.

The sensors allow studies of the water relations and drought responses of Eucalyptus obliqua as a keystone species in a number of satellite sites. It turns out the dry sclerophyll eucalypt forest is very resilient to changes in climatic conditions. It is a constant sink for carbon and the soil is a weak but also continuous sink for methane, while nitrous oxide fluxes are very low, indicating a tight nitrogen coupling.

We could detect almost constant stem growth throughout the year in dominant trees, while the canopy expands mainly in summer. For many years it kept its leaf area remarkably stable, a consequence of a good water supply. Only last year we saw a drop in leaf area in summer, an indication of signs of drought stress in the trees.”

Professor Stefan Arndt, the University of Melbourne

A timeseries plot of carbon flux data collected at the TERN SuperSite showing net ecosystem exchange (NEE), gross primary productivity (GPP) and ecosystem respiration (ER) over 10 years. Data are shown as a daily flux with a 30-day running mean (thick line).

An ecosystem in Australia’s fire red zone

Wombat Forest was declared a state forest in 1871. Prior to this, it was used to supply timber product for use in the gold rush: support for mine shafts; sleepers for railways; and pylons for pier construction. Today the forest is managed by the Victorian Government Department of Environment Land, Water and Planning (DELWP), which is responsible for scheduling control burns and selective harvesting. Wombat Forest is now a secondary regrowth forest – it was last commercially harvested in 1980.

The Wombat Forest covers approximately 70,000 ha of land in Australia’s southeast, in the cool temperate climate zone characterised by hot, dry summers and cold, wet winters (annual average rainfall is 844 mm and mean annual air temperature is 11°C).

The location of TERN’s Wombat Stringybark Eucalypt SuperSite in an area prone to drought and summer-season bushfires allows the data collected to be used by researchers to assess the carbon and water response of southeast Australian eucalypt forests to extreme heat events.

Recent research has shown that the trees at the Wombat Forest (primarily Messmate Stringybark (E. obliqua), Narrow-leafed Peppermint (E. radiata) and Candlebark (E. rubida)) are able to sacrifice water for carbon gain during heat extremes.  Such findings have significant implications for understanding long-term carbon and water balances of forest systems in the region and beyond.

“Understanding the response of the forest to fire is something we are very interested in. In 2015 we prepared the site for a planned burn and dismantled our satellite sites and some of our sensors.

We had negotiated the burn area with DELWP so that the entire footprint of the flux tower would have been burned. But in spring of that year a planned burn escaped in Lancefield, north of the Wombat Forest and destroyed some houses. This led to a re-evaluation of all fuel reduction burns and the planned burn at TERN’s SuperSite never happened.”

Professor Stefan Arndt, the University of Melbourne.

Views of the TERN eddy-covariance flux tower at the Wombat Stringybank Eucalypt SuperSite (credit: Nina Hinko-Najera)

Tasks for the next 10 years

Measurements are set to continue at TERN’s SuperSite so researchers can understand how the dry sclerophyll vegetation responds to increased environmental pressure from climate change. Long-term datasets, like the ones from Wombat Forest, are critical for detecting changes in vegetation structure and function over decadal timescales, but also for capturing a range of climate extremes that occur periodically – such as droughts and fire.

“Colleagues at the University of Melbourne have established a unique long-term experiment with the Fire Effects Study Area (FESA) experiment in the Wombat Forest, where parts of the forest are regularly burned in different intervals over the last 35 years. This allowed the investigation of the long-term effects of fire on carbon stocks and other ecosystem services. But the flux tower also allows us to study the more immediate short-term responses of the forest to a fire disturbance.”

Dr Nina Hinko-Najera, the University of Melbourne.

In addition to continued monitoring and sensor-enabled data collection, upcoming research activities using the site will include a more detailed investigation of the soil respiration responses using trenched plots and a new automated soil respiration system.

Another interesting study area will be the investigation of methane emissions from tree stems. Preliminary measurements indicate that while tree methane emissions are prevalent (and at times very high) in trees in wetland areas, they can also occur in upland forests, especially in older trees. Hence, this is a potentially costly carbon source that is currently not well understood.

A third area of interest to researchers is the adaptive responses of trees to changes in climate. The eucalypts in the study area show an incredible plasticity in functional trait expression in response to environmental stresses and the next step will be to link the plasticity in trait expression to the genetic profile of trees in the forest to evaluate the breadth of adaptive capacity of the eucalypts.

A view of the understorey at TERN’s Wombat Stringybank Eucalypt SuperSite (left) and the automated chambers that measure soil respiration (right) (credits: Nina Hinko-Najera)

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