When is an urban forest large enough?

Forested Canopy in Shenandoah National Park

In the 2007 Comprehensive Plan, the City of Charlottesville set for itself the ambitious goal of creating a 40% tree canopy. At least it seemed ambitious at the time. After performing an analysis of aerial photos this year, city officials realized that 46% of the area is actually already covered by trees. In response, a new Urban Forest Management Plan has been drafted that calls for yet more trees. City Council has reviewed the plan, and everyone who spoke wholeheartedly endorsed making this a reality.

The plan recommends several methods for increasing the scale of natural forest land within city limits, including protecting forested parkland and/or forested private lawns in perpetuity with conservation easements, increasing the tree planting requirements for all new development, and acquiring more public land to be converted into green space. To clarify, we're talking specifically about the 10 square miles within the limits of the City of Charlottesville.

I should stop here and make it clear that I love trees. I've spent two summers living in a national park, getting out into the mountains and forests as often as the opportunity allowed. Deforestation has been a major global problem for many years now, contributing to the rising levels of GhG in the earth's atmosphere due to loss in carbon sequestration potential. Street trees are a wonderful accoutrement to the urban fabric, enhancing the aesthetic value of streets, soaking up particulate matter, and providing energy-efficient shade. Trees are great.

From 2009 aerial-photo study (Enlarged Picture)

But ... and it's pretty obvious where this is going ... there are important differences between urban areas, rural areas, and wilderness areas. They each have their own ecological and economic function, form, and aesthetic properties. As enticing as it is may be to have the best of both worlds, with wilderness/rural/urban all mixed together across the landscape, this strategy of suburbanization has resulted in a number of unintended consequences throughout the 20th century. What was idealized as a rainbow of land types coexisting often turned out, in reality, to become a beige mush of none of the above. And acres of parking lots as a bonus.

Charlottesville happens to be an urban area, at least that's how it's conceived of by many of its residents. The current map of the urban tree canopy is pretty clearly an inverse of the population density map for the city, with the downtown the least treed and the more low-density residential districts showing more trees. Makes sense. More trees appear to mean less people. More forest would seem to mean less city.

So what's the right amount of trees for an urban area? There is a spectrum of options. Brooklyn, New York, recognized by many for it's tree-lined streets, has a 12% canopy coverage. So does Frederick, Maryland. On the other coast, The City of Vancouver has about 20% coverage throughout its jurisdiction. Portland, Oregon clocks in at 26%, mostly on account of its forested hillside parks west of downtown. As the Charlottesville plan notes, American Forests, a national forest advocacy group, recommends for towns and cities east of the Mississippi a 50% canopy coverage for suburban residential areas, 25% coverage for urban residential, and 15% coverage for the downtown.

This seems to me like a reasonable ideal to set. The more suburban a city would like to become, the closer to 50% tree coverage the better. The more urban a city would like to become, the closer to 15-25% tree coverage the better. But perpetual growth of the tree canopy with no limits would not seem to be a prudent way to plan for a city's future.