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Pollution from cars and power stations is changing the face of autumn, say British scientists.
Leaves are turning brown later in the year than they once did because of rising levels of carbon dioxide (CO2) in the atmosphere, a new study from researchers at the University of Southampton found.
The increase in CO2 levels allows leaves to continue for longer the process of photosynthesis - where they use CO2, water and sunlight to produce nutrients and oxygen. This allows leaves to delay when they start to change colour and, eventually, fall.
Over the past 30 years, this ageing process, known as autumnal senescence, has got later in Europe by between 1.2 and 1.8 days per decade. It is paralleled by the earlier appearance of spring, with buds bursting on oak trees, for example, up to 10 days earlier than they once did.
Until now it had been thought that both phenomena were triggered by rising temperatures. While it has already been established that there is a strong correlation between higher temperatures and earlier springs, analysis of autumnal senescence in 14 European countries shows there is only a weak correlation between temperature trends and the timing of the changes in leaves' colour and their fall. The new research shows it is the increasing CO2 which is slowing down the trees' ageing processes.
Over the past 30 years, atmospheric CO2 has risen by 13.5 per cent, affecting the physiology and functions of plants and influencing a wide range of internal processes.
The Southampton researchers undertook two large forest ecosystem experiments, in which poplar trees in separate plots were exposed either to regular or elevated levels of CO2, from the time of planting to full maturity.
The elevated CO2 concentration was at 550 parts per million, proposed as representative of concentrations that may occur in 2050. The present level is just over 380 parts per million.
It was found that the trees exposed to elevated CO2 levels retained their leaves for longer, and also experienced a smaller decline in end-of-season levels of chlorophyll, the green pigment found in leaves. This also resulted in a greener autumn canopy compared with the canopy of poplars grown in atmospheric conditions with lower levels of CO2.
"The research data provide compelling evidence in terms of both the leaf and canopy that autumnal senescence in such forest ecosystems will be delayed as the atmospheric concentration of CO2 continues to rise, independent of increased temperatures," said Professor Gail Taylor, of the University's School of Biological Sciences.
"Photosynthesis and canopy greenness are maintained for longer in elevated CO2. This is because a CO2-rich atmosphere allows the tree to generate carbon-rich compounds that are known to prolong the life of leaves," she said. But she warned: "Trees also need to set bud, and if they don't do that, it makes them susceptible to frost and other weather events."
A key question now, Professor Taylor added, is whether we should select trees which are better adapted to coping with increasing levels of CO2 rather than using locally sourced seed, as is present practice.
The study also provides the first insight into changes in the genetic make-up of poplar trees that can account for the shift to delayed senescence.
Of 20,000 genes studied, one suite was shown to be responsible for the change.
* A COLOUR FOR ALL SEASONS
Leaves are green because of the presence of pigments known as chlorophylls. In spring and summer green dominates and masks out other pigments present in the leaf.
In late summer, veins that carry leaf fluids are gradually closed off as a layer of special cork cells forms at the base of each leaf. Other pigments known as carotenoids, present during the leaf's life, begin to show through as yellows, browns, oranges and the many hues in between.
- Independent
