City Heat: Urban areas’ warmth affects plant growth

Satellite observations of eastern North America show that plants in and around urban areas bud earlier in the spring and retain their foliage later in the fall than do plants in nearby rural settings. Although that trend had been noted before, the new data suggest the differences are at least partially due to the phenomenon dubbed the “urban heat island.” Cities retain enough heat to raise their temperatures above those in surrounding regions.

Most studies of increasing temperatures have examined global warming over recent decades. It has disrupted biological life cycles worldwide, triggering earlier plankton blooms in marine ecosystems and earlier spring migrations of species ranging from squid to birds (SN: 3/8/03, p. 152: Spring Forward). Global warming has also lengthened the growing season of vegetation in mid- and high-latitude regions, says Xiaoyang Zhang, a geographer at Boston University.

Now, he and his colleagues have focused on urban heat islands, a separate warming process, and have linked increased temperatures in urban areas to longer growing seasons there.

The researchers used data collected by a satellite-borne instrument that can estimate land-surface temperature and monitor changes in vegetation over areas as small as 1 square kilometer. Data from that device, which is aboard NASA’s Terra satellite and known as the Moderate Resolution Imaging Spectroradiometer, indicate that each of at least 70 urban areas between northern Florida and southern Canada encompasses an area of more than 10 square km. These data suggest that those cities significantly affect local climate, says Zhang. He and his team report their findings in the June 28 Geophysical Research Letters.

Temperatures in eastern North American cities between January and May 2001 were, on average, 2.28°C warmer than they were at spots about 10 km away from each urban center. From September through December 2001, city temperatures were 1.48°C higher than they were in the nearby countryside.

Those increased temperatures appear to have influenced plant growth. Nonagricultural vegetation, such as trees, began to bud in the spring about 7 days earlier in cities than in nearby rural areas. Also, leaves stayed on city trees about 8 days longer in the autumn.

The linkage between the city-induced variations in temperature and vegetation growth also holds at intermediate locations. In spring 2001, temperatures in nonurban locations less than 3 km from urban areas in the New York–Philadelphia–Washington megalopolis averaged 1.8°C cooler than in the cities, and trees in those locations became green about 5.5 days later than did city trees. In areas 8 to 10 km away from the cityscapes, where average temperatures were 2.5°C cooler than they were in urban areas, trees greened up 8.7 days later.

Some lengthening of urban growing seasons might result from an abundance of ornamental trees, which are sometimes bred to keep their foliage longer, says Terry L. Root, an ecologist at Stanford University. Overall, however, Zhang’s team appears to have reached “reliable, robust conclusions,” she notes, adding that when plants warm up, their life cycles change.