Articles | Volume 13, issue 21
Biogeosciences, 13, 5947–5964, 2016

Special issue: OzFlux: a network for the study of ecosystem carbon and water...

Biogeosciences, 13, 5947–5964, 2016

Research article 01 Nov 2016

Research article | 01 Nov 2016

Carbon uptake and water use in woodlands and forests in southern Australia during an extreme heat wave event in the “Angry Summer” of 2012/2013

Eva van Gorsel1, Sebastian Wolf2, James Cleverly3, Peter Isaac1, Vanessa Haverd1, Cäcilia Ewenz4, Stefan Arndt5, Jason Beringer6, Víctor Resco de Dios7, Bradley J. Evans8, Anne Griebel5,9, Lindsay B. Hutley10, Trevor Keenan11, Natascha Kljun12, Craig Macfarlane13, Wayne S. Meyer14, Ian McHugh15, Elise Pendall9, Suzanne M. Prober13, and Richard Silberstein16 Eva van Gorsel et al.
  • 1CSIRO, Oceans and Atmosphere, Yarralumla, NSW, 2600, Australia
  • 2Department of Environmental Systems Science, ETH Zurich, 8092 Zurich, Switzerland
  • 3School of Life Sciences, University of Technology Sydney, Broadway, NSW, 2007, Australia
  • 4Airborne Research Australia, Flinders University, Salisbury South, SA, 5106, Australia
  • 5School of Ecosystem and Forest Sciences, The University of Melbourne, Richmond, VIC, 3121, Australia
  • 6School of Earth and Environment (SEE), The University of Western Australia, Crawley, WA, 6009, Australia
  • 7Producció Vegetal i Ciència Forestal, Agrotecnio Centre, Universitat de Lleida, 25198, Lleida, Spain
  • 8School of Life and Environmental Sciences, The University of Sydney, Sydney, NSW, 2015, Australia
  • 9Hawkesbury Institute for the Environment, Western Sydney University, Penrith, NSW, 2570, Australia
  • 10School of Environment, Research Institute for the Environment and Livelihoods, Charles Darwin University, NT, Australia
  • 11Lawrence Berkeley National Lab., 1 Cyclotron Road, Berkeley CA, USA
  • 12Dept of Geography, College of Science, Swansea University, Singleton Park, Swansea, UK
  • 13CSIRO Land and Water, Private Bag 5, Floreat, WA, 6913, Australia
  • 14Environment Institute, The University of Adelaide, Adelaide, SA, 5005, Australia
  • 15School of Earth, Atmosphere and Environment, Monash University, Clayton, VIC, 3800, Australia
  • 16Centre for Ecosystem Management, Edith Cowan University, School of Natural Sciences, Joondalup, WA, 6027, Australia

Abstract. As a result of climate change warmer temperatures are projected through the 21st century and are already increasing above modelled predictions. Apart from increases in the mean, warm/hot temperature extremes are expected to become more prevalent in the future, along with an increase in the frequency of droughts. It is crucial to better understand the response of terrestrial ecosystems to such temperature extremes for predicting land-surface feedbacks in a changing climate. While land-surface feedbacks in drought conditions and during heat waves have been reported from Europe and the US, direct observations of the impact of such extremes on the carbon and water cycles in Australia have been lacking. During the 2012/2013 summer, Australia experienced a record-breaking heat wave with an exceptional spatial extent that lasted for several weeks. In this study we synthesised eddy-covariance measurements from seven woodlands and one forest site across three biogeographic regions in southern Australia. These observations were combined with model results from BIOS2 (Haverd et al., 2013a, b) to investigate the effect of the summer heat wave on the carbon and water exchange of terrestrial ecosystems which are known for their resilience toward hot and dry conditions. We found that water-limited woodland and energy-limited forest ecosystems responded differently to the heat wave. During the most intense part of the heat wave, the woodlands experienced decreased latent heat flux (23 % of background value), increased Bowen ratio (154 %) and reduced carbon uptake (60 %). At the same time the forest ecosystem showed increased latent heat flux (151 %), reduced Bowen ratio (19 %) and increased carbon uptake (112 %). Higher temperatures caused increased ecosystem respiration at all sites (up to 139 %). During daytime all ecosystems remained carbon sinks, but carbon uptake was reduced in magnitude. The number of hours during which the ecosystem acted as a carbon sink was also reduced, which switched the woodlands into a carbon source on a daily average. Precipitation occurred after the first, most intense part of the heat wave, and the subsequent cooler temperatures in the temperate woodlands led to recovery of the carbon sink, decreased the Bowen ratio (65 %) and hence increased evaporative cooling. Gross primary productivity in the woodlands recovered quickly with precipitation and cooler temperatures but respiration remained high. While the forest proved relatively resilient to this short-term heat extreme the response of the woodlands is the first direct evidence that the carbon sinks of large areas of Australia may not be sustainable in a future climate with an increased number, intensity and duration of heat waves.

Short summary
Temperature extremes are expected to become more prevalent in the future and understanding ecosystem response is crucial. We synthesised measurements and model results to investigate the effect of a summer heat wave on carbon and water exchange across three biogeographic regions in southern Australia. Forests proved relatively resilient to short-term heat extremes but the response of woodlands indicates that the carbon sinks of large areas of Australia may not be sustainable in a future climate.
Final-revised paper