Climate change experts say we can expect higher temperatures, rising sea levels and more frequent storms, droughts and bushfires. But how will escalating carbon dioxide levels affect our trees?

Associate Professor Brian Atwell from the Department of Biological Sciences has more than a grudging respect for the 800 or so species of the genus Eucalyptus, despite the contempt eucalypt plantations in other countries sometimes attract. It's a respect born from decades studying the complexities of plants, from tiny rice shoots up to large trees.

"Eucalypts are conspicuous in many parts of the world: they are seen as extremely aggressive, high maintenance trees, but of course there are subtleties in the natural diversity of the eucalypts that we can use to understand tree adaptation," Atwell says. "Eucalypts are so diverse - some use large amounts of water but take high risks in doing it, and of course there are other eucalypts from desert areas that are highly conservative. There's also so much plasticity within eucalypts, they're able to respond to stresses well, so they grow brilliantly across the temperate zones of the world."

Atwell began working with eucalypts after arriving at Macquarie University in 1989, carrying on from the pioneering work of Professors Snow Barlow and Jann Conway who were studying how rising carbon dioxide levels impact upon tree growth.

"Snow and Jann, like many plant physiologists in the 1980s, were interested in connecting rising atmospheric carbon dioxide (CO2) levels on the planet with the fact that extra CO2 makes plants grow faster," Atwell recalls. "There's a tendency to think that most aspects of global climate change have just dawned on us, particularly those that are laid out so dramatically by the climatologists, but physiologists interested in photosynthesis were emphasising half a century ago that plants would grow quicker as CO2 rose."

In a time of dwindling forests and rising risks of starvation throughout much of the Third World, surely this ability of individual plants to grow faster - up to 30 per cent faster by the middle of this century according to a recent estimate - is an unexpected, but entirely welcome, benefit of climate change? Yes, and no, says Atwell.

"One of the escape routes for those who are apologists for the carbon fuel cycle is the belief that the good thing about climate change is that there'll be more food production, but there are an awful lot of qualifications that now go with that," he explains.

"The first problem is that plants won't universally grow quicker. Those plants that already have incredibly efficient CO2 uptake systems - such as most tropical grasses, for instance - won't respond much to higher levels of CO2. Neither will plants that are already constrained in their growth by other limitations, like freezing cold or intense heat or an extreme lack of nutrients.

"So while for food production faster growth is a good thing, ecologically it's a curate's egg - there are winners and losers in a mixed community. The other problem with faster growth in general is that plants only grow fast because they're mining more nutrients out of the soil. For example, there have been forest growth models which show that if forests were to grow quicker at high CO2 they'd have to lower their nutrient requirements, otherwise forest soils would become impoverished before the trees matured - the forest would hit a wall, as it were, when it ran out of nitrogen."

So while food crops like rice and wheat that are propagated within a closed system might prosper if provided with enough extra inputs - including nitrogen, phosphates and water - forests might provide no improved sink for carbon as they mine the soil for nitrogen and tie up more and more nutrients in their wood.

For a number of years Atwell has collaborated with members of the Co-operative Research Centre for Greenhouse Accounting, led by the Australian National University's Professor Marilyn Ball. Over the past few years, she has run a fascinating field experiment to see how snow gums, which grow very successfully in the high altitudes and poor nutrient soils of Australia's south-east, responded to higher CO2.

"Trees grew in open chambers near Canberra and were flushed continuously with CO2 and fertilised with high nitrogen - an embarrassment of riches for a lowly snow gum," says Atwell. "This CO2-rich, fertile state killed the trees in the heat of the summer. It's a good example of how if plants aren't selected or don't evolve to cope with high CO2 then climate change can potentially kill off whole ecosystems. Even the snow gums, which normally survive in a difficult climate, are defeated. With the extra nitrogen and high CO2 they roar along and look very impressive in spring and autumn until you get a 35 or 40 degree day in summer, and then they uniformly die. Curiously, they also become more susceptible to frost in winter."

Atwell and a number of other Sydney-based climate change researchers, including new Macquarie colleague Dr Belinda Medlyn, who models tree growth, are also involved with a project called the Hawkesbury Climate Change Experiment. This experiment at the University of Western Sydney in Richmond was established with more than $1 million from the Greenhouse Office and $2 million worth of growth chambers from the Royal Swedish Academy of Sciences and involves all the universities in the Sydney basin. The researchers will grow eucalypts at high CO2 for some years within large, completely closed chambers, and be able to analyse the entire growth process and carbon cycle - from numbers of shoots and roots to nutrient turnover and carbon release by microbes.

"Eucalypts from high-rainfall zones do everything with gusto, they grow very fast, they pump lots of carbon below ground, they tie up lots of nutrients, and so we'll be trying to quantify some of these things," Atwell says.

There is already mounting evidence that eucalypts in a high CO2 environment respond not just with altered patterns of water and nutrient use but also by branching differently and growing thicker leaves. Just what these resulting changes to canopies will mean for tree productivity, water use and landscape hydraulics is just one of many unknowns for our forests.

"Another really important thing we haven't worked on, and which we know nothing much about, is the amount of carbon that these systems, particularly eucalypt systems, naturally pump below ground," Atwell says. "If the planet's soils were to be heated up by one or two degrees it would liberate huge amounts of CO2 from soil microbial activity back into the atmosphere, and could speed up the Greenhouse Effect in a sort of positive feedback loop.

"I think the thing that's least often acknowledged in debates about climate change is that it's not necessarily the absolute values that are important - for example the Earth's CO2 levels have historically been much higher than they are today and plants have survived. Species can adapt if you give them enough time - but it's the rate of change that we're currently seeing that is so concerning, this is a very fast train and our plants are directly in its path. Trees have long life cycles and while they might be part of the solution because of the scale of forests, they are also especially vulnerable to climate change."