London tree surgeons are becoming increasingly concerned about the scientific reports being carried out to examine the impact on trees of global warming. Trees in the northern hemisphere could be growing faster now than they were 200 years ago as a result of climate change, according to a recent study. The impact of this on London’s eight million street trees, forests, extensive woodlands and parks could be enormous. Scientists suggest that northern forests may become increasingly important in terms of moderating the influence of man-made carbon dioxide on the climate.
Longer Growing Seasons Cause Oak Trees to Grow Faster
The trees appear to be growing faster because longer growing seasons and higher concentrations of carbon dioxide in the atmosphere. Dr Tom Tew, chief scientist at Natural England, said studies dating over the past 75 years show oak trees are coming into leaf three weeks earlier than they were in the 1950s. As a result, insects are shifting their emergence pattern to fit in, which deprives birds of food to feed their chicks.
More Carbon Dioxide in Forests
Scientists have measured the concentration of carbon dioxide in the air in forests and wooded areas and found that it has risen by 12 per cent. 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.
Warmer Winters Affect Tree Development
However, new research published in Nature by Dr. Trevor Keenan has found the advancing emergence of leaves in European forests is 40 per cent less — they are actually coming out just two days earlier instead of five for every degree of warming. So the rate of advance is slowing down. Leaves are still coming out earlier, but not as early as was first thought. One explanation for the observations is that global warming not only results in warmer springs, but also results in warmer winters, and trees don’t get the “chilling” they need. Temperature is the dominant factor in inducing the onset of spring leaf emergence in temperate deciduous forests.
If the winter is warmer, trees such as those commonly found in the London Boroughs of Wandsworth, Lambeth, Hammersmith and Fulham, Kensington and Chelsea, Merton and Kingston as well as the towns of Battersea, Clapham, Chelsea, Balham, Fulham, Putney, Earlsfield, Southfields, Wimbledon, Kingston, Stockwell, Roehampton and Streatham and Elmbridge Borough (which includes Esher, Claygate and Surbiton) need a longer winter to get the cold signal they need.
How Warmer Winters Affect Trees in London
So, while warmer springs may cause spring leaf emergence to be earlier, warmer winters could be causing leaf emergence to be later for the many tree species that require chilling, which may include species of native trees found in London.
Dr Keenan said if chilling was the reason for the slowing advancement of spring leaf emergence, then it was likely to affect the many other species that require winter chilling. But, he said it was not well known which species require chilling, and whether those requirements are easily met or not. For example, he said, some species only require a mild winter. The length of the day could also be playing a role in slowing down trees’ response to warming spring, Dr Keenan said. To make it worthwhile for a tree to put out its leaves, it needs a certain amount of sun and this could stop spring leaves emerging too early in the season.
It seems then that while climate change is bound to have an impact on London’s cherished trees, it is not yet clear how profound this will be and whether a significant reduction in pollution from fossil fuel vehicles will be enough to protect them.
