By definition, an urban forest is all the trees and associated plants that make up green cover in an urban or urbanizing area. This includes street trees, park trees, private yards, business parks, along stream courses, etc. The definition might even include trees in parking lots and on green roofs if these are managed properly. I like to think of a region’s forest as a continuum that starts deep in the recesses of our national forests and parks and extends through working farmland, urbanizing counties, the suburbs, and into downtown streets and parks.
The species of trees and plant associations certainly change along that continuum. Some would argue that plant associations in urban areas are not natural and thus the tree populations there should not be considered forests in the true sense. I would point out that plant associations in industrial forests are not natural either, being monocultures that are managed in a variety of ways (including chemically) to limit species diversity. I certainly consider a pine plantation a forest, but certainly not natural. Further, lack of species diversity is another characteristic it possesses. You might actually find a lot more species diversity in the urban forest.
Rural forests act as functioning ecosystems, but this is also true of urban forests. Within the last two decades, considerable scientific validation of the contribution urban trees make to environmental quality has been generated. Urban trees improve air quality, diminish storm water flows, mitigate urban heat islands, provide aesthetic appeal, help attract customers to retail business districts, assist with traffic calming, have positive impact on community psychological health, and attract wildlife.
It is helpful to think of urban forests as infrastructure. We’re all used to thinking of various layers of community infrastructure - roads and streets (transportation), phone and cable TV lines (communication), sewer and storm water drainage systems, the electric grid. These systems can be defined as gray infrastructure. Urban forestry proponents are working to develop the idea of urban forests as green infrastructure, an infrastructure that provides services as significant as those of gray infrastructure. In fact, this green infrastructure may actually provide some of the same services as gray infrastructure. This is true with regard to storm water management. Trees as green infrastructure are also becoming more important in terms of energy conservation and mitigating local climate change (urban heat islands).
Those of you used to thinking in terms of forests being associated with rural landscapes might want to consider the impact of urban trees. Urban trees are where the people are! Consider if you will, an expansive oak tree deep in the national forest. This tree is certainly valuable and no doubt provides all of us with many indirect benefits. Lets put that same tree in Central Park or on the National Mall in Washington. Think of how many people that tree benefits directly. Just try taking down a favorite tree on a city street or in a city park. You can bet people will notice (and it does make a sound!). This is precisely why urban forests are important - they are where the people are!