Focus on Arts and Ecology

In a newly published study, researchers at Germany’s Technical University of Munich (TUM) conclude that urban trees can grow up 25 percent faster than their country cousins.
This is a positive thing, right?
After all, trees growing in densely populated metropolitan areas do so much good: Among other things, they scrub the air of health-compromising pollutants, improve the moods of stressed-out city-dwellers, provide invaluable habitats to urban wildlife, mitigate stormwater runoff and cool the world’s concrete jungles by countering the urban heat island effect. Why would the fact that these multitasking miracle-workers are thriving and growing at an accelerated rate be construed as bad?
Per the study, published in the journal Scientific Reports, the clip that urban trees are growing at — easily viewed as a sign of health and vitality — is believed to be direct result of climate change, specifically the heat island effect. So yeah, not great.
Brought on by human activities such as development, this energy-draining and increasingly deadly atmospheric phenomenon is when a city is significantly warmer — sometimes as much as 22 degrees Fahrenheit — compared to surrounding areas that are less built-up. As mentioned, trees — along with green roofs, reflective pavements and other smart, heat-absorbing urban design strategies — can help to dramatically reduce urban heat islands.
In urban heat islands, higher-than-normal temperatures boost photosynthesis which, in turn, helps trees and other forms of vegetation grow faster. Researchers from TUM observed that in some cities, higher-than-normal temperatures have lead to growing seasons that are more than eight days longer than the norm. This all sounds beneficial, but here’s the kicker: While fast-growing city trees are busy sequestering carbon, soaking up floodwater and providing relief from the heat, they’re also aging and dying at a faster rate than rural trees. And as a result, researchers have found that these vital and industrious trees need to be replaced and replanted more frequently.
It’s a tricky arboreal quandary: Elevated temperatures are helping city trees flourish, enabling them to do what they do best, while also hastening their premature demise.

A trend that varies by climate zone

Paris' urban canopy makes the city safer and more livable, especially when the mercury rises. But the same phenomenon that makes trees grow faster is also limiting their longevity. (Photo: Christian Mueller/Shutterstock)
For the study, TUM researchers analyzed 1,400 healthy and mostly mature trees in 10 climatically diverse cities across the globe: Munich, Berlin, Paris, Houston, Hanoi, Vietnam; Cape Town, South Africa; Brisbane, Australia; Santiago, Chile; Sapporo, Japan and Prince George, a city in northern British Columbia. The team focused on predominant tree species found in abundance in both in city centers and in adjacent rural areas.
Based on tree ring analysis, researchers concluded that not only are city trees growing faster than their rural brethren, but they’ve been growing in "turbocharge" mode since the 1960s as a result of climate change. Prior to the 1960s, both city and rural trees grew at roughly the same rate. (In general, city and rural trees have been growing faster in recent decades; in most instances, the former are just growing at a speedier rate because of the urban heat island effect.)
"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," explains lead author Hanz Pretzch, a scientist in the department of Forest Growth and Yield Science at TUM, in a news statement. "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."
There were some exceptions to the findings, however. In Mediterranean climate zones, for example, Pretzch and his colleagues learned that urban and rural trees grew at roughly the same rate before and after the 1960s. The overall trend also didn’t apply to temperate European cities — in fact, the growth of city trees was somewhat stunted compared to rural trees in these areas likely due to factors such as poor soil quality. In sultry cities with subtropical climates like Brisbane and Hanoi, city trees grew at a faster rate prior to the 1960s but have since slowed.
While findings vary within individual climate zones, researchers conclude that, while not exactly imperiled, urban trees should be treated with extra care and consideration due to the accelerated aging process. "In order to sustain the green urban infrastructure, planning and management should adapt to this changing tree growth rate," concludes the study noting the valuable "ecosystem services" that urban canopies provide.
Pretzch and his team set out to perform the study largely in response to United Nation estimates that the world’s cities, so many of them already bursting-at-the-seams, will experience population growth of more than 60 percent by 2030. And with such rapid urbanization comes an urgent need for the lush, leafy goodness that make these cities better places to live.