The Gray-Green Urban Divide: How Wealth and Poverty are Visible from Space

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In many neighborhoods, income’s influence on a neighborhood can manifest itself physically in different ways.  Well paved roads and better maintained buildings are just two signs of a wealthier neighborhood.  The amount of vegetation, even in densely urban areas, can also be a predictor of the relative wealth of a neighborhood.  Known as the “gray-green divide”, the amount of trees and green space can be an indicator of the income level of the neighborhood.

What is the Gray-Green Divide?

The gray in the phrase, “gray-green divide”, refers to the dominant color from roads, sidewalks, and rooftops that strikes the viewer when looking at an aerial or satellite image of a lower-income neighborhood.  In contrast, higher-income neighborhoods tend to have more vegetation in the form of trees and landscaping that provide a “greener” view to the imagery.  The disparity is even more evident when viewing a lower-income neighborhood that back ups to a wealthier one.

For example, in this screenshot taken from Google Maps, the affluent neighborhood of Oyster Bay is seen on the right.  The neighborhood is far more verdant than the poorer neighborhood shown on the left. The blog, Per Square Mile, has a short list of examples of income inequality and vegetation.

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KCET, an independent television station that broadcasts in the Los Angeles area, started a section on its web site highlighting the disparity in tree canopy in Los Angeles.  A short introduction to the issue noted that Hancock Park, a wealthy neighborhood has up to 37% of its ground covered by trees, while poorer neighborhoods in the South LA only have 7% coverage. In the screenshot below, a South LA neighborhood is on the left while Hancock Park is on the right.

The urban canopy disparity in Los Angles. Source: KCET

The urban canopy disparity in Los Angles. Source: KCET

Quantifying Urban Vegetation Coverage

The disparity in vegetative cover has been studied by researchers.  In one study, 210 cities (representing all cities with a population of at least 100,000 people) across the United States were analyzed.  Using data from U.S. Department of Agriculture, the researchers applied economic data from the US Census Bureau to see what the correlation  is with urban tree coverage.  The researchers found that “higher income populations or residents will have more demand for urban forests. Urban forests are economic goods. When income increases the demand will rise as well. Rich communities have larger budget on public forests, and have larger private house lots where private trees mostly are grown. “ [1]  Quantitatively, for every one percent rise in income, demand for tree canopy coverage increased by 1.76 percent.  For every one drop in income, the demand decreased by 1.26 percent.[2]

How Neighborhood Income Affects Species Diversity

The gray-green divide also has an impact on biodiversity.  Researchers looking at species richness in Phoenix, Arizona and Baltimore, Maryland found both higher numbers and abundance of species in upper-income neighborhoods as compared to lower-income ones.  In the Arizona study which looked at bird diversity and numbers in city parks in a range of neighborhoods, Arizona State University ecologists Ann P. Kinzig explained[3], “We can’t explain bird diversity in the parks by the size of the parks, or the types or sizes of trees in the parks, which is what we might expect. Instead, the characteristics of the neighborhood, including the income of the residents, seem to play a significant role in influencing the number of species that live in the park.” A study in 2003 of 92 random plots in Phoenix also found that plant diversity increased with the area’s socioeconomic status.  [4] Kinzig theorized that a greater diversity of plants along with an attention towards cultivating more native landscaping in the form of desert plants was a factor in the increased total population and numbers of species.

Benefits of Urban Vegetation

The gray-green divide is troubling on many levels, given the documented benefits of greater urban green space.  In addition to the benefits trees and vegetation provide to reduce urban heat islands, lowering stormwater runoff, and storing carbon, studies have shown a link between the higher greeness in neighborhoods and reduced chronic diseases.  Urban green space promotes increased physical activity, leading to lower stress levels and increased community health.  A study based in Philadelphia also found that greener neighborhoods have a lower crime rate than lesser vegetated ones.  The authors of the study argued that the benefit of more green space it twofold[5].  First, it encourages residents to spend more time outside which deters crimes.  Second, more green space has a calming effect which in turn reduces crime.  As more city planners become aware of this inequality, efforts by local agencies and non-profits are being made to plant more trees in lower-income neighborhoods (e.g. Portland and New York City).

References

[1] Zhu, P., & Zhang, Y. (2008). Demand for urban forests in United States cities. Landscape and urban planning, 84(3), 293-300.

[2] De Chant, T.  (2012, May 17).  Urban trees reveal income inequality.  Retrieved from http://persquaremile.com/2012/05/17/urban-trees-reveal-income-inequality/

[3] Hathaway, J.  (2002, August 8).  City birds prefer rich neighbors.  Retrieved from https://www.asu.edu/news/research/citybirds_080802.htm

and

Kinzig, A. P., Warren, P., Martin, C., Hope, D., & Katti, M. (2005). The effects of human socioeconomic status and cultural characteristics on urban patterns of biodiversity. Ecology and Society, 10(1), 23.

[4] Hope, D., Gries, C., Zhu, W., Fagan, W. F., Redman, C. L., Grimm, N. B., … & Kinzig, A. (2003). Socioeconomics drive urban plant diversity. Proceedings of the national academy of sciences, 100(15), 8788-8792.

[5] Wolfe, M. K., & Mennis, J. (2012). Does vegetation encourage or suppress urban crime? Evidence from Philadelphia, PA. Landscape and Urban Planning, 108(2), 112-122.

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