Urban trees are growing faster than rural trees across the world, according to a major new study covering 10 different cities, and the urban heat island effect could be one of the reasons why.

With rising temperatures across the globe as well as increased urbanisation, it's important to understand how the heat trapped by cities is affecting our trees, and vice versa.

To take a closer look, a team from the Technical University of Munich (TUM) in Germany used samples to study the growth of 1,383 trees over the past 150 years. They found that while urban trees are winning the race overall, it also depends on where in the world they are.

"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," says one of the team, forest scientist Hans Pretzsch.

"We can show that urban trees of the same age are larger on average than rural trees because urban trees grow faster. While the difference amounts to about a quarter at the age of 50, it is still just under 20 percent at a hundred years of age."

city trees 02Researcher Jens Dahlhausen takes a sample from a tree in Paris. (TUM)

To get these figures, data was collected from trees in and around Berlin and Munich in Germany, Brisbane in Australia, Cape Town in South Africa, Hanoi in Vietnam, Houston in Texas, Paris in France, Prince George in Canada, the Chilean capital Santiago de Chile, and Sapporo in Japan.

These cities were chosen to cover four different climates: boreal, temperate, Mediterranean, and subtropical.

Across all those zones taken as a whole, growth rates of both urban and rural trees have increased a little since 1960s. And on top of that, city trees have grown as much as 25 percent faster than the ones in rural areas.

Split up the trees by zone though, and it's not so clear cut. The biggest lead in growth spurts for urban trees was found in boreal regions, but in temperate climates, urban trees are actually growing slower than rural trees.

There was hardly any difference in growth rates between the tree types in Mediterranean spots, while in the subtropics urban trees were growing faster before the 1960s but have levelled off to rural tree growth rates since.

Trying to tease out what's going on from those datasets is tricky, because the study only looked at growth rates and not any of the possible underlying factors, but the authors of the study have a few ideas.

The overall increase in growth since the 1960s is probably down to us and all the carbon dioxide we pump up into the atmosphere: as previous studies have found, as long as water shortage isn't an issue, a warmer environment is more conducive to tree growth.

Plants and trees can extend their growing seasons in warmer environments and can also increase their rate of photosynthesis thanks to the CO2 fertilisation effect.

As for the growth spurt in trees in some cities, the researchers are putting it down to the warmer air created by all those car parks and buildings soaking up the sunshine.

"The urban heat island involves higher temperatures in cities compared to the surrounding landscapes that may stimulate photosynthetic activity if the temperature optimum of a species is not yet reached and extend the growing season," write the scientists.

In cities where trees aren't prospering, factors like poor soil quality or a lack of water could be counterbalancing the heat benefits, suggests the study report.

And metropolitan planners need to understand urban tree growth in order to look after city dwellers – we know that foliage improves the health of a city, and the number of people living in urban areas is expected to keep on rising.

As well as growing faster, urban trees might be ageing faster, and need replacing more quickly, according to the researchers - that's one of the factors that local councils are going to have to take into consideration.

Ultimately more research is going to be needed to figure out the details of what's happening, and why we're seeing these shifts in tree growth.

Emily Meineke, an urban ecologist from Harvard University who wasn't involved with the research, told Maddie Stone at Earther that this study was a useful step towards that goal.

"This study is important in that it shows that tree growth has increased over time, potentially due to climate change, and that effects of urbanisation on tree growth depend on where in the world a tree is living," she said.

"It will be interesting for future work to investigate why."

The research has been published in Scientific Reports.