Tree-lined streets essential to urban neighborhoods

Harrisburg Patriot News
Sunday, Aug. 17, 2007

By Thomas Hylton

In recent years, there has been a growing recognition of the value of street trees to enhance pedestrian communities.  Trees not only beautify our streets, they cleanse the air, absorb carbon dioxide, and lower ambient temperatures.

That’s why the state Department of Conservation and Natural Resources has planted 20,000 trees throughout southeastern Pennsylvania in recent years and plans to add 20,000 more in western Pennsylvania as part of its TreeVitalize program.

Twenty years from now, these trees will likely provide 20 times more environmental benefit than they do today – if they survive.

Recently, Camp Hill Borough sent notices to 15 property owners to cut down mature street trees to facilitate sidewalk repairs.  One homeowner, a lawyer, refused to comply, and the ensuing controversy generated an article in the Patriot-News.  (His tree still stands.)

After reading the article, I decided to see for myself whether all this tree removal was necessary.  By the day I visited, many of the “Camp Hill 15” had already been chopped down and their stumps ground into sawdust.  But a few trees remained.  Some had clearly seen better days, but others were perfectly healthy.  I arrived just as a tree surgeon was about to cut down a lovely, adolescent pin oak on South 29th Street.  The adjacent sidewalk had already been replaced, but the homeowner told me the borough feared the sidewalk might be lifted again.  So the tree had to go.  However, I had a chance to measure it while it was still standing. 

The New York City Parks Department has devised a formula, by measuring the square inches of a tree trunk, to calculate how many young trees are needed to equal the environmental benefits of a mature shade tree. 

Two years ago, the city of Harrisburg planted 33 new elm trees on State Street between Front Street and the Capitol.  Using New York City’s formula, I calculated it would take 28 of Harrisburg’s fledgling elms to equal the cooling power of the pin oak I saw being needlessly removed in Camp Hill.

Rather than remove mature trees because they lift concrete sidewalks, perhaps it’s time to rethink the kind of sidewalks we use.

Concrete sidewalks typically consist of 4-inch-thick slabs poured as large blocks over a bed of gravel.  Over time, these slabs become uneven and create lips that cause people to trip.  Trees are one reason – but far from the only reason -- that concrete sidewalks become uneven.  Tree roots can grow under concrete slabs and lift them up or crack them.  But normal freezing and thawing of the ground, and settling caused by subsurface pipes, can also throw concrete slabs out of kilter.

Sidewalks don’t have to consist of rigid concrete.  There are two walking surfaces, time-tested and readily available, that are far superior: brick and asphalt.  Asphalt is nearly always used to construct pedestrian greenways and bike paths.  In fact, Camp Hill has an asphalt pathway for pedestrians and bicyclists running through its Willow Park.  So why not asphalt sidewalks?

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 jack-hammering it out and pouring a new slab at considerable expense.  Because asphalt is flexible, tree roots can grow right through it, at worst causing bulges rather than lips, which can easily be fixed. 

It you don’t like the color, asphalt can be pigmented or painted.  Asphalt sidewalks are common in New England towns, which have much more severe winters than we do.

A more historic walking surface is brick pavers, which date back to colonial times.  Because bricks are laid with open joints, rain water can seep into the ground between them, making it less likely tree roots will rise to the surface in search of moisture.  When tree roots do appear, the average handyman can fix a brick sidewalk by lifting up the affected bricks, pruning the roots with a hatchet, and replacing the bricks.  Although brick sidewalks can become wavy over time, they rarely create a tripping hazard, and they are easy to make plumb again.

Nor is it written in the stars that sidewalks must run in straight lines.  If a tree has outgrown its space, the sidewalk can be detoured around it if space is available.

If we want environmentally friendly communities, we need to preserve our trees.  Trees and sidewalks can safely coexist.  We just need to be flexible in our thinking and our construction.