The older and larger the tree, the more environmental benefits it provides. One 50-year-old canopy tree, for example, can bestow more cooling power than 100 freshly planted saplings. Big street trees are especially important in high-density areas with few green spaces.
Street trees can be killed by insects, disease, or errant cars, but by far the most common cause of premature demise is lifted sidewalks. Everyone's an environmentalist until his sidewalk buckles. Then, too often, the response is to remove the tree - just as it's providing its maximum ecological benefit.
Rather than remove mature trees, perhaps it's time to rethink sidewalks. Concrete sidewalks typically consist of 4-inch-thick slabs poured in large blocks over a bed of gravel. Over time, these slabs can become uneven, creating lips that cause people to trip. Tree roots growing under the concrete slabs can lift them up and crack them, but they're certainly not the only culprit. The normal freezing and thawing of the ground, as well as settling caused by underground pipes, can also throw concrete slabs out of kilter.
Joys of asphalt
But sidewalks don't have to be made of rigid concrete. In fact, one time-tested and readily available material is far superior: asphalt.
Because asphalt is poured in one continuous ribbon, there are no slabs to become uneven. When a concrete slab is lifted, the only solution is to jackhammer it out and pour a new slab, at considerable expense. Asphalt is flexible, though, so tree roots can grow right through it, at worst causing bulges rather than lips. And cracked or lifted sections of asphalt can be easily cut out and replaced, unlike concrete.
Asphalt is commonly used for streets, parking lots, and walking paths throughout the region. Large portions of the Schuylkill River Trail, from Philadelphia to Valley Forge and beyond, are made of asphalt. Walkways at colleges and universities, from Bryn Mawr to Princeton, are also made of asphalt. And parks throughout the region have asphalt paths.
So why not sidewalks? Asphalt sidewalks are common throughout Europe. They can also be found in a wide swath of New England towns, from Providence, R.I., to Freeport, Maine, which have much more severe winters than we do.
If you don't like the color, asphalt can be pigmented or painted. New York City has colored miles of asphalt bike paths, and portions of streets that have been closed to vehicles, with a pigmented epoxy that comes in all colors.
In Pottstown, where I live, the nonprofit Trees Inc. raised nearly half a million dollars in the early 1980s to plant more than 1,500 street trees. Thirty years later, the trees have beautifully transformed the appearance of the downtown area and surrounding neighborhoods. Some now soar over two-story houses and shade entire blocks.
But they have also lifted sidewalks. And we've found that the easiest, most cost-effective way to create safe walking surfaces without removing trees is to replace concrete with asphalt.
Two years ago, as a demonstration project, Trees Inc. replaced severely lifted concrete sidewalks around 15 street trees with asphalt. The wells in which the trees grow were also expanded as much as possible, so few roots had to be pruned. The newly poured asphalt was then coated with a permanent epoxy solution pigmented to blend in with the concrete. Today, the asphalt remains in pristine condition and is almost indistinguishable from the concrete.
Meanwhile, the Pottstown Borough Council passed an ordinance allowing asphalt sidewalks to be used in areas where trees are planted. Replacing lifted concrete with asphalt makes sense, but it makes even more sense to avoid concrete sidewalks in the first place.
If we want environmentally friendly communities, we need to ensure the trees we plant last for their entire natural life spans. And mature trees and sidewalks can safely coexist. We just need to be flexible in our construction and our thinking.
Thomas Hylton is the author of "Save Our Land, Save Our Towns" and a cofounder of Trees Inc. He can be reached at firstname.lastname@example.org.