Trees in the urban environment provide a variety of benefits. Various researchers have touted the benefits of tree canopy in cities: cleaner air, stormwater reduction, carbon sequestration, energy savings, higher property values and health benefits.i   Some have even found a reduction in crime associated with tree canopy.ii   If distribution of these benefits are affected by income, ethnic, or racial disparity it may be viewed as an issue of environmental justice.

 Numerous studies have found strong positive correlations between household income and tree canopy. One major study published in PLoS|ONE  in April 2015 looked at seven major U.S. Cities and found, “Money may not grow on trees, but this study suggests that in a way, trees grow on money. Our findings show that high-income neighborhoods in our selected cities are more likely than low-income neighborhoods to have high tree canopy cover.”iii   This same study found that “correlations between the distribution of benefits and socio-economic variables can vary across cities.” Race and ethnicity correlated with the amount of tree canopy in some cities, but not others. 

Tualatin Riverkeepers took a look at how tree canopy in cities of the Tualatin Basin compares with median household income for those cities and percentage of residents that identify themselves as Latino or Hispanic.  Census data were matched with tree canopy data calculated by Nathan Herzog using i-Tree on 2014 aerial photos from Google Earth to make these comparisons.

Tualatin Riverkeepers’ results are consistent with the PLoS|ONE study. While there is a weak negative correlation between Latino population and tree canopy, there is a very strong positive correlation between Per Capita Income and the percentage of the landscape covered with tree canopy.

 

Cities
of the Tualatin Basin
Latino % Population  Per Capita
Income  
     % Tree
Canopy
King City 4.5% $27,536 15.0%
Cornelius 50.1% $15,290 13.0%
Rivergrove 2.4% $31,546 37.3%
Tigard 12.7% $25,110 25.0%
Hillsboro 22.6% $21,680 17.1%
Forest Grove 23.1% $16,992 16.0%
West Linn* 4.0% $34,671 33.7%
Beaverton 16.3% $25,419 25.6%
Sherwood 7.0% $25,793 21.0%
Tualatin 17.3% $26,694 22.9%
Lake Oswego 3.7% $42,166 47.6%
North Plains 11.0% $18,794 15.3%
Durham 13.8% $29,099 49.0%
Banks 7.0% $21,354 11.2%
Gaston 11.0% $17,758 15.5%
Sources:  U.S.Census 2010, i-Tree analysis by Nathan Herzog on Google Earth aerial photos.


Lake Oswego
Trees grow on money in Lake Oswego.

Targeting tree planting programs to lower income areas has its challenges. Research by Geoffrey Donovan of the U.S. Forest Service found that residents of lower income areas in Portland Oregon were less receptive to tree planting programs than in higher income areasiv.  Some of this can be explained by lower home ownership in low income areas. Renters tend to be more transient and may not be able authorize tree planting.

End notes
i McPherson, E. Gregory, et al. “Million trees Los Angeles canopy cover and benefit assessment.” Landscape and Urban Planning 99.1 (2011): 40-50.
ii Troy AR, Grove JM, O’Neill-Dunne JP. The relationship between tree canopy and crime rates across an urban-rural gradient in the greater Baltimore region. Landsc Urban Plan. 2012;106: 262–270. doi: 10.1016/j.landurbplan.2012.03.010
iii Schwarz K, Fragkias M, Boone CG, Zhou W, McHale M, Grove JM, et al. (2015) Trees Grow on Money: Urban Tree Canopy Cover and Environmental Justice. PLoS ONE 10(4): e0122051. doi:10.1371/journal.pone.0122051
iv Donovan, Geoffrey H. and John Mills, Environmental Justice and Factors that Influence

Participation in Tree Planting Programs in Portland, Oregon, U.S Arboriculture & Urban Forestry

2014. 40(2): 70–77

Cornelius
Cornelius Oregon.