Site of the Month - Alice Mulga and Ti Tree East

To celebrate the first month of Spring, and another site birthday milestone, we're off to the red centre of Australia. This month's story is a paired feature from TERN's Alice Mulga SuperSite External linkAlice Springs Mulga External link and its partner site Ti Tree East External link. Both sites are part of the OzFlux network and provide crucial high temporal resolution ecosystem carbon, water and energy flux information from dryland arid ecosystems that dominate the heart of interior Australia.

The TERN Alice Mulga SuperSite External link was established in 2010, so like last month's feature Wombat Forest, the site is celebrating its 10th birthday this year! The Ti Tree East partner site was established in 2012 as part of the NCRIS Groundwater initiative External link, which aims to provide long-term groundwater monitoring across key sites in Australia. The two sites are managed by Dr Jamie Cleverly External link from The University of Technology Sydney, who is also the Associate Director of OzFlux and TERN's Science Partnerships Liaison.

A milestone birthday for flux monitoring in Australia's red center

Over 10 years of flux measurements in Australia's red centre, the Alice Mulga and Ti Tree East OzFlux sites have observed a lot. With more than 100 publications (and counting) between the two sites (Alice Mulga Google Scholar External link & Ti Tree East Google Scholar External link), their value to Australian and international research communities is enormous. From assessing the seasonal patterns of carbon and water flux to rainfall pulses External link in Acacia spp. savannas, to enhancing our technical understanding of component carbon fluxes External link, and understanding the impact of interacting climate modes External link that can lead to large contrasts in inter-annual carbon uptake External link, observations from these two sites have revealed a wealth of information about how climate has shaped, and continues to alter, these arid/semi-arid ecosystems.

Alice Mulga flux towerview from the top of the Ti Tree East tower

The eddy-covariance flux tower at the TERN Alice Mulga SuperSite (left, credit: Jamie Cleverly) and a top of canopy view from the Ti Tree East flux tower (right, credit: Jamie Cleverly).

Two towers tell the tale of life in the desert

Both towers are located on the Pine Hill cattle station, approximately 200 km north of Alice Springs in the Northern Territory, over iconic vegetation Mulga and hummock grasslands, which cover about 50% of Australia's land area. A small amount of grazing occurs External link at the Alice Mulga SuperSite, which is dominated by Acacia aneura (i.e. Mulga). In contrast, the Ti Tree East site experiences very little to no cattle grazing due to its unpalatable hummock grasses (Triodia schinzii) surrounding its small, 4.85 m tall Mulga patch.

'The water table at Alice Mulga is quite deep compared with Ti Tree East - 49 m compared with 8 m, respectively. It's this contrast that led to establishment of the Ti Tree East site as part of the NCRIS Groundwater initiative, as it was believed the site vegetation may have access to this groundwater. However, it was later found that the Mulga patches are isolated from groundwater by a hardpan, whilst the widely spaced bloodwood trees use groundwater only in those brief periods when the air isn't too hot and dry, just after rainstorms.' - Jamie Cleverly

Annual precipitation is typically low (307 mm per year on average), but it can be highly variable, ranging from 54 mm recorded in 2019 to 750 mm in 2010 (Bureau of Meteorology). This highly dynamic nature of rainfall extremes and the length of time between drinks leads to extraordinary variations of carbon flux in these ecosystems. This dynamic was most pronounced during the strong La Niña event of 2011, where higher than average rainfall caused a greening of the southern hemisphere External link, and central Australia in particular External link.

The Alice Mulga SuperSite successfully recorded this dramatic increase in ecosystem productivity External link. Similarly, record-breaking rains fell in January 2017, when more than 350 mm of precipitation fell in a single month, and ecosystem productivity exceeded that of the 2011 land carbon uptake sink. These sites have given us the fortune to witness the incredible ability of Australian vegetation to come alive for flooding rains.

plot of 10 years of NEP and rainfall for Alice Mulga

Net ecosystem productivity (NEP) and rainfall across the 10-year measurement period from the Alice Mulga SuperSite. Negative fluxes represent loss from the ecosystem due to respiration and degradation, and positive fluxes represent carbon uptake into the system through photosynthesis.

The harsh reality of desert life in the future

Periodic drought is a harsh reality these ecosystems must face in the semi-arid interior of Australia. Despite the greening pulses of 2011 and 2017, ecosystem productivity was relatively short lived due to a drought event External link that closely followed them. With 2019 dubbed the hottest year on record in Australia External link, these ecosystems were pushed to their brink. The Alice Mulga SuperSite received much lower than average rainfall from April 2017 through to the end of 2019 – the latest big drought of the region. Arid areas can receive patchy rainfall, which was the case in 2019, when the Alice Springs Mulga tower recorded only 35 mm of rainfall.

In August 2020, TERN's Ecosystem Surveillance External link field team conducted detailed surveys and sampling at the Alice Mulga SuperSite and at tens of other sites in the Northern Territory. Early anecdotal reports from the TERN's Field Lead and Senior Botanist, Emrys Leitch, indicates large swathes of Mulga die-back across the region – colloquially referring to it as a 'mulgapocalypse'.

Dr Cleverly recalls a similar Mulga die back event during the 2012-2013 drought, whereby the return of significant rainfall to the area revitalised vegetation at both sites.

'With this current "mulgapocalypse", up to half of the "dead" trees could be zombies, waiting for favourable conditions to revive' – Jamie Cleverly

The data and samples collected during the winter 2020 TERN field survey campaign are currently being curated and will be publicly released via TERN in the coming weeks. The data, samples, and photographs from the campaign are expected to enable significant ongoing research into the observed Mulga dieback event.

The research being enabled by such surveys combined with the comprehensive long-term data records from the two flux monitoring sites are critical in understanding the harsh reality of desert life now and in the future.

While the research continues, we patiently wait and hope for much needed rain to return and bring these zombie-like trees back to life. With La Niña and a negative Indian Ocean dipole setting up, we have our fingers crossed for another big wet.

Wombat Understory

Soil Chambers

Panoramic photos from the Ti Tree East site showing the difference in vegetation health from 2012 (top, credit: TERN Ecosystem Surveillance ) to 2020 (bottom, credit: TERN Ecosystem Surveillance).

Modified: 7/10/21