Urban forests are worth billions of dollars, thanks to their ability to purify air, cool overheated streets, filter runoff, and raise property values. That’s why many U.S. cities are pushing hard to plant more of them.
But a new study from the University of Southern California says cities, and trees, may be better served by preserving the waning leaf coverage that exists. Tree cover declined at alarming rates in L.A. County’s 20 largest cities between 2000 and 2009, during which time officials called on homeowners to plant tons more. One major disconnect can be traced to housing policy, as compact residences swelled into McMansions and sucked up trees in the process.
Oddly enough, this research—led by USC spatial scientists Su Jin Lee, Travis Longcore, John P. Wilson, and Catherine Rich of the Urban Wildlands Group, forthcoming in Urban Forestry and Urban Greening—casts 1950s suburban development as a friend to environmental health.
The post-war, one-story bungalow might be the avatar of evil in many an anti-sprawler’s view, but the authors write that these units were actually pretty decent at conserving trees and grass, thanks to the “cultural value of appreciation for greenery and shade” embedded in the suburban promise. Previous research has shown this appreciation helped drive an overall increase in tree coverage between the 1920s and 2006 in certain parts of L.A., largely thanks to trees planted on private land.
But that trend reversed in the first decade of the 2000s. Studying the footprints of residential lots based on satellite imagery and data from the L.A. County assessor’s office, the researchers found that about one-third of existing green cover was lost during the average home “improvement” on single-family properties between 2000 and 2009. Over that time period, an additional nine percent of residential lots across those 20 cities in the study were covered by development’s spreading footprint, and tree cover dropped by roughly 14 percent.
These trends held true across the income spectrum. The authors surmise that hardscape increase and tree loss in poorer parts of L.A. might be the outcome of tree removal or densification, while the driver in wealthier areas seems to have been speculative development and home expansions, bloating single-family homes into floor-area ratios gone wild.
Ironically, in 2006, then-mayor Antonio Villaragosa started the “Million Trees L.A.” campaign to encourage private homeowners to plant more trees, an effort that’s morphed into a public-private tree-planting initiative called City Plants. These efforts represent a nice sentiment, write the authors, but whatever effect they’ve had on increasing green space in L.A. “were more than offset by tree removal to accommodate additional hardscape and larger homes.”
This research sheds even more light on how deep the disconnect between policy and reality is inside L.A.’s urban forest: by comparing assessor data to satellite imagery, the authors inadvertently found “widespread increases in building footprint for parcels where no legal increase in square footage had been reported to the Assessor.” That’s concerning from a tax-base perspective: increases in property sizes are one of the few opportunities local governments in California have to raise more public funds, and apparently Los Angeles County has been missing out.
More to the point, the loss of trees undermines the city’s ability to “adapt to increased urban temperatures, manage urban stormwater, and maintain urban nature and quality of life,” the authors write.
The tree-killing housing boom of the early 2000s may have slowed down during the recession, but the economic recovery of the past five years has seen housing prices and development pick right back up. Home sizes are continuing to swell, and the resulting loss of tree cover—coupled with the drought- and disease-fueled loss of undeveloped forestlands—isn’t likely to abate. Ad hoc tree planting efforts may be shoveling sand against the tide. Cities committed to green space may want to start by tracking trees lost to new construction, and shoring up what they already have.