A study published by two researchers with the USDA Forest Service analyzed paired aerial photographs taken between 2009 to 2015 in order to quantify changes in tree cover and urbanization across all 50 States and the District of Columbia (D.C.). The study found that, nationally, tree cover is declining at a rate of 175,000 acres per year which is the equivalent of about 36 million trees lost each year. The study found that the loss of tree cover was statistically significant in 23 of the states/district. The states with the greatest decline in tree cover in urban and community areas (as delineated by the 2010 Census) were: Oklahoma (−0.92%/yr), District of Columbia (−0.44%/yr), Rhode Island (−0.40%/yr), Oregon (−0.38%/yr) and Georgia (−0.37%/yr).
Researchers in this study calculated estimated changes in tree cover by pairing 1,000 locations in each state by using a set of aerial imagery dating taken as close to five years apart as possible. Trained photo-interpreters then classified the location of each point from each set of photographs as being one of the following: trees/ shrubs (woody vegetation), grass or herbaceous cover, bare soil, agri- culture (crop areas), water, impervious (buildings), impervious (roads), or impervious (other). To calculate the cover percentage, the researchers applied the following calculation:
cover class (p) = the number of sample points (x) hitting the cover attribute divided by the total number of interpretable sample points (n) within the area of analysis (p = x/n)
Using this established method for calculating canopy coverage in urban and community areas, the researchers found that tree cover in urban/community areas ranged from 68.1% in Maine to 10.1% in North Dakota. Factoring in all of the 50 states and D.C., tree cover declined from 42.9% to 42.2%, an annual decline of 0.12%. In comparing aerial images, the decline was mostly attributed to a change to grass/herbaceous (−0.8%) or impervious cover (−0.3%). For urban areas, tree cover ranges from 61.6% in Connecticut to 10.1% in North Dakota. Urban areas, as compared to what the researchers defined as urban/community areas which may encompass more suburban or rural lands, are more densely populated and tend to have more impervious areas. Researchers concluded that the percent tree cover was greater in urban areas (−1.0 percent) than in urban/community areas (−0.7 percent) due to pressures exerted by increased population densities and urbanization.
Tree cover has been associated with many benefits such as reducing ambient temperatures (which, in turn, reduces energy needed for cooling buildings), removing carbon dioxide from the air, reducing rainfall runoff, absorbing pollutants, and providing social and mental health benefits. A recent study showed that one immediate and crucial tool for mitigating climate change would be to add 1 billion of hectares of trees globally. Conclusions from this research unfortunately suggest that the number of trees in the United States is on the decline in urbanized areas.
Nowak, D. J., & Greenfield, E. J. (2018). Declining urban and community tree cover in the United States. Urban forestry & urban greening, 32, 32-55. https://www.fs.fed.us/nrs/pubs/jrnl/2018/nrs_2018_nowak_005.pdf