Root cause of late autumn revealed

23 November 2007

If you thought that autumn started later this year, you were probably right. It seems that rising levels of carbon dioxide (CO2) aren't just causing climate change - this greenhouse gas is also keeping trees greener for longer and could change future forests.

Antenna leafs through the evidence...

Many leaves stop photosynthesising, die and turn from green to brown during autumn - the scientific term is senescence.

Image: Leo Reynolds

Scientists have been keeping tabs on nature's cycles for hundreds of years. Natural events usually happen bang on time, but the last few decades have seen European seasons changing in length: autumn leaves have been falling slightly later each year and spring has been springing into action sooner.
Earlier signs of spring have been strongly linked to global warming. But it hasn't been quite so clear whether rising temperature is the most important factor in delaying autumn. Is there another, more important, cause? Now a big international team of scientists have an answer to this question.

Phenology is the study of natural cycles - according to phenologists, leaves are dying later each year, but why?

Image: University of Southampton

'Autumn is definitely influenced by temperature, but other factors, such as rainfall and sunlight, make it difficult to see any direct effects of rising temperature,' says researcher Gail Taylor. 'We did a "controlled experiment" where holding all other factors constant enabled us to tease out the effects of CO2.'

Gail Taylor, environmental expert, University of Southampton.

Image: University of Southampton

The trees were grown until they were mature - the scientists had to build scaffolds to reach the treetops.

Image: Gail Taylor

In the 1990s the team began growing two forests. By running pipes through the trees, they could pump in different levels of CO2 to see how it affected the life span of the leaves. Trees were exposed to CO2 both at today's levels and at the level expected for 2050 to see what might happen in the future.
The team found that CO2 keeps leaves looking green and discovered that as levels rise, autumns get later. 'We found compelling evidence that autumn will be delayed as the concentration of CO2 in the atmosphere continues to rise,' says Gail.
So while temperature and other factors play a part, it turns out that the root cause of global warming - CO2 - is what's really keeping leaves youthful. But how is it doing it?
To find out exactly how CO2 was affecting the trees, the scientists took a closer look at the leaves' genes. They saw that increased levels of CO2 being absorbed by the leaves were directly changing the effects and activity of genes, allowing the leaves to live for longer.
Normally, the activities of a few hundred different genes alter to make leaves stop photosynthesising and die off in autumn. But in higher concentrations of CO2, far fewer genes changed what they were doing and other 'anti-ageing' genes became busier, protecting the leaves.

This display shows the activity of leaf genes at higher CO2 concentrations. Red dots are genes which are expressed more and green dots are genes which are expressed less.

Image: Gail Taylor

But does it matter if autumn gets later and later? The answer is it might. If trees don't get enough 'downtime' before the coldest winter months, their buds don't have time to build up their strength, frost is more likely to kill them and the tree will suffer.
However, the new research could help us to plan how we plant future forests in the UK so they can cope with climate change. If our forests are to adapt to shorter winters, we will need trees that 'switch off' and lose their green leaves at higher CO2 concentrations.

Buds need to spend the autumn months toughening up - if they aren't on tip-top form by the start of winter frosts, they may never open in the spring.

Image: Zen Sutherland

Or it could be a good thing that autumn will fall later each year. If the trees manage to pull through each year, then keeping leaves greener for longer could help ward off the impacts of climate change.
'Longer growing seasons could mean that trees will be able to lock away more CO2 in a CO2-enriched world, which would be good news for climate change,' says Nick Collinson at the Woodland Trust. 'This research reminds us that the natural world is always more complex than we think it is.'

Nick Collinson, Head of Conservation Policy, Woodland Trust.

Image: The Woodland Trust

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