What is an Urban Forest?

Nature doesn't stop at the boundaries of a city.  A web of living things weaves in and out of every human environment we have created; through city blocks, along railroad tracks, within farm fields and backyards, crisscrossing the boundaries of private and public property. Trees are the largest organisms living within the urban environment and create the biostructure below which all other layers of the urban forest fall. Some trees are planted intentionally while 'volunteers' spring up along fencelines, in abandoned lots and other places where seed get deposited by birds,animals, wind and water.

Part of Washington, DC's urban forest as viewed from the National Cathedral

Part of Washington, DC's urban forest as viewed from the National Cathedral

To understand why the urban forest is important, it's critical to understand the concept of Ecosystem Services.  Ecosystem services are benefits that humans receive from the surrounding environment. These benefits can be categorized into four types of services: supporting (such as formation of soil), regulating (filtering of water and air), provisioning (food or timber), and cultural (such as recreation). Trees are powerhouses of ecosystem services, improving our quality of life every day through functional processes.

  • Trees pull carbon from the air and produce oxygen, improving local air quality and reducing carbon in the global atmosphere.
  • Trees reduce the impact of rainfall in a community by actively absorbing water and by slowing runoff as it clings to the surface area of each leaf, stem, branch and trunk.
  • Rainwater clings to the leaves, branches, stems and bark of trees, slowing demand on infrastructure.
  • Trees improve the quality of our drinking water by filtering pollutants and material, reducing the need for pre-treatment of municipal supplies.
  • Trees reduce heating and cooling costs by shading homes in summer and blocking wind in winter.
  • Trees reduce heat island effect, a phenomenon which can raise local temperatures when the summer sun heats concrete and asphalt.
  • Trees increase property values: the American dream is lined with leafy, sun-dappled neighborhoods.
Tree plantings are an integral part of the streetscape in downtown Shirlington, Virginia

Tree plantings are an integral part of the streetscape in downtown Shirlington, Virginia

  • Trees reduce crime by introducing a neighborhood watch effect: people spend more time outside, so there is more visibility and sense of ownership of one's community.
  • Trees promote biodiversity: birds, mammals, insects, fungi and microbes form a distinct web that increases quality of life for humans, from harvesting aspirin from willow bark to housing birds and bats that eat mosquitoes.
  • Trees provide food security by offering habitat to pollinators of crops.
  • Trees feed us: the harvesting of edible fruits, nuts and berries for consumption is called permaculture.
A grove of established trees in a neighborhood park in Rock Island, Illinois

A grove of established trees in a neighborhood park in Rock Island, Illinois

'Trees are the answer' is a phrase I borrowed from a Virginia Tech professor to indicate the myriad ways that our communities benefit from a healthy urban forest. Trees are the single biggest investment cities can make in themselves.  But it has to be a real commitment to investment, willing to provide the space and resources trees need to thrive and produce reliable benefits we can enjoy.

The urban forest grows powerful through economies of scale.  Yes, a tree in front of a home can create a shady spot, hold some rainwater that clings to its surfaces, uptake some water and remove some carbon from the air. But the impact is massive when one thousand, ten thousand trees or more grow together.  If you look down from above onto a city that is 20% covered by tree canopy, that is 1/5 of the city that is reducing demand on infrastructure while enriching the lives of its residents at the same time. A strong canopy is money in the bank for a city and the taxpayers who fund it in terms of utility management, property values and human health.

Future posts will point out challenges to maintaining a healthy urban forest by discussing the limitations of trees. Limitations include failing to thrive under stress or resource deprivation, and being unable to break up with us when we treat them poorly or move when they don't like their neighbors.

Summertime at the National Arboretum in Washington, DC

Summertime at the National Arboretum in Washington, DC