The principal advantage of preserving natural vegetation is protecting desirable trees, vines, bushes, and grasses from damage during project development. Vegetation provides erosion control, stormwater detention, biofiltration, and aesthetic values to a site during and after construction activities. Other benefits of preserving natural areas are because natural vegetation
- Can process higher quantities of stormwater runoff than newly seeded areas
- Does not require time to establish
- Has a higher filtering capacity than newly planted vegetation because above ground and root structures are typically denser and using living root systems helps to hold soil in place
- Reduces stormwater runoff by intercepting rainfall, protecting soil surface from the impact of raindrops, holding soil particles in place, maintaining the soil's capacity to absorb water, promoting infiltration, and lowering the water table through transpiration
- Provides buffers and screens against noise and visual disturbance
- Provides a fully developed habitat for wildlife
- Usually requires less maintenance (e.g., irrigation, fertilizer) than planting new vegetation
- Enhances aesthetics
Preserving natural vegetation is applicable to all construction sites where vegetation exists in the predevelopment condition. The practice can be especially beneficial for floodplains, wetlands, stream banks, steep slopes, and other areas where erosion controls would be difficult to establish, install, or maintain. Clear only the land needed for building activities and vehicle traffic.
Siting and Design Considerations
Designers should be aware of and respond to local the climate and other conditions, including project scheduling, that may influence the use of natural vegetative stabilization measures. Before clearing activities begin, clearly mark the vegetation that is to be preserved. Prepare a site map with the locations of trees and boundaries of environmentally sensitive areas and buffer zones to be preserved. Plan the location of roads, buildings, and other structures to avoid these areas. This requires careful site management to minimize the impact of construction activities on existing vegetation. Protect large trees near construction zones because damage during construction activities could result in reduced vigor or death after construction has ceased. Extend and mark the boundaries around contiguous natural areas and tree drip lines to protect the root zone from damage. Obviously, direct contact by equipment damages trees and other vegetation, but compaction, filling, or excavating land too closely to the vegetation also can cause severe damage.
When selecting trees for preservation, consider the following factors:
- Tree vigor. Preserve healthy trees that will be less susceptible to damage, disease, and insects. Indicators of poor vigor include dead tips of branches, stunted leaf growth, sparse foliage, and pale foliage color. Hollow, rotten, split, cracked, or leaning trees also have a lesser chance of survival.
- Tree age. Choose older trees because they are more aesthetically pleasing as long as they are healthy.
- Tree species. Preserve species that are well-suited to present and future site conditions. Keeping a mixture of evergreens and hardwoods can help to conserve energy--specifically, keeping evergreens on the northern side of the site to protect against cold winter winds and keeping deciduous trees on the southern side to provide shade in the summer and sunshine in the winter.
- Wildlife benefits. Choose trees that are preferred by wildlife for food, cover, and nesting.
Other considerations include following natural contours and maintaining preconstruction drainage patterns. Changing the hydrology might kill preserved vegetation because their environmental requirements are no longer met.
The following are basic considerations for preserving natural vegetation:
- Do not nail boards to trees during building operations.
- Do not cut tree roots inside the tree drip line.
- Use barriers to prevent equipment from approaching protected areas.
- Keep equipment, construction materials, topsoil, and fill dirt outside the limit of preserved areas.
- If a tree or shrub that is marked for preservation is damaged, remove and replace it with a tree of the same or similar species with a 2-inch or larger caliper width from balled and burlaped nursery stock when construction activity is complete.
- During final site cleanup, remove barriers from around preserved areas and trees.
Preserving vegetation is limited by the extent of existing vegetation in preconstruction conditions. It requires planning to preserve and maintain the existing vegetation. It is also limited by the size of the site relative to the size of structures to be built. High land prices might prohibit preservation of natural areas. Additionally, equipment must have enough room to maneuver; in some cases, preserved vegetation might block equipment traffic and can constrict the area available for construction activities. Finally, if grading is not done properly, it could result in changes in environmental conditions that kill vegetation. Consider the hydrology of natural or preserved areas when planning the site.
Even if workers take precautions, some damage to protected areas might occur. If this happens, repair or replace damaged vegetation immediately to maintain the integrity of the natural system. When planning for new vegetation, choose kinds that enhance the existing vegetation. Ensure that new structures do not harm protected areas. If fertilization is needed, use the following practices to minimize adverse water quality affects:
- Apply fertilizers at the minimum rate and to the minimum area needed.
- Work the fertilizer deeply into the soil to reduce exposure of nutrients to stormwater runoff.
- Apply fertilizer at lower application rates with a higher application frequency.
- Limit hydroseeding, which is simultaneously applying lime and fertilizers.
- Ensure that erosion and sediment controls are in place to prevent fertilizers and sediments from being transported offsite.
Natural vegetation (existing trees, vines, brushes, and grasses) can provide water quality benefits by intercepting rainfall, filtering stormwater runoff, and preventing sediments and other pollutants from leaving the site.
Preserving natural vegetation could require additional labor to maneuver around trees or protected areas.
Smolen, M.D., D.W. Miller, L.C. Wyall, J. Lichthardt, and A.L. Lanier. 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission; North Carolina Department of Environment, Health, and Natural Resources; and Division of Land Resources, Land Quality Section, Raleigh, NC.
USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Industrial Activities: Developing Pollution Prevention Plans and Best Management Practices. U.S. Environmental Protection Agency, Office of Water, Washington, DC.