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Urban Canopies

February 24, 2014

Last week, Metro Connection, DC’s NPR affiliate’s excellent show that weaves regional stories together, had a piece on our urban forest, prompted by a symposium that Casey Trees (a great local organization whose mission is to increase our urban tree canopy) sponsored recently. Metro Connection spoke with Richard Olsen, a plant geneticist at the National Arboretum (a local treasure, if you’ve never been there).

Olsen discussed some factors that urban foresters need to consider when deciding what trees to plant. In the past, we often relied on monocultures of trees, based in large part on what we thought was attractive. American elms are a prime example here, and were the cause of a hard lesson learned when Dutch elm disease wiped out large parts of city canopies in the US. These days, we’re also contending with pests such as the emerald ash borer, which is hitting ash trees hard.

At the same time, you need species that are able to survive — and even thrive — with the unique stressors facing them in an urban environment. Olsen used the example of the salt and other substances we lay down on roads every time it snows (or even when snow is forecasted — we’ve certainly all seen salt laid down only to have a storm turn out to be a bust). Unless rains washes the salt away relatively quickly, it’s going to end up seeping into the ground and will ultimately hit trees’ roots. 

Aesthetics also play a role — we want trees that look good, according to our sensibilities. We also don’t want trees that bear lots of fruit (that then needs to be cleaned up when it falls on the ground), or trees that are extra hard for those with allergies to cope with.

At the same time, he points out, we have a new concern for how trees play into the larger urban environment — will they support urban biodiversity? Olsen describes this as a changing aesthetic, which is an interesting way of thinking about it: “We’ve gone from purely ornament, a design feature in the landscape, an aesthetic, to this new aesthetic, which is wouldn’t it be great and wonderful if it supported all sorts of wildlife.”

However, there are barriers to a diverse urban canopy — for example, as Olsen explains, “you can only plant what’s available in the nursery industry…[and] the nursery industry is only going to produce what they can sell.” So perhaps one question we need to ask is: can we encourage nurseries to grow a more diverse set of species? This applies not only to trees, but also other plants — and also begs the question of whether or not we can encourage nurseries to stop selling invasive ornamental species.

This reminds me of a discussion we had in the class I’m teaching this semester last week, although in class we weren’t specifically talking about nurseries and plants.  Can you create consumer demand for environmentally-low impact products (in this case, plants and trees) through the products that a business chooses to sell? Or do you need to create consumer demand to get businesses to sell those trees and plants in the first place? In other words, can a top-down approach (business influencing consumers) work? Or is a bottom-up approach (consumers influencing business) more realistic? In a world where time and resources are limited, which approach is more efficient?

American Elm in Jamaica Plain, MA. Courtesy of the OSU Special Collections and Archives via WikiMedia Commons

American Elm in Jamaica Plain, MA. Courtesy of the OSU Special Collections and Archives via WikiMedia Commons

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