Preserving Natural Vegetation
Definition
Minimizing exposed soils and consequent erosion by clearing only where construction will occur.
Where Practice Applies
Natural vegetation should be preserved whenever possible, but especially on steep slopes, near perennial and intermittent watercourses or swales, and on building sites in wooded areas.
Advantages
Preserving natural vegetation will:
- Save money on site stabilization
- Help reduce soil erosion.
- Save money on landscaping costs.
- Provide areas for wildlife.
- Provide buffers and screens against noise. Preserving natural vegetation also moderates temperature changes and provides shade and cover habitat for surface waters and land. Increases in stream water temperature tend to lower the dissolved oxygen available for aquatic life.
Disadvantages/Problems
Saving individual trees can be difficult, and older trees may become a safety hazard.
Planning Considerations
New development often takes place on tracts of forested land. Building sites are often selected because of the presence of mature trees. Unless sufficient care is taken and planning done, however, much of this resource is likely to be destroyed in the interval between buying the property and completing construction. It takes 20 to 30 years for newly planted trees to provide the benefits for which we value trees so highly. Natural vegetation can be preserved in natural clumps or as individual trees, shrubs and vines.
Selection
Examine the area to identify trees to be saved: trees with unique or unusual form, trees which may be uncommon in the area, desirable shade trees and trees for screening purposes. Look for healthy trees with full green crowns. The length of the annual twig growth gives an indication of the general vigor of the tree. Trees with broken tops or with many dead branches are usually not good risks. Badly scarred trees are also unsuitable.
In selecting trees to be retained, care must also be used to make certain they will not interfere with the installation and maintenance of utilities such as electric and telephone lines, water and sewer lines and driveways. Preserving individual plants is more difficult because equipment is generally used to remove unwanted vegetation. Points to consider when attempting to save individual plants are:
- Value: Is the plant worth saving? Consider the location, species, size, age, vigor, and the work involved. Local governments may also have ordinances to save natural vegetation and trees.
- Desirability: Is the tree or shrub a desirable plant? Is it shallow-rooted, do the roots seek water, or are insects and disease a problem? Shallow- rooted plants can cause problems in the establishment of lawns or ornamental plants. Water-seeking roots can block sewer and tile lines. Insects and diseases can make the plant undesirable. This is especially true with aphids on alder and maple.
- Age and size: Old or large plants do not generally adapt to changes in environment as readily as young plants of the same species. Usually, it is best to leave trees which are less than 40 years of age. Some hard- woods mature at approximately 50 years of age. After maturity they rapidly decline in vigor. Conifers, after 40 years of age, may become a safety hazard due to the possibility of breakage or blowdown, especially where construction has left only a few scattered trees in an area that was formerly dense woods. While old large trees are some- times desirable, the problem of later removal should be considered. Local governments, however, may have requirements to preserve older, larger specimen trees.
Tree Preservation
Clearly flag or mark areas around trees that are to be saved. It is preferable to keep ground disturbance away from the trees at least as far out as the dripline.
Barriers
If possible, place a barrier around the trees. Bulldozers are notorious for damaging trees. Besides skinning bark from tree trunks, their tracks severely damage tree roots which are close to the surface. Place a simple wooden fence around the tree. Inexpensive or scrap lumber will do. Snow fencing, although more expensive, is easy to install. The fence should enclose an area at least five feet out from the tree trunk. Erect the fence before the bulldozer arrives and leave it up until the last piece of equipment has left the area.
Marking
If erecting a barrier around the trees is impractical, marking the trees may help save them from damage, if equipment operators are forewarned and reliable. A band of bright colored cloth, ribbon, or tape may be used to identify trees to be protected. The band should be placed around the trunk high enough to be seen from a distance and from all angles. It is important that all people involved be informed of the meaning of the marking.
Grade Changes
Filling
Tree roots need air water and minerals to survive. Few trees can survive with more than six inches of earth fill over the roots. The tree roots are literally suffocated with earth fill. The coarser the fill material, the better the chance for survival. Construction of a dry well around the tree trunk will provide some air circulation for the trees. Installation of a drain system in conjunction with the dry well is even better. Four-inch drain pipe is placed in a spoke-like fashion to drain water away from the tree before filling takes place. The dry well may be built of stones, brick, tile, concrete blocks or other material. It should be at least 12 to 18 inches away from the trunk of a large, slow-growing tree and up to 36 inches for younger fast-growing trees.
Lowering
Lowering the natural ground level can seriously damage trees and shrubs. Most of the plant roots are in the upper 12 inches of the soil and cuts of only 2-3 inches can cause serious injury. To protect the roots it may be necessary to terrace the immediate area around the plants to be saved. If roots are exposed, construction of retaining walls may be needed to keep the soil in place. Plants can also be preserved by leaving them on an undisturbed, gently sloping mound. To increase the chances for survival, it is best to limit grade changes and other soil disturbances to areas outside the dripline of the plant.
Excavations
Protect trees and other plants when excavating for tile, water, and sewer lines. Where possible, the trenches should be routed around trees and large shrubs. When this is not possible, it is best to tunnel under them. This can be done with hand tools or with power augers. If it is not possible to route the trench around plants to be saved, then the following should be observed:
- Cut as few roots as possible. When you have to cut - cut clean. Paint cut root ends with a wood dressing like asphalt base paint.
- Backfill the trench as soon as possible.
- Tunnel beneath root systems as close to the center of the main trunk as possible to preserve most of the important feeder roots.
Common Trouble Points
Some problems that can be encountered with trees are:
- Maple, Dogwood, Eastern hemlock, Eastern red cedar and Douglas fir do not readily adjust to changes in environment. Special care should be taken to protect these trees.
- Maples, and willows have water-seeking roots. These can cause trouble in sewer lines and filter fields. On the other hand, they thrive in high moisture conditions that other trees would succumb to.
- Thinning operations can cause serious disease problems. Disease can become established through damaged limbs, trunks, roots, and freshly cut stumps. Diseased and weakened trees are also susceptible to insect attack.
Maintenance
Inspect flagged areas regularly to make sure flagging has not been removed. If tree roots have been exposed or injured, re-cover and/or seal them.
References
Washington State Department of Ecology, Stormwater Management Manual for the Puget Sound Basin, Olympia, WA, February, 1992.
U.S. Department of Agriculture, Soil Conservation Service, Amherst, MA, Guidelines for Soil and Water Conservation in Urbanizing Areas of Massachusetts, October, 1977.