London: A new study by scientists has determined that global warming is making trees across North America grow at a rate faster now than they have done at any time in the past 200 years.
According to a report in The Independent, the trees in the northern hemisphere appear to have accelerated growth rates due to longer growing seasons and higher concentrations of carbon dioxide (CO2) in the atmosphere.
Scientists have documented the changes to the growth of 55 plots of mixed hardwood forest over a period of 22 years, and have concluded that they are probably growing faster now than they have done at any time in the past 225 years – the age of the oldest trees in the study.
Geoffrey Parker, a forest ecologist at the Smithsonian Environmental Research Centre in Edgewater, Maryland, said that the increase in the rate of growth was unexpected and might be matched to the higher temperatures and longer growing seasons documented in the region.
The study suggests that northern forests may become increasingly important in terms of moderating the influence of man-made carbon dioxide on the climate.
Dr Parker and his colleagues have carried out a detailed census of the trees on a regular basis since 1987, measuring every tree and sapling that has a diameter of more than 2 cm (0.78 in).
They calculated that the forest is producing an additional two tonnes of wood per acre each year, which is equivalent to a tree with a diameter of two feet sprouting up in the space of a year.
The scientists identified a series of plots with trees at different stages of growth and found that both young and old trees were showing increased growth rates.
More than 90 percent of the tree groups had grown by between two and four times faster than the scientists had predicted from estimates of the long-term rates of growth.
The scientists said that if the trees had grown as quickly throughout their lives as they had shown in recent years they would be much larger than they are now.
They based their conclusions on 250,000 measurements taken over more than 20 years.
During the same period, the scientists measured the concentration of carbon dioxide in the forest air and found that it had risen by 12 percent.
The average temperature had increased by three-tenths of a degree, and the growing season had lengthened by 7.8 days.
The scientists believe that all three factors have played a role in helping the trees to grow faster.