Trees that live in cities grow faster, scientists discover

Trees in metropolitan areas have been growing faster than trees in rural areas worldwide according to the first study looking into the impact of the urban heat island effect on tree growth.

Headed by the Technical University of Munich (TUM), the analysis found that urban trees had been exposed to changing climatic conditions for a long period of time, whereas global warming has only recently started to impose the same effect in rural areas.

While the effects of climate change on tree growth in forests have been extensively studied, there is little information available so far for urban trees,” said Professor Hans Pretzsch from the Chair for Forest Growth and Yield Science at TUM.

A central motivation for the study is the prevailing trend towards global urbanisation. According to calculations by the United Nations, the urban population worldwide is expected to increase by more than 60 per cent by 2030 – with a continuing upward trend.

Urban trees already improve the climate in cities and contribute to the well-being and health of city dwellers, and these forecasts show that their significance for the quality of life in cities will increase even further in the future.

Heartwood samples from the metropolises of Berlin, Brisbane, Cape Town, Hanoi, Houston, Munich, Paris, Prince George, Santiago de Chile, and Sapporo were collected and analysed. The cities were selected to cover different climate zones.

The spectrum ranged from boreal to temperate, Mediterranean, and subtropical climates. In total, the TUM research team focused on almost 1400 mostly mature trees. A typical and predominant tree species was selected for each city and examined in both the city centre and surrounding rural areas.

“We can show that urban trees of the same age are larger on average than rural trees because urban trees grow faster”, said Pretzsch.

“Further observation showed that the relative difference in size between urban and rural trees decreases with increasing age, but still remains relevant. While the difference amounts to about a quarter at the age of 50, it is still just under 20 per cent at a hundred years of age.”

The researchers believe that the growth acceleration of urban trees is due to the so-called heat island effect.

This effect leads to a stronger heating-up and thus higher temperatures in urban centres. Compared to the surrounding rural area, this increase in temperature can amount to between three and ten degrees Celsius.

Higher temperatures can increase the growth of trees in two ways: by stimulating photosynthetic activity and prolonging the vegetation period, which extends the time of the year during which trees can grow.

However, the initial positive effect is also accompanied by accelerated ageing of the trees. According to Pretzsch, accelerating the life cycle may mean that city administrations will have to replace ageing and dying trees sooner.

Regardless of the growth advantage of urban trees, the study shows that both urban and rural trees have been growing faster since the 1960s as a result of climate change. This observation reflects a pattern that has already been reported for forest trees in comparable international studies.

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