Tree and Impervious Cover Change in U.S. Cities

By David J. Nowak and Eric J. Greenfield

Syracuse, NY (January 17, 2012)- Paired aerial photographs were interpreted to assess recent changes in tree, impervious and other cover types in 20 U.S. cities as well as urban land within the conterminous United States. National results indicate that tree cover in urban areas of the United States is on the decline at a rate of about 7900 ha/yr or 4.0 million trees per year.

Tree cover in 17 of the 20 analyzed cities had statistically significant declines in tree cover, while 16 cities had statistically significant increases in impervious cover. Only one city (Syracuse, NY) had a statistically significant increase in tree cover.

City tree cover was reduced, on average, by about 0.27 percent/yr, while impervious surfaces increased at an average rate of about 0.31 percent/yr. As tree cover provides a simple means to assess the magnitude of the overall urban forest resource, monitoring of tree cover changes is important to understand how tree cover and various environmental benefits derived from the trees may be changing. Photo-interpretation of digital aerial images can provide a simple and timely means to assess urban tree cover change to help cities monitor progress in sustaining desired urban tree cover levels.

During the periods studied, the change in tree/shrub cover in each city was:

  • Albuquerque, NM: -2.7%
  • Atlanta, GA: -1.8%
  • Baltimore, MD: -1.9%
  • Boston, MA: -0.9%
  • Chicago, IL: -0.5%
  • Denver, CO: -0.3%
  • Detroit, MI: -0.7%
  • Houston, TX: -3.0%
  • Kansas City, MO: -1.2%
  • Los Angeles, CA: -0.9%
  • Miami, FL: -1.7%
  • Minneapolis, MN: -1.1%
  • Nashville, TN: -1.2%
  • New Orleans, LA: -9.6%
  • New York, NY: -1.2%
  • Pittsburgh, PA: -0.3%
  • Portland, OR: -0.6%
  • Spokane, WA: -0.6%
  • Syracuse, NY: +1.0%
  • Tacoma, WA: -1.4%

Although tree planting and natural regeneration are occurring in urban areas, net tree cover is on a general decline in urban areas of the United States. It is apparent that tree planting and natural regeneration are insufficient to offset the current losses of established urban tree canopies. However, without various tree planting efforts in cities, tree cover loss would be higher.

Sustaining tree cover not only includes establishing new trees, but also limiting the loss of existing canopy, particularly large trees that provide substantial amounts of canopy per tree. Sustaining tree health and protection of healthy tree canopies from human removal (e.g., development) or natural mortality forces (e.g., insects and diseases) can also help sustain existing tree cover and associated environmental services.

While individual campaigns are helping to increase or reduce the loss of urban tree cover, more widespread, comprehensive and integrated programs that focus on sustaining overall tree canopy may be needed to help reverse the trend of declining tree cover in cities. Developing coordinated healthy tree canopy programs across various land ownerships can help sustain desired tree cover levels and better manage cover change.

Related Resource:
Urban Forestry & Urban Greening- Tree and Impervious Cover Change in U.S. Cities