This is a fascinating table compiled by Fiona Watt and Bram Gunther (both with the New York City Deparment of Parks) that we learned about at the ASCE Low Impact Development Conference.

City Tree Cover (%) Date Method/Source Atlanta, GA 36.7 1998 UFORE/USFS Houston, TX metro area 30.0 Regional Ecosystem Analysis/American Forests Austin, TX 34.0 early 1990s Regional Ecosystem Analysis/American Forests Boston, MA 29.0 2005 Flyover–USFS Dallas, TX 28.0 UC Davis–USFS National average 27.1 2004 UFORE/USFS Atlanta, GA 27.0 early 1990s Regional Ecosystem Analysis/American Forests Syracuse, NY 26.6 1999 UFORE/USFS Seattle, WA 25.0 early 1990s Uncertain estimate Baltimore, MD 25.0 early 1990s Regional Ecosystem Analysis/American Forests New York City, NY 24.0 2005 Satellite imagery–USFS Boston, MA 22.3 1998 UFORE/USFS Boston, MA 29.0 2006 Satellite imagery–USFS Baltimore, MD 21.0 1998 UFORE/USFS New York City, NY 20.9 1998 UFORE/USFS Los Angeles, CA 18.0 UC Davis–USFS Philadelphia, PA 15.7 1998 UFORE/USFS San Francisco, CA 12.0 UC Davis–USFS Jersey City, NJ 11.5 1998 UFORE/USFS Milwaukee, WI 11.0 early 1990s Regional Ecosystem Analysis/American Forests Chicago, IL 11.0 1991 uncertain/USFS Miami Dade County 10.0 early 1990s Regional Ecosystem Analysis/American Forests

(Table credit: Watt & Gunther, 2010)

As you can see, the national average of tree cover in major U.S. cities is 27.1. American Forests recommends an average of 40% tree canopy coverage in areas East of the Mississippi (in the Southwest and dry West, that number is 25-35% depending on the type of development). In the past 20 years, American Forests estimates that tree canopy cover has been reduced by about 30%. It’s distressing to see these already-low numbers and realize that they are part of a broader trend. Egads.

Stopping the decline of the urban tree canopy — and setting goals to increase it and get it closer to the recommended coverage — is important for several reasons. Large, healthy, long-living trees act as “green infrastructure” in our communities. In an increasingly warm world, they help keep temperatures low and reduce heat-island effect. They also keep the air clean and free from pollution and, if integrated into the streetscape thoughtfully, provide extremely effective on-site stormwater management.

The loss of this functionality due to reduction in tree canopy carries very real costs that affect our communities. While “More Trees!” (like in Los Angeles and New York City, to name just two) is an easy cause to endorse, policy makers and municipalities should keep in mind that trees need adequate soil volumes in order to make a significant environmental and financial impact. A tree with a 30″ (77 cm) diameter delivers SEVENTY TIMES the environmental benefits of a tree with a 3″ (8 cm) diameter. We estimate that a tree (dependent upon species, climate, etc.) needs around 1,000 cubic feet (28 cubic meters) to have a trunk diameter of 16″ (40.5 cm) and a canopy diameter of 32′ (9.7 m). That amount of soil can store about 200 cubic feet (5 cubic meters) of water in addition to the water uptake and evapotranspiration provided by a healthy tree.

For more information about to calculate the economic and environmental value of the trees in your community, check out CITYgreen software.