Growing Trees in Future Tents
Probing the Response of Plants to Climate Change
How will our plants grow in a greenhouse future? It’s projected that our atmosphere will contain elevated levels of carbon dioxide (CO2). Carbon dioxide is essential for plant growth, so does having more of it around mean plants will grow faster? And if they do, will they absorb greater amounts of carbon from the atmosphere? The answers to these deceptively simple questions have massive implications for agriculture and our understanding of climate change. Plant scientists in the Research School of Biology (RSB) (in collaboration with partners at the University of Western Sydney) are attempting to throw more light on these issues by studying how trees respire when raised in an atmosphere of the future. This is achieved by growing a whole tree in a massive transparent tent – a Whole Tree Chamber – in which CO2 is present in concentrations expected to be experienced in 50 to 60 years time.
Understanding the interaction of CO2 and plants is central to our understanding of the global carbon cycle. Humans currently release around 6-7 gigatonnes of carbon into the atmosphere every year but plants take up around 20 times that amount through photosynthesis. A significant proportion of this carbon is then released in plant respiration, the process of growth that uses the stored energy captured by photosynthesis.
“To effectively model the future carbon economy we need a thorough understanding of how plants photosynthesise and respire at elevated levels of CO2,” says Associate Professor Owen Atkin from the Functional Ecology Group at the SoB. “While there are many ways of estimating this, one of the best is to grow a whole plant in an atmosphere with elevated CO2 levels. Other methods include sealing up leaves and branches in bags containing a modified atmosphere but the gold standard is looking at the whole plant.”
And this is exactly what’s being attempted in the Hawkesbury Forestry Experiment, a unique national facility that has been established at the University of Western Sydney (UWS). It involves growing blue gum trees, a fast growing plantation species, in large chambers in which the CO2 and moisture can be controlled.
“The trees were planted in 2007 and they were placed in chambers which enabled the environment around individual trees to be manipulated,” explains Atkin. “They have 12 chambers, six of which have ambient atmospheric CO2 concentrations and the other six have elevated CO2 concentrations. The elevated levels simulate CO2 concentrations that we’ll have later this century, around 640 parts per million.
“But the experimental facility is looking at more than just CO2 levels because one of the expected impacts of climate change is an increased frequency of drought. So the experiment will also look at this. In the ambient and elevated chambers half the trees have been subjected to drought conditions, and the other half to a well watered regime.”
While the experimental facility is being managed by UWS and associated partners, it is a national resource established to study a global phenomenon. Researchers from around the country have been invited to participate and apply their special research strengths on the encased trees. Associate Professor Atkin’s interest is in plant respiration under varying environmental conditions, and the opportunity to work with the trees has allowed him to fill in an important information gap in modelling carbon exchange and respiration.
“The process of respiration releases a huge amount of CO2,” says Atkin. “Anywhere between 20-80% of the carbon that comes in through photosynthesis is respired everyday by whole plant respiration. Half of it takes place in leaves and the other half largely happens in the roots. So it’s a big player in terms of the carbon economy of an individual plant, and it’s also a big player from the point of view atmospheric CO2 concentrations.
“Most of the global circulation models that predict future climate have a photosynthesis component and a respiration component. But the respiration component has several weaknesses in its underlying assumptions. For example, one assumption is that respiration increases exponentially with rising temperature but we know that it doesn’t. Respiration doesn’t just keep going up with temperature; it acclimates, it seasonally shifts its temperature response curve as you get a warming.
“And large scale models are unable to predict accurately respiratory rates that are occurring in forest trees. Without that we can’t properly model how quickly those trees will grow and the contribution those trees will make to atmospheric CO2 either in a negative or positive way.
“So, it’s extremely important that we understand how environments impact on this process of respiration in plants. This experiment was very useful because it enabled us to access whole plants that were going to experience future elevated levels of CO2. Plus we could study the impact of drought.
“I was excited to take part in the Hawkesbury Forestry Experiment because it’s the only facility of its type in Australia. It enables us to quantify the rate of carbon uptake by entire canopies through time. And the Whole Tree Chambers also have a partition between the above and below ground part of the tree that allows them to separate the shoot processes from the soil and the roots so we can quantify CO2 release from the below ground part as well.”
Working with ANU-based postdoctoral fellows Kristine Crous and Joana Zaragoza-Castells, and colleagues at UWS (Professors David Ellsworth and David Tissue) Atkin has been travelling up to visit the enclosed trees every 4-6 weeks in the latter half of 2008. Each visit lasted several days during which they measure respiration rates from 5am in the morning through till 11.30pm at night.
They found that the trees growing with elevated CO2 levels were exhibiting elevated rates of photosynthesis and were respiring at higher rates. This was expected but they also found that the leaves were thicker and there was a change in leaf chemistry with lower levels of nitrogen being present.
“We’ve found that elevated CO2 affects the plant’s respiration rates,” explains Atkin. “It enhances it on an area basis, though not so much on a mass basis.
“Drought has a big impact on respiration on elevated and ambient CO2 trees. Significantly, the decrease under drought was quite pronounced under elevated CO2. Under drought conditions, respiration rates come right down to the same basal rates of the ambient level plants. So, they both have dropped their rates, but one set of trees (the plants growing in elevated CO2) start a bit higher.
“It makes sense when you consider that the plants have to respire; if the leaves don’t respire they’re dead. So there’s a certain basal rate they must maintain in order for their tissues to remain viable. Remaining viable during drought means that when water becomes available they can start taking advantage of it. In a modelling context, drought has a much bigger impact on the respiratory fluxes of a CO2 elevated plant; they start from a higher point but they come down to a similar point. These kinds of empirical data are critical if our models are to be valid.”
The first crop of trees grown in the enclosures has now been harvested and the hope is that the Hawkesbury Forestry Experiment might grow several more crops over the coming years to better explore trees and carbon exchange.
“Carbon sequestration is a big strategy for managing global carbon but there’s so much we don’t know on how climate change impacts on the rate of carbon movement in and out of trees,” says Atkin. “Working with experimental facilities such as this will be critical if our efforts to effectively manage carbon with trees works over time.”