By Ian Leahy, Director of Urban Forest Programs

 “Urban forests provide critical social and environmental benefits for 83 percent of the US population living among 136 million acres of urban forest land.”– Ten-Year Urban Forestry Action Plan

136 million acres. This number has been common boilerplate in press releases and reports in our field for quite a few years now. It is generally trotted out to subtly draw favorable comparison with the 193 million acres of national forests that are managed under much larger federal budgets. For better or worse, that official number may soon be creeping a little closer to the scale of national forests.

When a landscape is converted from one dominated by natural systems to one teeming with many small landowners, businesses, multi-lane roads and dense population, the way in which that land must be managed fundamentally changes. That’s where urban forestry comes in. Knowing what scale we’re dealing with is an important factor.

U.S. Forest Service scientist and American Forests’ science advisory board member David Nowak has submitted for peer review an updated estimate of 141 million acres nationwide. That number will, of course, continue to grow over time as the Regional Plan Association projects 90 percent of Americans will be living in urbanized areas by 2050.

But, what does this 141 million acres comprise?

For starters, it’s important to realize that only 68 million of those acres exist in actual urbanized areas or clusters. As defined by the U.S. Census Bureau (2013), an urbanized area has a population of at least 50,000 people. Urban clusters are more flexible, possessing a population between 2,500 and 50,000 people.

The remaining 73 million acres fall into the “community” part of “urban and community forestry.” This includes anywhere that is an incorporated or designated place. By overlapping measurements mixing population density with geopolitical delineations, we capture the cities, suburbs, exurbs and small towns where 83 percent of Americans live.

Once an area falls within this 141 million acres, urban forestry professionals and conservation planners often kick into action, seeking to develop an interconnected system of green infrastructure at different scales that can mimic, as closely as possible, the natural functions that have been lost. Or, in arid climates where forests are not a natural occurrence, urban forests are created out of nothing to serve a population now living in that landscape and needing the services a robust tree canopy can provide. This was the case in Denver, which claims an almost entirely manmade urban forest.

At the smallest scale, highly-engineered bioretention installations in sidewalks and streets serve as a last-chance filter before water floods the underground storm sewer lines. With slightly more open space, street trees, yard trees, gardens and landscaped boulevards can take root. Taken together, they serve as a front line defense against particulate matter and management of both water quality and quantity. These components of the urban forest integrated into residents’ daily life also provides the most tangible interaction many people have with nature on a daily basis, generating higher rates of social, mental health and economic benefits.

Green spaces, particularly interconnected ones, such as urban parks, river and coastal corridors, greenways, shelter belts of trees and working trees absorbing toxins from abandoned industrial lands, begin to mimic nature without intervention, providing the added benefit of easily-accessed recreational, economic, critical infrastructure protection and other open space benefits.

Lastly, it is the large nature preserves which have the potential to impact air quality, water quality, wildlife viability and quality of life throughout a region. These landscape-scale assets provide resilience to climate change and storm surges, protect drinking water sources and provide the types of large-scale recreational opportunities that often attract people to live in a community.

Integrated together, these different scales of green infrastructure each play a critical role in building livable, sustainable built environments from small towns to big cities.