Friday, July 13, 2012

Does Income Impact How Green Your Neighborhood Is?

The blog Colorlines recently posted an interview with environmental journalist Tim De Chant highlighting his use of aerial imagery to show the stark difference between the number of trees found in wealthier and lower-income neighborhoods. In short, De Chant found significantly more trees in the wealthier areas he reviewed than in the lower-income areas. A Boston-area resident, De Chant mapped the disparities, using satellite images from Google Earth.

The images are certainly striking and they may offer a visual aid to understanding one way that income inequality can affect populations. But the methodology is somewhat imprecise and complicating factors are not considered. For one, population density can also impact the number of trees in an area so that a wealthy neighborhood, say, in Boston's Back Bay will likely have fewer trees than a lower-income neighborhood in the suburbs and vice versa due to lack of space. In his comparison, De Chant's high- and low-income neighborhoods were West Cambridge vs. Ball Square, and he referred to Somerville as a "neighborhood." Somerville is in fact the most densely populated city in New England, even more densely populated than Cambridge.  

This got us thinking about what we might find here in Somerville with more precise stats.
So we tested the relationship between trees and household income using a computer program called GIS. The City keeps a full inventory of trees and the U.S. Census provides income data, so it was relatively easy to do this. We found that there is no significant correlation in Somerville between wealth and tree density; that is, wealthier Somerville neighborhoods do not necessarily have more trees than poorer ones. See for yourself in the maps posted here.

The green map above shows tree density in Somerville. The darker the green, the more trees in the area. The red map shows income. The darker the red, the higher the median income for the area. If wealth and tree density were significantly related, you would see two maps that roughly look the same, but they don't.

In fact, with the support of federal Community Development Block Grant funding, the City has been steadily greening Somerville's lower-income neighborhoods. The City has made a concerted effort to plant trees in neighborhoods such as East Somerville, where the average median income is lower than the rest of the city. The scatterplot below compares trees per acre to median income. You'll notice there is no clear pattern, which again supports the conclusion that in Somerville at least, income level does not determine how leafy your neighborhood is. 

That's not to say that De Chant is not onto something here. The correlation may apply in other cities or on a greater scale. What are your thoughts or have you seen similar comparisons?


  1. Keep the green trees coming in East Somerville! It helps with air quality, privacy, shade in the summer, property values and pleasing views. My landlord refuses to put them in because he says they'll interfere with ridiculously old pipes under the sidewalk. Others can't afford the purchase. We hope to get more especially near main streets like lower Broadway, Pearl St and sections of Washington St. around MacGrath & towards Sullivan Sq. Thank you!

  2. I was told several months ago the the tree in front of the Hanson Memorial (which was killed when the road work was done on Medford St. 2 years ago) would be replaced. When?

  3. Very interesting effort at analysis. But isn't the city's tree inventory limited to city-owned trees? Would the results change if privately-owned trees were factored in?

  4. Hi Dana,

    I'll look into the tree in from of Hanson Memorial.


    Good point! That might affect the analysis to have privately-owned trees in the model. In that case, you might see a correlation between neighborhood income and, say, the number of trees in people's backyards. Wealthier neighborhoods tend to have larger lots, which would allow for more space for trees, so it makes sense.

    That said, I think that the important social equity question is, does the city provide more trees for wealthy neighborhoods? And I think the answer is clearly no.

  5. I don't believe the original blogger was trying to say that a city provides more trees to wealthier neighborhoods, but that wealthier neighborhoods enjoy more trees- because of land-use histories and issues of environmental (in)justice (wealthier people have bigger yards= more green and more trees- many privately owned; they have the resources for saving or replacing trees, etc. His blog actually supports the program in Somerville of adding more trees to poorer neighborhoods. Yay trees!


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