Trees spring into action earlier in cities

Trees turn green 6 days earlier in cities compared to rural areas...

That's the finding from a new study by University of California, Berkeley, researcher Lin Meng and published in the journal PNAS.

The effect occurs, she says, because cities are warming at faster rates than the rest of the planet since artficial materials ubiquitous in urban settings, like concrete, absorb heat more readily than natural materials like wood. The temperatures in the cities she investigated were, on average, 1.3 degrees warmer than surrounding rural areas, although Meng found the difference could even be as large as 3 degrees.

Inspired to study the effect of warming environments on tree greening due to a personal experience with cherry blossoms in Beijing, China, when an unexpected snowstorm wiped out all the flowers she had hoped to see in full bloom, Meng used satellite data to compare the start of tree greening in 85 major cities and their neighbouring rural areas across the United States, including New York City and Washington DC. Trees in built up areas, she found, typically start to grow earlier during the spring and stop growing later into the autumn. Their growing period over the day is also extended too, owing to the effects of artificial light.

While the key finding of earlier greening was consistent across all investigated cities, Meng noted that there was some variation. In particular, trees in southern cities near the coast were more susceptible to earlier greening compared to northern cities that are dryer and more inland. “This is because trees in northern cities [are] mainly regulated by temperature change, but trees in southern cities [are] more in response to water condition changes,” Meng explains.

Furthermore, while the effects of urban warming and climate change are thought to be largely negative, Meng points out that it is not clear whether the earlier greening of trees in cities is a good or a bad thing. The longer growing season “could increase the risk of spring frost”, and these changes “could also impact the timing and severity of pollen season,” leading to a higher risk of pollen allergy for humans. In contrast, the earlier greening could  be beneficial, because “trees could absorb more carbon dioxide from the atmosphere,” and they could have a greater cooling effect in urban environments as they would be leafy for longer stretches of the year and thus be able to counteract some of the urban warming locally.