Vegetation Retention
Vegetation and Tree Retention
- When starting with a wooded site, preserve as much of the natural vegetation and undisturbed land as possible. The vegetation and soils in the preserved areas will help capture and control rainfall. Water will infiltrate instead of running off.
- Establish specific targets for retaining mature trees and native vegetation at the pre-development stage. Keep these targets in mind when designing and laying out the structures to be built on your site. When you are ready to begin construction, flag the target vegetation and work with your contractor, subcontractors, and/or engineers to ensure your goals are met.
- Impacts to vegetation, especially certain tree species, may not show up until several years after the development has been completed. Some of the activities that can result in tree damage include changes to the natural drainage patterns on the site, reduction of water tables, and damage to tree roots from soil compaction, paving, and other activities on the property. To prevent this impact, consider the indirect effects of construction and post-construction activities to trees during the design and planning phase of the development. Establish a tree protection zone around key trees.
- Natural vegetation along a stream, pond, or lake is particularly important because it provides shade that keeps water temperatures cool (necessary for aquatic life) and helps reduce water pollution by filtering out sediment, chemicals and nutrients from storm water runoff. While it is always a good idea to leave a wide, naturally vegetated buffer along these waters, some streams, ponds, and lakes have regulated buffer widths under shoreline or critical area ordinances. Check with Whatcom County Planning and Development Services, 676-6907, to see if the waterbodies on your property have a required buffer width.
Establish a Tree Protection Zone
- Work with an arborist or other person knowledgeable on tree health to complete a tree survey to determine which trees are suitable for retention.
- Based on the outcome of the survey, establish a tree protection zone for the trees you are retaining. Factors such as the species’ tolerance to development impacts and the tree’s age should be considered when establishing a tree protection zone.
| Methods for Establishing a Tree Protection Zone | |
|---|---|
| Dripline Method | Protect the area within the dripline for broad-canopied trees, or up to 1 ½ times the dripline for narrow-canopied trees |
| Tree Height Method | Protect a circular area with the radius equal to the height of the tree. |
| Trunk Diameter Method | For every inch of trunk diameter at 4 ½ feet above grade, allow 1 to 1 ½ feet of spaces from the trunk. |
| (Trees and Development: A Technical Guide to Preservation of Trees During Land Development, 1998) | |
- Once your tree protection zone is established, erect a fence that encloses the root area of the tree or group of trees you are retaining.
- Within the tree protection zone, restrict construction activity including material storage, dumping of excess materials, and fires.
- Adjust paving sections, paving materials, and finish grades to minimize root interference.
- Conduct root pruning before excavating near trees.
- Protect the soil from surface compaction by equipment.
- Avoid moving equipment across root areas. When unavoidable, apply a surface mulch to the area to help distribute the weight and minimize soil compaction: 6 inches of wood chips or 4 inches of 3/4 inch crushed gravel placed over the soil surface significantly reduces compaction at depths up to 4 inches.
| Relative Tolerance of Selected Species to Development Impacts |
|||
|---|---|---|---|
| Common Name | Scientific Name | Relative Tolerance | Comments |
| Western red cedar |
Thuja plicata |
Poor-good |
Relative windfirm. Intolerant of changes in water table/soil moisture. Intolerant of fill. |
| Western hemlock |
Tsuga heterophylla |
Poor-moderate |
Prone to windthrow and decay Intolerant of grade change. Poor compartmentalization. |
| Dougals-fir | Pseudotsuga menziesii | Poor-good |
Tolerant of fill soil if limited to one- quarter of root zone. However, may decline slowly following addition of fill. Tolerates root pruning. Intoler- ant of poor drainage. Susceptible to bark beetles following injury. |
| Vine maple |
Acer circinatum |
Good |
Best retained as clumps. |
| Bigleaf maple |
Acrer macrophyllum | Poor-good | Select specimens with good crown structure. Tolerant of root pruning and injury but not of fill. Declines following addition of fill. |
Vegetation Retention References
Storm Water Management Manual for Western Washington, BMP Reference (2005):
Available at: http://www.ecy.wa.gov/programs/wq/stormwater/index.html
- Preserving Natural Vegetation – BMP T5.20, Volume V, Runoff Treatment BMP’s
- Preserving Natural Vegetation – BMP C101, Volume II, Construction Stormwater Pollution Prevention
- Buffer Zones – BMP C102, Volume II, Construction Stormwater Pollution Prevention
Low Impact Development Technics for Puget Sound, Puget Sound Action Team
Available at: http://www.psat.wa.gov/Programs/LID.htm
-
Native Vegetation Protection – 4.1 Vegetation Protection, Reforestation, & Mgmt
Whatcom County Development Standards (County Code)
Available at: http://www.co.whatcom.wa.us/
- Water Resources Special Mgmt Area (Land Clearing) – WCC 20.80.735
City of Bellingham Development Standards (Municipal Code)
Available at: http://www.cob.org/web/bmcode.nsf/
- Seasonal Restrictions on Clearing and Earthwork – BMC 16.80.120
