Saving Trees During Construction, continued Many builders use a bulldozer to clear unwanted trees and brush away from trees that they intend to preserve. This is a terrible idea. Even if the operator is skilled enough to skim off unwanted vegetation without cutting roots or changing the grade, the weight of the machine will harm trees by compacting the soil. Clearing unwanted trees by hand may be slower and more expensive, but it's far less damaging (Figure 6).

Figure 6. The area in the foreground has been leveled for a new lawn, but the trees beyond have been left untouched and will survive. If necessary, the area can be thinned later by hand, minimizing the risk of mechanical damage and soil compaction.

Grade changes. Lowering the grade around a tree is another surefire way to destroy its root system. This often results in a virtually rootless tree on a little mound, looking something like the lone palm tree on a cartoon desert island (Figure 7). If the grade change is unavoidable, it's better to cut down the tree immediately, rather than leaving it to be toppled by the first strong wind.

Figure 7. Lowering the grade around this oak has left it with practically no root system. The thin and discolored foliage shows that it's already dying. The inhabitants of the blue house at right might want to sleep in the basement on windy nights.

Raising the grade by piling soil around the base of a tree destroys its roots by depriving them of oxygen. A good guide is to maintain the root flare at the bottom of the tree. Some professionals claim you can add 2 or 3 inches of well-draining topsoil to an older, well-established tree and get away with it, but our experience suggests that it's best not to add any soil. If there's no way to avoid substantially raising the grade around a tree, it's sometimes possible to save the tree by enclosing it in a tree well. Unless this is done correctly, though, it's a waste of time and effort. Simply building a stone wall around the trunk won't do it, because the rest of the root zone will be deeply buried and suffocated. A properly constructed tree well must include a network of vertical bell tiles that act as snorkels to supply air to the entire root system (Figure 8). Because this typically costs several thousand dollars per tree, it's seldom a practical option. You're better off working with the existing contour of the land if at all possible.

Figure 8. If there's no way to avoid raising the grade around a tree, it might be saved by constructing a root-aerating tree well around it. This is an expensive option and one that is best left to an experienced arborist.

Paving and drainage. Paving over a tree's roots is another common cause of root suffocation. If a drive or walkway must be located in the root zone, consider surfacing it with brick, gravel, or crushed stone instead of asphalt. Another option is to use "porous pavers" — paving blocks with ornamental cutouts that allow for the passage of air and water. Finally, make sure that storm runoff won't pool around a tree, or you risk drowning it. And make sure that no one washes down equipment near desirable trees. Trees are sensitive to chemicals, and concrete residue washed from a truck affects the pH of the soil. Petroleum washed from equipment also hurts, as does the calcium chloride that is often used to control dust. Like other salts, it draws water from plants and seals the soil's surface, smothering roots.

Following Through

Critical decisions are made during the design phase, but follow-through makes or breaks a project. Tree preservation is an unusual concern for many workers, and you can be sure that some will think the extra care required is a bunch of baloney. Meet workers as they arrive on the site, explain the ground rules, and keep an eye out to ensure compliance. If you'll be working with an arborist, he or she can oversee field implementation, but the most effective policy is to have the arborist advise a fully invested site supervisor.