The growth rate of urban trees is speeding up worldwide, according to a study published in Scientific Reports. Trees in urban areas are growing up to 25% faster than those in rural areas, and have been experiencing accelerated growth since the 1960s.
Led by Hans Pretzsch, the Chair for Forest Growth and Yield Science at the Technical University of Munich, the researchers sampled nearly 1,400 trees in ten cities around the world. They chose cities that represented different climate zones, including boreal, temperate, Mediterranean, and subtropical climates on six continents. In each city, they sampled trees growing in the city center and in nearby rural communities.
They drilled into the trees to take samples of heartwood – the mature, nonliving wood in the tree core. By analyzing tree rings, they were able to determine how fast the trees were growing each year.
They found that while both urban and rural trees are experiencing faster growth rates because of climate change, urban trees are growing even more rapidly than their rural counterparts. Since the 1960s, climate change has added to the effect, causing tree growth in cities to explode. The results were not uniform across all the cities: in Mediterranean climates, trees in rural and urban areas grew at roughly the same rate, and in subtropical climates, urban tree growth has been slowing down since 1960.
The reason for the faster tree growth observed in some cities is a phenomenon called the urban heat island effect. Compared to areas with natural vegetation, dark-colored city streets and rooftops absorb more solar heat during the day, waterproofed surfaces reduce the potential for evaporation, and energy consumption generates additional waste heat. The result is that cities are often two or three degrees Celsius warmer than surrounding rural areas.
Higher temperatures in cities enhance photosynthesis and can even extend the growing season by up to eight days every year, and over time the effect is significant. The researchers found that not only are trees growing more rapidly in cities, the growth is also accelerating as a result of climate change.
“If we take the general climate effect, which seems to boost both urban and rural trees, we see that recently the urban trees profited mostly less than the rural ones,” said Dr. Pretzsch. “This in turn could be an indicator for an approaching limit where so far favorable conditions for urban trees turn into stressful ones, when the warming continues.”
Of course, faster tree growth is not necessarily a bad thing. Trees provide plenty of benefits for city dwellers – human and otherwise. I asked Dr. Pretzsch what the findings mean for the average urban dweller. “Clearly, trees are an ever-important factor for the life-quality of urban people,” he said. “So, the message that urban trees often cope surprisingly well with their environment is not bad news. As we have to expect climate change to go on, increasing the number of trees in the city centers is always a good idea, as they counteract the urban heat island effect as long as they are vital.”
But these positive effects are mitigated by the fact that accelerated growth means trees age faster, too. The researchers said urban planning and management should take this into account, and prepare to replace and replant trees earlier. The long-term effects of climate change must be considered as well, they said.
“Maybe it is more important that urban people – and not only them – think about what they could contribute to mitigate climate change,” said Dr. Pretzsch. “If urban ecosystems overheat in future climates, there will be limits for trees’ and people’s vitality.”
Featured photo credit: Tanuja Jagernauth via Instagram