Forget aesthetics. Tree-lined streets increase property values (and, in turn, tax revenue), help reduce crime rates, and aid in stormwater management. As a matter of fact, says Thomas Brady, town arborist for Brookline, Mass., by covering 30% of your watershed with tree canopy, you can reduce stormwater runoff by 14%. This is because each tree holds up to 240 gallons of water a day - water that's not discharged into a stormwater drainage system.
But trees - and their root systems - are also fragile, susceptible to damage by physical impact or injury, construction activities, soil compaction, improper aboricultural methods, and plain lack of understanding of how a tree functions. Any of these factors can cause a tree to die and/or fall, often years after the damage occurred.
Here's a few tips to avoid damaging your street trees while performing sidewalk or road maintenance:
Don't remove structural roots. These large roots keep the tree standing and provide water, nutrients, and energy to the tree. Most people are familiar with these thick roots because you can see some growing from the base of the tree.
Any root larger than a 2-inch diameter is a significant structural tree root and should be left intact whenever possible. When smaller-diameter roots must be cut, make a clean cut. Whereas a torn root will rot and decay, a clean cut will regenerate new roots.
If damage does occur to structural roots, notify your municipal arborist or other authority. If left unchecked, the tree will fall and can cause property damage, injury, and even death.
Do not substantially cut or raise the grade in the root zone of a tree. The critical root zone of a tree extends beyond the branch tips (drip line) - sometimes extending 2.5 times the diameter of the drip line - and the majority of nutrient-absorbing roots exist in the upper 6 to 12 inches of soil.
Cutting the grade in the critical root zone reduces water and nutrient intake, eliminates stored energy, and may compromise the stability of the tree. It also eliminates the tree's nearly invisible fibrous roots. Adding topsoil or dirt to the grade, on the other hand, will smother roots.
Consider removing trees that have sustained critical root zone loss of 30% or more.
Reduce soil compaction. A tree's fibrous root system consists of very fine, hair-like rootlets that absorb water. The tree needs this root system to survive, and the root system needs pore space, which is filled with air and water, in soil to thrive. Soil compaction from heavy equipment traffic and materials storage reduces the amount of pore space, which inhibits root growth.
To minimize compaction, spread a 12-inch layer of wood chips over the soil within the critical root zone. Compaction can also be partially alleviated by drilling a series of 2-inch-diameter holes to a depth of 12 to 18 inches.
Avoid creating wounds on the branches, trunk, or roots. This can happen during excavation, trenching, and other construction-related activities, as well as through improper pruning. Avoid trenching inside the drip line by going around the tree than under it. Or, use tunneling rather than trenching inside the drip line.
Collisions and scrapes from construction equipment, snowpolows, and other vehicles can also damage a tree. Although a healthy tree is capable of sealing off small wounds to localize injury, larger wounds will decay and can serve as entry points for disease.
To protect the street trees in your jurisdiction, consider implementing a tree protection and management plan. Require certified arborists to review tree protection plans, and work with local planning boards to ensure tree protection is included in each filing.
American Public Works Association 2010 Congress
"Challenges and Issues Which Face Us in the Management of Our Urban Forest"
Thomas D. Brady, MCA
Tree Warden/Town Arborist
Town of Brookline, Mass.
Tues., Aug. 17, 2010
3:45 - 5 p.m.