Urban Sprawl’s Impact on Vegetation

With these “before” and “after” data in hand, Imhoff could then establish to what degree urbanization altered the natural vegetation in the United States. “Three primary patterns kept coming up again and again,” says Imhoff. He explains the most common pattern is best represented in and around Chicago, which was built on a mixed forest type of ecosystem. “Here urbanization elongates the growing season through urban heating, but you have lower overall annual productivity,” says Imhoff. As can be seen on any nightly newscast, temperatures are usually a degree or two higher within urban areas. Consequently, urbanization causes vegetation to sprout earlier in the spring and turn brown later in the fall. During the summer months, however, urbanization lowers the total amount of plant growth substantially. Imhoff points out that overall, urbanization has resulted in a loss of a minimum of five full days of peak season plant growth a year in Chicago. In other words, when a mixed forest ecosystem is completely urbanized, plant growth is reduced by at least the equivalent of five full days a year.
 

But this was not the only pattern Imhoff and his team uncovered. In cities such as Miami, the native foliage is savanna, which unlike the mixed forest of Chicago is active and therefore green pretty much all year around. Here, the addition of urban heating was of no value and urbanized areas lowered the amount of plant growth all year long, resulting in the loss of 22 full days of peak season plant growth a year. And in cities such as Denver, you see the opposite effect. Denver is built on open shrub land that is sparsely vegetated. People have planted lawns and trees throughout the city and its surrounding suburbs, adding a substantial amount of greenery. For most of the year, the plant growth in the urbanized area is greater than it is in the rural dusty, tumbleweed-laden plains surrounding the city. So urbanization actually adds 11 days of productivity to Denver.

“These days many of the open spaces right outside the urban areas are farmland,” says Imhoff. “In many cases, they are right in the path of development.” So in addition to measuring the difference in vegetation between non-urban and urban areas, Imhoff also recorded the difference between urban and peri-urban areas, where most farmland can be found. The results were in some ways more disturbing. On average, when cities encroach on farmland, the area undergoes a loss of 10 days of peak season plant growth. So in areas such as Chicago, the loss in primary productivity is greater when urban areas replace farmland than when urban areas replace a mixed forest ecosystem.

“The impacts on production are variable, but they’re generally negative. And the impacts may be disproportional to the area converted,” says Imhoff. “So while only three percent of the of the land area in the United States is urbanized, that three percent used to be the most productive soils we had.” Simply put, most of the cities in the United States are located on or near some of the most fertile soils where plant productivity is the highest. As urban areas expand onto peri-urban and non-urban areas, more of this productive soil will give way to development.

In the future, the loss of vegetation due to urban growth will likely accelerate and may affect carbon dioxide levels globally. Right now Imhoff and his colleagues are working on a way to calculate exactly how the reduced plant growth, caused by urbanization, affects carbon dioxide. “Our next step with carbon modeling is that we want to look at the impact of urbanization on photosynthesis in terms of grams of carbon fixed as plant organic matter per square meter of land over a period of months and years,” says Imhoff. With such data, those who construct climate models to forecast global warming and climate change could obtain more precise measurements on how much less carbon dioxide the land can absorb as our population and our cities expand.

• References
• Intergovernmental Panel on Climate Change, 2001: Summary for Policymakers, A Report of Working Group 1 of the Intergovernmental Panel on Climate Change. Cambridge, University Press, Cambridge, UK, 2-17.
• World Resources Institute, 1996: World Resources 1996-97. Washington, DC.

 Viewing Vegetation in a New Light