Trees in cities grow 25% faster than their country cousins, according to new research by an international team of scientists led by the Technical University of Munich in Germany.
Researchers analysed the growth of about 1400 trees in 10 cities around the world, chosen to give a wide range of climates. They included Paris, France; Houston in Texas; and Sapporo, Japan. The scientists used tree rings to estimate their age and growth and mapped how fast they grew over the last 150 years.
They found global warming due to climate change is having an effect on trees everywhere, increasing growth by up to 17% since 1960. But in cities, the effect was much more pronounced, with trees in towns shooting up by as much as 25% more than those in the country. The team believe this could be because of the urban heat island effect, which causes temperatures in the city to rise by as much as 10°C above those in surrounding areas.
The difference between urban and rural trees decreases as the trees get older, suggesting the effect may ‘wear off’ after time – perhaps because of other checks on growth in a city, such as lack of water and compacted soil around the roots. But the scientists found city trees are still just under 20% ahead even at a century old.