LONDON, Nov. 14 (Xinhua) -- Pollution from cars and power stations which causes an increase in atmospheric carbon dioxide (CO2) levels is changing the face of autumn, according to a new study by British scientists.

    Leaves are turning brown later in the year than they once did because of rising levels of CO2 in the atmosphere, researchers at the University of Southampton found.

    The Independent newspaper reported Wednesday that the 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 current level is just over 380 parts per million.

    They 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.

    The elevated CO2 concentration also resulted in a greener autumn canopy compared with the canopy of poplars grown in atmospheric conditions with lower levels of CO2.

    The researchers said 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.

    Over the last 30 years, atmospheric CO2 has risen by 13.5 percent, affecting the physiology and functions of plants and influencing a wide range of their internal processes.

    Over the same period, the aging process, known as autumnal senescence, has begun later in Europe by between 1.2 and 1.8 days per decade, parallel to the earlier appearance of spring with buds bursting on oak trees, for example, up to 10 days earlier than they once did.

    Both phenomena until now had been thought to be triggered by rising temperatures. The new research showed it is the increasing CO2 which is slowing down the trees' aging processes. Autumnal senescence in such forest ecosystems will be delayed as the atmospheric concentration of CO2 continues to rise, independent of increased temperatures, the researchers said.