Forest researchers David Nowak and Eric Greenfield of the U.S. Forest Service’s Northern Research Station evaluated changes in city tree canopy by analyzing differences between the recent digital aerial photographs and past imagery from early- to mid-2000.
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. Current planting campaigns may increase tree cover now and in the future, however Nowak and Greenfield found that tree cover is decreasing at a rate of about 0.27 percent of land area per year in U.S. cities, which is equivalent to about 0.9 percent of existing city tree cover being lost annually. An estimated 4 million trees are lost every year in urban areas.
In the photo at left a street tree damaged by Asian longhorned beetles is being removed. Photo credit: USDA Agricultural Research Service Archive, USDA Agricultural Research Service, Bugwood.org
For each city, paired digital aerial photographs were obtained for the most recent date possible and imagery as close to five years prior to the most current date as possible. A minimum of 1,000 random points were analyzed across each city.
Nineteen of the 20 cities analyzed showed a reduction in tree cover, 17 of those cities had a statistically significant net reduction. New Orleans, as expected, had the largest percent reduction in tree cover between 2005 and 2009 (-9.6 percent of city area or about one-third of existing tree cover lost), which is most likely due to the devastation of Hurricane Katrina in 2005. Photo credit: Katy Armstrong, US Forest Service
To find out how the percent of tree cover and impervious cover are changing in U.S. cities, Nowak and Greenfield selected 20 cities from across the United States. Two cities were specifically selected to determine the effect of recent suspected tree cover change: 1) New Orleans, La., (effect of 2005 Hurricane Katrina), and 2) Detroit, Mich., (effect of recent infestation of emerald ash borer). Image shows a Detroit street before and after tree removals due to emerald ash borer damage. Photo credit: Michigan Department of Agriculture
Of the 20 cities analyzed, tree cover ranged from a high of 53.9 percent in Atlanta to a low of 9.6 percent in Denver; while total impervious cover varied from 61.1 percent in New York City to 17.7 percent in Nashville. Photo credit: TreesAtlanta
Tree cover loss due to the emerald ash borer in Detroit was lower than expected. Since 2002, this beetle has killed more than 30 million ash trees in Southeastern Michigan. However, the loss of tree cover in Detroit between 2005 and 2009 (-0.7 percent of the city area, or about 3 percent of existing tree cover) was less than the average loss from the sampled cities. This difference could be due to ash trees not comprising a major component of overall tree cover in Detroit and/or new trees being established through tree planting programs or natural regeneration that help offset the loss of ash and other trees. Photo credit: Therese Poland, US Forest Service Northern Research Station
Only one city (Syracuse, N.Y.) exhibited an overall increase in tree cover. However, the increase was dominated by European buckthorn (Rhamnus cathartica), an invasive small tree / shrub from Europe, and the most of the city’s cover increase is likely due to natural regeneration. Photo credit: David Nowak, US Forest Service Northern Research Station
Sixteen of the 20 cities had statistically significant increases in impervious surfaces (e.g., buildings, roads). Cities with the greatest annual increase in impervious cover were Los Angeles (average of 550 ha/yr), Houston (400 ha/yr) and Albuquerque (280 ha/yr). Image at left is a Google Earth aerial view of downtown Los Angeles, California. Photo credit: Google Earth
Photo-interpretation of paired digital images offers a relatively easy, quick and low-cost means to statistically assess changes among various cover types. To help in quantifying the cover types within an area, a free tool (i-Tree Canopy) is available (www.itreetools.org) that allows users to photo-interpret a city using Google images.