The movement of equipment and the storage of materials on a construction site can compact the soils, resulting in major changes in the soil air and water characteristics. An "ideal" mineral soil is about half solid material (45 percent mineral and 5 percent organic material) and half pore space. Under ideal growing conditions about one-half of the pore space is air and one-half water. When compaction occurs, the solid material near the soil surface (commonly the surface four to eight inches is most affected) is compressed into a more dense mass. This results in possible damage to existing plant roots and fewer and smaller pores in the compacted soil layer. Finer textured soils are commonly affected more than sands; soils without a surface litter layer (leaves, twigs, etc.) are commonly affected more than those with a surface litter layer. Compaction commonly results in water moving into and through the soil more slowly. Surface runoff and erosion are increased. Less water moves into the soil to become available for absorption by tree roots, and soil aeration may be reduced. Plant roots also often have more difficulty growing in and through compacted soil since they grow through pore spaces in the soil.
Because soil compaction is extremely difficult, expensive, and often impractical to correct once it has occurred, the emphasis is on prevention. While it may not be practical to eliminate compaction from a construction site, a number of things can and should be done to minimize its impact on trees. Vehicular traffic over the entire site should be kept to a minimum and routed away from trees. Construction work should be performed with equipment having good flotation and yet able to achieve the objective. Unnecessary traffic, such as workers' personal vehicles, should be prohibited on the site. Movement over the site by delivery vehicles should be restricted as much as possible. A storage area for construction materials should be identified that is well away from trees and located so as to minimize the traffic required to retrieve and use the materials.
The most effective method of minimizing the effects of soil compaction on trees is to establish a fenced tree protection zone around each tree or group of trees within which no construction activity of any kind can occur. Several methods have been used to define the size of the protection zone including using the tree's dripline, height, or diameter as a guide. When the dripline method is used, an area equal to the extent of the tree's dripline is protected for broad-crowned trees like sugar maple or American beech, and an area equal to approximately 1.5 times the dripline is protected for narrow-crowned trees such as tulip tree or cedar. When tree height is used as a guide, the protected area around the tree is a circular area with a radius equal to the tree's height.
While both the dripline and tree height guides can produce adequate protection zones when applied with judgment by a knowledgeable arborist, the use of tree diameter as a guide routinely produces the most dependable results. Table 5 provides the guidelines for using trunk diameter (measured 54 inches above the ground) to establish the size of the tree protection zone currently recommended by the International Society of Arboriculture. The species tolerance to construction impacts is found in the Appendix. Relative tree age, in Table 5, is based on the tree's life expectancy with young being <25% of life expectancy, mature being 25-75% of life expectancy, and overmature being >75% of life expectancy. Distance from the trunk in Table 5 provides the radius of the tree's protection zone for each inch of tree trunk diameter. Applying this tree diameter guide to an 80-year-old (mature) American beech (tolerance to construction impacts poor) 20 inches in diameter would result in a tree protection zone of 20 * 1.25 or 25 feet in radius around the beech.
| Table 5. Tree Preservation Guidelines as Published by the International Society of Arboriculture.2 | ||
|---|---|---|
| Species Tolerance (*) | Distance from Trunk | Tree Age (**) (feet per inch trunk diameter) |
| Good | Young | 0.5 |
| Good | Mature | 0.75 |
| Good | Overmature | 1.0 |
| Fair | Young | 0.75 |
| Fair | Mature | 1.0 |
| Fair | Overmature | 1.25 |
| Poor | Young | 1.0 |
| Poor | Mature | 1.25 |
| Poor | Overmature | 1.5 |
| * Species tolerance to construction is given in the Appendix.
** A young tree is a tree that has used less than 1/4 of its life expectancy. A mature tree is a tree that has used 1/4 to 3/4 of its life expectancy. An overmature tree is a tree that has used more than 3/4 of its life expectancy. | ||
In areas of a construction site where compaction-causing activities cannot be eliminated, mulching with a six-inch-thick layer of wood chips or a four-inch-layer of inert gravel can substantially reduce the compaction. Again, this would be particularly important in plant root zones and areas where plantings are planned. The mulch can be retained or removed after construction is completed. Organic mulches, such as wood chips, are generally more desirable because they are easier to remove, and what remains after cleanup will deteriorate over time while gravel will not. The use of a geotextile under gravel will help simplify its removal. If an organic mulch is used in a tree root zone, the trees may benefit (and certainly won't be hurt) from fertilizing prior to spreading the mulch. Use a rate of one to two pounds of actual nitrogen per 1,000 square feet of soil surface. If large gravel is used as a mulch, be sure it is not limestone or dolomite. These materials are liming agents and could dramatically alter the pH of the soil to the detriment of the trees.