Climate change and global warming not only decrease air quality and cause rising sea levels, but also affect how plants grow. An international team of researchers headed by the Technical University Munich (TUM) demonstrates for the first time that trees in urban areas grow faster than in rural areas. For the study, which was published in the journal “Nature Scientific Reports”, the researchers analysed samples from different metropolises and climate zones: from Berlin to Brisbane and from Hanoi to Houston. In total, the TUM research team examined almost 1400 trees. A typical and predominant tree species was selected for each city and was studied in both the city centre and surrounding rural areas. Trees in metropolitan areas have been growing faster than trees in rural areas worldwide since the 1960s due to warmer urban temperatures. Similar effects due to changing climatic conditions are now also occurring for trees in rural areas.

How do trees cope with urban climate?

„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 Hans Pretzsch, Professor for Forest Growth and Yield Science at TUM. However, such information is becoming more and more pivotal, as the UN estimates the global urban population to increase by more than 60% until the year 2030. Urban trees are supposed to improve climate and air quality within metropolises and contribute to the well-being and health of city dwellers, but how trees are coping with the city life has not been investigated yet. The current study was supported by the Bavarian State Ministry for Environment and Consumer Protection as well as by the Audi Foundation for the Environment.

Heat island effect causes trees to grow faster

„We can show that urban trees of the same age are larger on average than rural trees because urban trees grow faster“, said Pretzsch. However, this difference decreases with increasing age, while still remaining significant. „While the difference amounts to about a quarter at the age of 50, it is still just under 20% at a hundred years of age.“ According to the researchers the growth acceleration of urban trees is due to the so-called heat island effect: it causes stronger heating-up and thus higher temperatures in urban centres. Compared to the surrounding rural area, this increase in temperature can be up to between three and ten degrees Celsius. Higher temperatures in turn affect the growth of trees in two ways: They stimulate photosynthetic activity and prolong the vegetation period, i.e. the time of the year during which trees can grow. However, this accelerated aging of the trees also causes an acceleration of the life cycle of the trees.

Climate change accelerates tree growth in general

In addition to the accelerated tree growth in cities the researchers also show that rural trees have been growing faster since the 1960s as a result of climate change. „The general acceleration of growth in all trees by about 20%, which we report in the current study, is comparable to previous findings on forests. This effect has also been observed in agricultural production“, says Pretzsch. Evidently, there are a number of changes in environmental conditions that promote accelerated tree growth across different climate zones. „In this context, in addition to global warming, fertilization effects due to the rising atmospheric CO₂ concentration and increased nitrogen depositions are discussed as potential driving forces.“

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