Whether they are ringed by wrought iron or suspending a swing, urban trees are first and foremost trees. In fact, they are all working trees.
Consider, for example, carbon storage. From New York City’s Central Park to Golden Gate Park in San Francisco, America’s urban trees store an estimated 708 million tons of carbon, valued at $50 billion. Annually, these trees absorb an estimated 21 million tons of carbon, a value of $1.5 billion.
A team of U.S. Forest Service scientists led by Dave Nowak of the agency’s Northern Research Station used field data from 28 cities and six states along with new estimates of urban land area and urban tree cover to estimate national carbon storage. Their study was recently published in the journal Environmental Pollution.
The monetary value of carbon storage and sequestration is an estimate of the damages associated with incremental increases in carbon emissions in a given year. This includes, among other things, changes in net agricultural productivity, human health, property damages from increased flood risk, and the value of ecosystem services due to climate change.
Nationally, carbon storage by trees in forestlands was estimated at 22.3 billion tons in a 2008 Forest Service study; additional carbon storage by urban trees bumps that to an estimated 22.7 billion tons. While forests in rural areas occupy a larger land area and store more carbon, urban trees and forests can make a difference in climate change by altering the environment where most Americans live.
For example, when urban trees shade your house, your air conditioner is running less and burning less energy. Similar benefits from trees add up to significant reductions in carbon emissions throughout urban areas, which are a significant source of carbon emissions.
Between 1990 and 2000, the increase in urban land within the lower 48 states was roughly the size of Vermont and New Hampshire combined. If that growth pattern continues, by 2050 the nation’s urban land cover could expand by an area greater than the state of Montana.
That increase may or may not translate to more urban forests storing carbon, however. Last year, Nowak and Eric Greenfield, a forester with the Northern Research Station and a study co-author, found that urban tree cover is declining nationwide at a rate of about 20,000 acres, or 4 million trees, per year.
“With expanding urbanization, city trees and forests are becoming increasingly important to sustain the health and well-being of our environment and human population,” according to Nowak. “Carbon storage and sequestration is just one of many potential benefits provided by urban trees and forests.”