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.
