Trees in metropolitan areas have been growing faster than trees in rural areas worldwide since the 1960s. This has been confirmed for the first time by a study on the impact of the urban heat island effect on tree growth. The analysis shows that the growth of urban trees has already been exposed to changing climatic conditions for longer, which is just beginning to happen for trees in rural areas.
Researchers from eight countries sampled the heartwood of 1,383 trees in 10 cities in a range of climate zones. Their root space may be more cramped, their leaves may be assaulted by more pollution, but the limes along Unter den Linden in Berlin, and the London planes of Paris, France are flourishing.
They found that urban trees were responding to the notorious heat island effect - inner cities can be between 3°C and 10°C warmer than the surrounding countryside. They also found the city trees are also enjoying a longer growing season as a consequence of higher atmospheric levels of carbon dioxide, which follow from human combustion of fossil fuels.
Although soil carbon was lost when streets were paved, the remaining urban soils were systematically enriched by peat, compost and new plants. “The results of our study show the potential of urbanisation for the increase of soil organic carbon stocks. This process, in turn, would probably mitigate the effects of climate change,” said Vyacheslav Vasenev of the agro-biotechnology department of the RUDN, the People’s Friendship University of Moscow.
“The optimistic conclusion of our study should be further explored by land-use planners and scholars worldwide, since urbanisation will be progressively more important in the future.”