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Continuing with the theme of urban tree canopies and green spaces, I’d like to comment on a Washington Post article that was sent to me following that post. The newspaper article analyzed data of tree canopies in Washington, D.C., compiled by the University of Vermont’s Spatial Analysis Laboratory and interpreted by the newspaper. The article uses the data to show that low-income neighborhoods have a less dense tree canopy than higher income neighborhoods (see Figure 1 below). The darker green represents a more dense cover, the lighter green signifies a less dense tree canopy, and the purple shows the lowest levels of tree canopy. Figure 2 provides the income distribution in the city (obtained from an different source), with yellow representing higher income and purple for lower income. It’s clear that zone 1 (marked in red in Figure 1) has a denser tree canopy and higher income (as seen in Figure 2). This comparison leaves me wondering if the municipal government is responsible for the entire city and if green spaces are a public good, then what can explain the prevalence of a denser tree canopy in higher income areas?

Comparing tree cover with income levels

Figure 1: Map from the Washington Post, April 25, 2013, “Tree canopy’s density indicates wealth of D.C. neighborhoods.”

 

Tasa de ingreso familiar medio en D.C.

Figure 2: Median Family Income in Washington, D.C. Map from http://www.city-data.com/

One possible explanation, as discussed in the article by representatives of environmental organizations, is that low-income areas have a higher number of renters (see Figure 3) and a lower number of homeowners. Tenants are thought to be less likely to plant trees and owners are more likely to do so, as well to put pressure on their governments to plant trees in public parks and on the streets near their homes. Even if that is a valid reason, it only explains part of the story. It falls short against a public policy argument as municipalities are still responsible for planting trees in public spaces, and certainly the situation should be more balanced between city neighborhoods. On the other hand, what reasons exist about the benefits of trees and the responsibility of community members to maintaining them?

 

Porcentaje de unidades de vivienda ocupadas por arrendatarios en D.C.

Figure 3 : Percentage of units occupied by renter in Washington, D.C. Map from http://www.city-data.com/

 

I wondered if the same phenomenon occurs in other cities. The answer is yes. Aerial photos of many cities, such as Rio de Janeiro, Beijing, and Boston, which can be seen in this blog post, seem to show similar results. I continued to explore this and wondered what happens in some of the cities in the Emerging and Sustainable Cities Initiative (ESCI) of the IDB and found an example in Mar del Plata, Argentina. While we don’t have quantitative statistics to study, we analyzed satellite images that were taken during a study (see Figure 4) and there seems to be a better tree canopy in the areas of higher income of the city (Polygon 3, situated 730 meters or 0.45 miles from the ocean).

 

figure 4. BID

Figure 4: Measured Density from the ocean in Mar del Plata, Argentina. Source: IDB

 

As we move away from the sea (see Figure 5), 730 meters (0.45 miles), 2200 meters (1.36 miles), and 4800 meters (2.98 miles), there is a noted reduction in the tree canopy and an increase in density, by 152 people /block, 360 people /block, and 440 people /block, respectively. There is also an augment of impermeable surfaces, 34%, 52%, and 68%, respectively (see figure 6). This case shows a clear example of a model of development that brings about greater density with greater levels of impermeability or hard surfaces, which all translates to less surface for planting trees. The relative distance from the coast also has a relationship similar to the decrease in income we see in the example in Washington, D.C.

figure 5. BID

Figure 5: Density measured in blocks, Mar del Plata, Argentina. Source IDB.

 

figure 6. BID

Figure 6: Measured Density in blocks of the city of Mar del Plata, Argentina. Source: IDB.

 

ESCI participating cities receive assistance to prepare them to satisfy their future needs, and to emerge as a need not only able to analyze aspects such as density and permeability, but also green infrastructure/green spaces and their relationship to the urban environment. In fact, a new question that ESCI should ask itself is whether we should add up the number of trees as a new indicator of urban sustainability.