Side project: tree canopy coverage in Baltimore
It's well-known that Baltimore's history of redlining has impacted the current state of street tree density, diversity and age. To better visualize this effect, I generated the following maps in R using publicly-available data from Baltimore Neighborhood Indicators Alliance. Mapped onto the 1930s Home Owner's Loan Corporation (HOLC) categories for the area, it appears that tree canopy cover (shades of teal) is inversely related to the area’s previous HOLC category, from A (“best”, green) to D (“hazardous”, red). Indeed, quantifying tree canopy cover reveals that the communities previously categorized as A or B (“still desirable”, blue) have a higher average percent area covered compared to the redlined communities (C: “definitely declining”, yellow; D: “hazardous”, red).
Fig 1: Tree coverage is lowest in Baltimore's formerly-redlined communities. In 1A (above right), map shows tree canopy coverage (shades of teal) overlaid on the 1930s map of HOLC categories (A, green; B, blue; C, yellow; D, red). Toggle either map on and off with the buttons in the top right and hover over a community to see its tree canopy percentage.
Data from 1A are quantified in 1B (left). Individual data points represent the tree coverage for a single community. Communities were assigned the HOLC category that covered the largest area within its bounds, as has been done in similar studies.
Similar patterns can be seen when the same tree coverage data are overlaid with each community’s current socioeconomic status, shown in Figure 2 by the percentage of family households living below the poverty line (shades of pink). The inverse relationship between tree canopy cover and poverty level is immediately apparent, highlighting the enduring impact of redlining. For example, some of the highest tree coverage communities (i.e., Mount Washington/Cold Spring: 66.6% of area covered by trees) have some of the lowest poverty rates (Mount Washington/Cold Spring: 2% of households). These communities appear more teal on the merged map. On the opposite end of the spectrum (more pink on the merged map), some of the communities with the highest percentage of households living below the poverty line (i.e., Upton/Druid Heights: 43%) have a much lower area covered by trees (Upton/Druid Heights: 15.5%).
Fig. 2: Tree canopy coverage is lowest in Baltimore communities experiencing the highest rates of poverty. Tree canopy coverage (shades of teal) is overlaid with the each community's percentage of family households living below the poverty line (shades of pink). Unclick "merge" to toggle between each overlay and hover over a community to show its data.
Fig. 3: The number of trees planted in Baltimore ranges widely between communities and poverty rates. Trees planted by TreeBaltimore in 2020 (shades of blue) overlaid with each community's percentage of family households living below the poverty line (shades of pink). Unclick "merge" to toggle between each overlay and hover over a commynity to show its data.
So what can we do about this? There are many benefits to having more and larger trees in urban communities, which are not equitably shared across income levels in Baltimore. Nonprofit organizations like Baltimore Tree Trust aim to increase Baltimore's total tree canopy coverage by planting street trees in lower-income areas. The City program TreeBaltimore also exists to increase tree canopy, but are they doing so equitably? Figure 3 shows the percentage of households living below the poverty line in shades of pink, as above, this time for the year 2020, along with the number of trees planted by TreeBaltimore for the year in shades of blue. The number of trees planted ranged from a low of 2 (Mount Washington/Cold Spring; 2.9% of households below poverty line) up to 447 (Allendale/Irvington/S. Hilton; 14.2% of households below poverty line). With "Merge" toggled on, we can see that some communities have both low levels of poverty and received few new trees in 2020 (closer to white; i.e., Mount Washington/Cold Spring, Fells Point, South Baltimore; all 0-3.3% households below poverty and 2-14 trees planted). Others had relatively higher levels of poverty and gained relatively more trees (shades of purple-blue; i.e., Pimlico/Arlington/Hilltop, Oliver/Johnson Square, Belair-Edison; 18.9-25.8% households below poverty line and 223-393 trees planted). Still, though, many of the communities with the highest levels of poverty (i.e., Poppleton/The Terraces/Hollins Market, Upton/Druid Hills, Oldtown/Middle East; 41.4-45.8%) had relatively few new trees planted (37-96).