Extreme droughts, temperatures and death in forests

An ACEAS working group is moving beyond the meteorological definition of droughts to an ecological one, so we can better understand the impact of climate change on Australia’s forests

Among the many ways that climate change is affecting the world’s forests, one that particularly worries scientists around the globe is the increase in the number of trees that die during droughts.

In Australia, regular droughts and associated high temperatures in forest ecosystems may lead to reductions in net primary productivity, changes in the growth of seedlings, and reductions in canopy cover. In extreme cases, enough trees may die that the ecosystem may enter a ‘state change’ where, for example, woodlands are converted to grasslands. As the frequency of drought increases, so does the rate of tree mortality – and as a result the capacity of forests to store carbon may be reduced.

Researchers are trying to understand the impacts of climate change on forest structure, ecosystem services and carbon sequestration, but to comprehend the effects of so many complex variables they need to synthesise existing knowledge so they can understand what drives changes in these processes, and their consequences.

A multi-disciplinary group, brought together through the Australian Centre for Ecological Analysis and Synthesis (ACEAS), a facility of TERN, is analysing understanding of forest mortality to determine how vulnerable our forests will be in the future.

Their preliminary results indicate that there are ‘hot spots’ of tree mortality in Australia – places where forest ecosystems are particularly susceptible to drought and extreme temperatures. By mapping these locations, they will help ecosystem managers and researchers focus their efforts on developing adaptation mechanisms to make it more likely that Australia’s natural and managed forests will thrive in coming decades. The research will also be useful in determining the potential viability of landscapes managed for carbon and biodiversity plantings.

The group brings together scientists from 14 universities, research institutions and government agencies in Australia and overseas to analyse and synthesise data from across disciplines. The principal investigator of the group is Dr Anthony O’Grady, from CSIRO’s Ecosystem Sciences.

‘Drought-induced forest mortality has always happened in Australia, so one of the first questions our group is addressing is how to characterise droughts in an ecological sense, not purely a meteorological one,’ Anthony says.

‘We want to know the characteristics of droughts that are likely to result in tree mortality compared with trees that don’t die.

‘Another aspect of our work is characterising how sensitive different forest species are to drought. This information will be invaluable in predicting changes in ecosystem structure and function under a changing climate.’

The group has put together a trait database that captures the different physiological responses that Australian plant species have to drought.

‘Taking a step back and looking at the key traits and strategies that govern how different species survive drought allows us to better predict what species may be vulnerable into the future,’ says Anthony.

During the recent so called ‘angry summer’ of 2012–13, drought in southern Tasmania was severe enough to cause mortality in plant communities across the landscape. Dr Patrick Mitchell from CSIRO, a member of the working group, has been studying drought-induced tree deaths.

‘Events such as this are being documented from sites across Australia. They form a drought mortality database, from which the patterns and climatic drivers associated with extreme droughts can be explored,’ Patrick says.

The ACEAS working group, ‘Forest drought and mortality’, held its final meeting recently, and is writing journal papers for publication.

Tree mortality near Hobart this summer (Photos courtesy of Patrick Mitchell, CSIRO)



Published in TERN e-Newsletter May 2013

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