Summer can sometimes be a miserably hot time for city dwellers, but new research shows that an urban setting allows plants to bask in a hothouse environment that keeps them greener longer. Recent NASA-sponsored research from a team of geographers in Boston University’s Center for Remote Sensing shows that the growing season for vegetation in about 70 urban areas in North America is, on average, 15 days longer than that in rural regions surrounding the cities studied. Led by Xiaoyang Zhang, a research assistant professor in BU’s Geography Department, the team found that, like many urban-dwelling humans, urban greenery lives at a more intense pace, getting as much as a seven-day jump-start on spring and up to eight additional days before winter dormancy than vegetation in surrounding rural areas.
This happens largely because the asphalt, steel, exhaust, and other environmental changes introduced by humans contribute to what is known as the urban heat island effect. The researchers found this influence to be far-reaching. Taken together, the heat island effect for the areas studied ripples beyond urban boundaries to create an ecological “footprint” 2.4 times greater than that of urban land use in eastern North America.
Their analyses also show that changes in land surface temperatures and when vegetation first becomes green are not significantly related to urban size, leading the team to speculate that factors related to population density may play the more important role in the observed effects. The data do, however, show that “green” changes in surrounding rural areas are linked to city size — the larger the city, the longer the reach of the extended green effect.
To date, more than one-third of the land surface of Earth has been transformed by human activities. These changes have not only altered the look of the land, but, according to a growing body of research, have also affected climate and significantly changed Earth’s ecosystems. In an attempt to determine the effect that the urban heat island phenomenon has on the growing season for urban vegetation, the BU team used satellite data to evaluate when vegetation in urban and rural settings first became green and then when it entered dormancy, thereby ending one season’s growing cycle.