Preserving the natural condition of the site is important. In Green Building projects, the site is considered first and evaluated for all characteristics. The site has a crucial role in future performance of the building and enjoyment of occupants. Preserving native vegetation can greatly reduce water and pesticide use, and large existing trees add to property value. A recent study determined that Austin's urban forest contributes $133,600,000 annually in real benefits. These benefits affect air quality, stormwater run-off mitigation, and direct energy savings from trees.

The protection of trees and shrubs can provide wind and solar protection for on-site buildings. Using indigenous vegetation also reduces the amount of energy and water needed to grow and transport materials. By maintaining habitat for native species a crucial environmental balance can be preserved.

Keeping natural grades undisturbed can minimize erosion and water pollution. There are several ways to minimize the adverse environmental impacts related to site work.

Before design begins, thorough ecological and anthropological surveys should be conducted of existing site conditions. Then develop plans for protection or remediation of any sensitive areas. These issues are best addressed in design before construction is considered.

General climatic data (insolation, temperature, humidity and wind patterns) should be analyzed in conjunction with specific site elements (i.e. topography, vegetation, water conditions on-site, existing built forms, natural drainage patterns) in the selection of building location, orientation, form, envelope construction, and size and location of apertures.

Consideration should be given to mapping the individual elements discussed above on overlays to get a composite view of their effects for making design decisions. Consider hiring a professional arborist or tree specialist for the surveys.

If the property is a large tract of land, and measures have been taken to promote wildlife habitat, the property may be eligible for a Texas tax exemption (1-d-1W valuation).

Site Clearing

For a sustainable project, valuable existing vegetation, particularly understory species, should be identified and preserved when clearing and grubbing. Small woody and herbaceous plants should not be removed indiscriminately without first identifying any potentially valuable species.

Felled large trees should be stripped of branches. Branches and leaves should be chipped or pulped by a machine capable of reducing it all to mulch for use on site. This material makes excellent mulch for trees on construction sites. It also reduces soil compaction when used adequately.

Marketable (large diameter) trees that must be removed should be offered for bid to saw mills. Resources in this regard include Texas Kiln Products and regional mesquite mills. Other trunks can be cut into firewood or planks. If there is space on site, stumps should be piled in the open to form wildlife cover.

Tree Protection

Existing trees should be viewed as valuable natural and aesthetic resources that are not readily renewable. They should be valued as natural shading devices, which can reduce a building's energy costs and reduces the heat-sink impact on paved areas. Existing trees that are to remain on site need to be protected during construction. Wooden slats attached to tree trunks will not provide adequate protection. Much of the damage that occurs to trees during the construction process is due to soil compaction from heavy equipment and materials being driven or stored under trees. Roots, which extend out far beyond the trunk, are damaged when soil becomes compacted. This will cause trees to go into a gradual decline and eventually die. Trees should be protected from damage during construction with clearly visible fencing located below the outermost branches and flags in overhanging branches.

Site Protection, Fig. 1

Locate fences in line with the tree's outermost branches.

Filling or cutting in tree root areas will also kill trees; therefore, tree wells or decks should be used to work around tree trunks, roots, and slopes. Any activity in the root zone of a tree has potential to cause problems. Fill soils and soil compaction must be avoided. Severing of roots (grade cuts) is the most serious injury. Be sure to cleanly cut any exposed roots in trenches; this promotes regrowth.

When trees are of a significant size or value, an arborists' or nurseryman's assessment of the values should be made. The tree should be clearly labeled with this value during construction. The nominal unit price per tree can be recorded in Division 1 section of specifications. Anyone on the site damaging the labeled tree should be expected to compensate the assessed amount to the owner.

Guidelines for Drawings

Existing trees, 8" caliper and larger, should be incorporated into the project as much as feasible. Future growth of the trees should be allowed for in the design of structures and utility lines.

Trees to remain should be clearly labeled and designated on the drawings; they should be clearly distinguishable from the trees to be removed.

Tree protection fencing should be shown as in Figure 1 if applicable.

Tree roots extend much further than the dripline of the tree. If possible, the design should avoid trenching in tree root zone areas. The term "critical root zone" (CRZ) is used in the Land Development Code and is defined as a radius of 1 foot for every inch of diameter of trunk size as measured at 4.5 feet.

Keep in mind that symmetry of the zone is less important than the total area. If a building on one side of a tree encroaches into the CRZ, make up the difference in the area by extending tree protection further on the side to encompass the same amount of area. If it is necessary to install buried utility lines in a tree root area, tunneling is the preferred method.

If an area around a tree must be paved, pervious materials are recommended. If impervious materials must be used, large holes, installed at regularly spaced intervals within the tree root zone, are recommended.

Tree Pruning

Pruning existing trees will assist their health. Guidelines issued by the International Society of Arboriculture should be followed for proper tree pruning and Oak Wilt protection measures. Pruning, while a valuable tool for tree health, is frequently overemphasized and over done. Pruning trees under stress of construction can be detrimental, not beneficial.

Earth Work

Topsoil is a valuable living ecosystem that should not become mixed with subsoil or other materials. If topsoil is stripped during construction, it should be stockpiled for reuse on the site. Bare topsoil should never be allowed to wash off the site. Topsoil should be piled carefully, clear of all pollutants. The foot of the pile should be diked to prevent erosion and creepage. Slopes should be covered with mulch or seeded with annual grasses if a growth season is likely to occur during the life of the stockpile.

Recycled concrete should be considered for use as backfill material. In addition, recycled concrete is an acceptable substitute for quarried stone aggregate in nearly all site applications. It can be used as subgrade or base course for roads, embankments, and retaining structures.

Site Grading

Extreme slope modification should be avoided in order to reduce soil erosion and increase infiltration. When modifying drainage, preference should be given to sheet flow rather than concentrated flow. Provide swales and temporary on-site areas, so that rainwater and roof run-off can be absorbed on-site.

PLATEAU Integrated Land and Wildlife Management
Christina Allday-Bondy
P.O. Box 1251
Dripping Springs, TX 78620
(512) 894-3479
Tododia@swnell.net

Urban Forestry Resources
Dennis Brown
9800 Westward Dr.
Austin, TX 78733-3144
(512) 263-2798
Quercus@texas.net

International Society of Arboriculture
P. O. Box 3129
Champaign, IL 61826-3129
(217) 355-9411
www.ag.uiuc.edu

References:

Urban Ecological Analysis for Austin, Baltimore, Milwaukee, July 1996.

American Forest Report prepared for the National Urban And Community Forestry Council.