Expert Commentary

Tree and impervious cover change in U.S. cities

2012 study from the U.S. Forest Service and the SUNY College of Environmental Science and Forestry on the change in trees and greenery in 20 U.S. metropolitan areas.

A sunny day, a sweltering street, a shady tree. Studies have shown that residents of cities and towns benefit both monetarily and psychologically from tree-filled neighborhoods and quiet parks: Air quality improves, stress is lower and houses are worth more. The greening of urban spaces has even been shown to have a relationship with reduced crime rates and better health and safety outcomes.

But despite the abundant benefits of urban greenery, trees are often the losers as the ongoing financial crisis cuts into local budgets, drought hits home and cities become ever more congested.

A 2012 study published in Urban Forestry & Urban Greening, “Tree and Impervious Cover Change in U.S. Cities,” looked at 20 U.S. metropolitan areas. The researchers, from the U.S. Forest Service and the SUNY College of Environmental Science and Forestry, analyzed aerial photographs spanning periods from four to six years to determine shifts between greenery and impervious surfaces such as buildings, parking lots and roads.

  • In the cities examined, tree and/or shrub cover ranged from a high of 53.9% in Atlanta (2005) to a low of 9.6% in Denver (2009).
  • The highest rate of impervious surface coverage was 61.1%, in New York City (2009), and the lowest 17.7 %, in Nashville (2003). The highest rate of building surface area was 27.1% in Chicago (2005) and the lowest in Kansas City, 4.8% (2003). Roads, parking lots and other impervious surfaces covered 36.2% of Miami and just 12.3% of Nashville (both 2003).
  • Over the periods studied, cities’ tree, shrub and soil cover generally fell, while other categories of cover tended to increase. “Nineteen of the 20 cities analyzed showed a reduction in tree cover, 17 of those cities had a statistically significant net reduction.”
  • Excluding New Orleans and Detroit, which were expected to have reduced greenery, the average tree cover dropped by 1.1% during the varying periods of analysis (the 20-city average was 1.5%). “The greatest decreases in coverage [were] in New Orleans (-9.6%), Houston (-3.0%) and Albuquerque (-2.7%). The relative reduction in tree cover was as high as -29.2% in New Orleans, but averaged -3.8% (-5.0% for 20-city average).”
  • The city with the greatest annual loss in tree cover was New Orleans, with an average loss of 1,120 hectares per year. Next were Houston and Albuquerque, with declines of 890 and 420 hectares per year, respectively.
  • The greatest per-capita decline was also in New Orleans, which lost 24.6 square meters each year for every city resident. Next were Albuquerque (-8.3) and Nashville (-5.3). The average loss was 1.9 square meters per person per year (-3.0 for the 20-city average).
  • Most of the areas originally with trees were converted to grass (47%), followed by impervious cover such as buildings, roads and parking lots (29%), and bare soil (23%).
  • The greatest annual increase in impervious cover was in Los Angeles, with an average of 550 hectares per year. Next came Houston (400) and Albuquerque (280). Per-capita increases were highest in Tacoma (6 square meters per person per year), Kansas City (5.9) and Albuquerque (5.). The average increase in impervious cover was 2.2 square meters per person per year (2.1 for 20-city average).
  • “Loss of tree cover was slightly correlated to increased population density in the 18 cities.”

“While cities expend resources to plant millions of new trees, land development, storms, old age and other factors are reducing the number of older, established trees in cities,” the researchers state. They suggest that “developing coordinated healthy tree canopy programs across various land ownerships can help sustain desired tree cover levels and better manage cover change.”

Related research: A 2013 study in Psychological Science, “Would You Be Happier Living in a Greener Urban Area? A Fixed-Effects Analysis of Panel Data,” using data from more than 10,000 individuals to explore the relationship between urban green space, well-being and mental distress for the same people over time.

Keywords: cars, conservation, urban greenery

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