Warmer temperatures help city trees grow eight times faster.
Cities, with their hard surfaces and black streets, are really good at soaking up solar energy. These sorts of surfaces retain heat, making them markedly warmer than less paved areas, in a process known as the urban heat island effect – a name that calls to mind a hellish slow-roast in an oven of urbanity. It's not exactly deadly, but higher temperatures in cities have been blamed for greater energy use and, by extension, an increase in CO2 emissions and the proliferation of climate change – all of which makes it hard to look at the urban heat island effect as anything but a big problem.
But there may be at least one surprising side-benefit to this urban insulation: higher temperatures in cities have been found to greatly increase the growth of some trees. Red oak seedlings planted in New York City have been found to grow nearly eight times faster than the same trees planted in more suburban and rural areas, according to new research published in the journal Tree Physiology.
In the spring seasons of 2007 and 2008, researchers planted sets of red oak seedlings in four different locations: within New York City's Central Park, in a plot in the suburban Hudson valley, in a rural forest near the town of Palisades, New York, and in a remote site in the Catskill foothills about 100 miles north of the city. The altitudes of the sites ranged from about 28 meters above sea level in Central park up to 233 meters above sea level in the foothills, but that range had little impact on differences in tree growth. "The difference in altitude was really quite slight," says Stephanie Searle, lead author of the study. "The only real effect would be a further increase in temperature."
Searle and her team watered and fertilized each set of seedlings every week under the same conditions. By August the city seedlings had developed about eight times the biomass as the other two sets of trees.
Temperature seemed to be the determining factor. On the hottest days, temperatures near the city trees were an average of 4 degrees warmer than their country cousins. On the coldest days, city temperatures were an average of 8 degrees warmer. Despite other conditions that might have influenced this faster growth, the researchers have determined that the hyper-growth speeds are largely attributable to the higher temperatures in the city. They confirmed this hypothesis with seedlings grown in a lab under similar temperatures and conditions.
Trees can provide a number of benefits to urban areas. Their positive impact on property values has been documented extensively. Urban trees have also been found to provide a significant economic benefit to cities due to their role in stormwater treatment, energy use reduction, air quality improvement and carbon sequestration.
Trees have also been found to help counter the urban heat island effect that is apparently helping them grow much faster – a negative feedback loop that suggests planting more trees in the city makes a lot of environmental sense. The warmer temperatures caused by the urban heat island effect are certainly causing problems in cities, but they're also creating what have turns out to be ideal conditions for tree planting.
Photo credit: Lucas Jackson / Reuters