Trees grow on money: Economist gets to the root of urban tree cover

A study conducted by Boise State economist Michail Fragkias and others looks at the correlation between the number of city trees and overall income levels in seven U.S. cities: Baltimore, Maryland; Los Angeles, California; New York, New York; Philadelphia, Pennsylvania; Raleigh, North Carolina; Sacramento, California; and Washington, D.C. Results were published at PLOS ONE.

Titled "Trees Grow on Money: Urban Tree Canopy Cover and Environmental Justice," the study notes the increasing popularity of urban tree cover in city sustainability plans as a way to mitigate the environmental impact of human activity and to improve aesthetics.

Trees offer cooling shade and help with carbon capture. Recent research has shown that they also have lesser-known benefits, such as decreasing cognitive fatigue, improving worker attitudes and reducing stress, anger, depression and anxiety.

"As an economist, I'm interested in the relevance of ecosystem services for human well-being and the economics of land use, particularly in cities," Fragkias said. "It seemed to me that the distribution of tree canopy in urban areas bridges those interests quite well."

Given the current state of environmental equity, researchers expected to find more trees in higher income areas, and fewer trees in areas with predominately minority populations.

The numbers support that African Americans, Hispanics and Latinos are more likely than whites to live in leafless neighborhoods that are highly vulnerable to the urban heat-island effect -- which shady trees can mitigate. However, study results showed that race isn't really the main factor here, money is. Simply put, wealthier neighborhoods, regardless of their ethnic makeup, are more likely to have more and denser trees.

"It's important to know how the valuable services provided by our ecosystems are distributed across inhabited areas and experienced by different members of society," Fragkias said. "If all cities had an awareness of the distribution of tree canopy (and the associated ecosystem services it provides) they could target programs such as tree planting and other green infrastructure projects that would help ameliorate inequity."

Study results did vary for Los Angeles and Sacramento, where the arid climate requires that trees be irrigated in order to survive. That's because the additional cost of watering trees can create a burden that exceeds the benefits.