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).
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
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
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.
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
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.
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.
Paul Fisetteis director of the Building Materials and
Wood Technology Program, andDennis Ryanis director of the Arboriculture and
Urban Forestry Program, both at the University of Massachusetts