Low-income blocks in the US have up to 30% fewer trees and are 4°C (7.2 °F) hotter -- and this is probably not a coincidence.
Trees do a lot to help the local environment, especially in urban areas -- their impact goes far beyond just producing oxygen and absorbing pollutants from the air. Cities are notoriously hotter than their surroundings, and trees provide much-needed cooling. Trees also help with regulating water runoff, prevent erosion, and help the local ecosystem. Increasingly, studies are showing that urban tree cover is linked with better cardiovascular and lung health as well as mental health.
But studies also show that urban tree cover is unequally distributed, with low-income and minority communities often having fewer trees than their richer counterparts.
To see if this is the case, Robert McDonald of The Nature Conservancy in Arlington, Virginia, and colleagues quantified just how much tree cover and urban temperatures vary with income in 5,723 cities, towns, and villages in the US (collectively home to 167 million people).
They started with digital images from the National Agriculture Imagery Program to examine tree cover in the 100 largest urban areas of the U.S. and used satellite imagery to compare summertime temperatures in these communities. Out of the studied areas, 92% of low-income communities had less tree cover than their high-income counterparts.
Low-income communities had, on average, 15.3% less tree cover and were hotter by an average of 1.5°C. But some areas showed much higher differences.
"The greatest difference between low- and high-income blocks was found in urbanized areas in the Northeast of the United States, where low-income blocks in some urbanized areas have 30% less tree cover and are 4.0⁰C hotter," the study authors write.
Even when compensating for population density and built-up intensity, the association between income and tree cover is significant. The researchers also found that blocks with a greater proportion of people of color had less tree cover and hotter summer temperatures.
"We had expected low-income and minority neighborhoods to have less tree cover and so be more at risk during summer heat waves, but we were still surprised and troubled at how widespread this tree inequality was, with 92% of communities having lower tree cover in low-income than in high-income areas," the authors add.
The researchers also calculated how this disparity could be addressed. Overall, there are 62 million fewer trees in low-income blocks than comparable high-income blocks. This could be addressed by a $17.6 billion investment in tree planting and natural regeneration, which could benefit 42 million people. Another way to look at this is that if you're living in a high-income community in the US, you're likely getting a lot of quantifiable benefits from the extra tree cover.
This is not the first study of this kind, and the problem is not only US-specific: both in the US and in other parts of the world, low-income communities are prone to low tree cover and heat.