Some residents and local authorities think trees are too much trouble. They require attention to get started and are vulnerable to vandalism in their early years. Their roots interfere with services and foundations; their branches snag power lines; and their leaves, bark and seeds clog gutters.
Certain species drop death-sized branches from time to time. Natives perform well in drought conditions but block sunlight to houses in winter. In established areas with narrow streets new plantings will sometimes be at the expense of parking spaces.
However the benefits of trees far exceed their cost. Urban designer Dan Burden reckons it costs between $250 and $600 in the US to establish a tree over three years, but the direct benefits over its lifetime exceed $90,000 (excluding aesthetic benefits).
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The NY State Department of Environmental Conservation estimates each mature tree removes around half a tonne of carbon dioxide and about 100 kg of pollutants from the air each year. Trees can lower home and vehicle cooling costs, improve residential amenity, and increase property values.
Trees support bird life and some even claim they lower crime. I suspect the latter is primarily the result of a selection effect but nevertheless there’re lots of good reasons to have more trees in our streets.
I found this extraordinary map of street trees in San Diego County (second exhibit) via Kain Benfield’s blog. What’s remarkable is you can click on each individual tree to get information about its species, location, size and any alerts concerning its welfare (see here).
The inventory was produced by San Diego County Trees, an initiative of the California Center for Sustainable Energy. But all the data on individual trees is crowd-sourced – it comes from residents who care about their trees.
Here’re a couple of similar maps for Washington DC. The DC street trees map shows the location and species of trees planted by the District’s Urban Forest Administration. The Casey trees planting map shows trees planted by a non-profit organisation, which also publishes an annual report card.
It’s hard to think of a more cost-effective way of improving urban streetscapes than planting trees. Yet while almost all established suburbs have at least some trees (e.g. here, here and here), in many cases they don’t seem to be incorporated as a “designed” part of the streetscape.
Existing trees are often small – presumably so they don’t interfere with power lines in older suburbs – and not planted closely enough to create a strong sense of an avenue or a canopy. To my eye, some of the species also look scrappy with thin foliage. Whatever their merits as individual trees, they don’t contribute as effectively to the whole as alternative (native) species would.
There’s plenty of green along the main roads through Melbourne’s sprawling middle ring eastern and south-eastern suburbs, yet they’re often cited as the epitome of boring and featureless suburbia. A key reason, I think, is there’s little sense of visual containment. That could be provided by a “wall” of taller buildings or, more feasibly in the short to medium term, by a “wall” of taller trees.
Undergrounding power lines in established suburbs would be expensive, but it could be done in a limited number of locations – such as along major roads – so taller trees could be planted at closer intervals. The trade-off might be slower traffic speeds.
What I’d like to see more generally, though, is greater attention given to the potential of trees to create streetscapes. I’d like to see a larger role given in decisions on tree selection, location and spacing to those with a designer’s eye for the public realm.