Mature trees provide many benefits to development sites. They enhance the aesthetic character of the area, give scale to the new buildings, provide shade, give a look of maturity to the landscape, and provide habitat for wildlife. it may be possible to retain trees with minimum forethought. However, preserving specimens that will survive and perform well in the new environment requires thorough planning, careful implementation, and adequate maintenance. Effective tree protection is a long process. First, evaluate the trees to determine which specimens are suitable for preservation. Next, work with the planners and engineers to design improvements so that suitable trees are preserved. Then, monitor construction around the trees to see that the trees are not injured. Finally, routinely evaluate the trees to identify maintenance needs.

Identifying Trees to be Protected Deciding which trees to preserve, and designing development around them often seems like a 'chicken or the egg' problem. The trees suitable for preservation must be identified before designing around them. On the other hand, the design strongly influences the selection of trees to be preserved. One way out of this dilemma involves a preliminary evaluation of the trees to determine those that are suitable for preservation. Compare that information with conceptual site plans. Identify the trees suitable for saving, and modify the site plan to accommodate the desirable trees. Finally, work out the details of grading and preservation for trees that will remain (Fig. 14). In most cases the costs to preserve trees are significant. Costs accrue because the land dedicated for tree preservation is unavailable for building, structures, and techniques to minimize damage to trees require extra design and construction attention, and affected trees must be maintained to support long-term health. These costs will be borne by the public, whether through increased taxes to pay for trees in public rights-of-way, higher homeowner association dues, greater commercial rents, higher home prices or direct payment for maintenance. Trees should be selected carefully, keeping their value and contribution to the new environment in mind. In determining which trees are to be preserved, four factors must be considered:

—	Suitability of the species to the new land use;
—	Tree health and structural stability;
—	Species tolerance to changes in environment; and
—	Level of maintenance that will be provided following impact.

Species Suitability - The suitability of the species to the location and use of the site are important factors in determining whether to save a tree or not. For example, species that normally grow along streams, such as willow and alder, would perform poorly in a parking lot. The tree should survive in the new landscape for a considerable length of time to warrant the cost and effort of protection. An oak that can be expected to survive for many decades has a greater value than a mature acacia that may only live for a few years. Tree Health - Young, vigorous, healthy trees are the best candidates for protection, because they grow new tissue quickly and adapt readily to new environments. However, it is large, old trees that are most often the focus of preservation. Of course, it is possible to preserve old trees as long as they are healthy, but younger ones may give the best return on investment. Vigorous trees usually have full canopies and healthy leaves. Three conditions indicate poor tree health. First, the leaves are small and pale for the species. Second, some of the branches are dead. Finally, most of the foliage arises from short twigs along the major limbs, known as epicormic growth. Trees with large cavities or other structural weaknesses are not good candidates for preservation, unless the problems can be alleviated by pruning, cabling or bracing. Tolerance To Changes In Environment - The ability of the tree to tolerate injury and changes in the environment is another important factor to consider. Trees that regenerate roots quickly and have adaptations to control water loss seem to be better able to tolerate construction impacts. For example, coast live oak (Quercus agrifolia), which has a stiff, cupped leaf with a thick cuticle, tolerates greater root injury than California black walnut (Juglans hindsii), which has a smooth, thin leaf. On the other hand, willows, which also have smooth, thin leaves, typically adapt well to new environments, because they have the ability to regenerate new roots quickly. Environmental changes can cause tree decline even when the tree is not directly injured. Significant changes in water table levels or rechannelization of streams or runoff can seriously weaken or kill trees. Road fills placed over stream beds can raise water levels upstream. It is important to consider large scale alterations in the overall ecology of the area, as well as the specific changes that will occur next to trees. Level Of Maintenance That Will Be Provided - Trees stressed by construction require considerable maintenance to prevent loss of vigor and attack by harmful insects and diseases. Irrigation, fertilization, pest and disease control, and pruning are all aspects of this maintenance. For example, Monterey pine trees (Pinus radiata) stressed by root injury may require regular irrigation and spraying for bark beetles or other insects. If those treatments cannot be provided, it may be better to remove the trees rather than trying to preserve them. Responsibilities for maintenance of affected trees should be established early in the planning phase. If the trees will not be maintained, it is important to minimize stresses to the trees that would affect tree health. Preparing a Tree Location Map A tree location map is critical to successful preservation. Use a topographic map showing vegetation lines as a base. Early in the design process locate the trunks approximately. Later, the engineer's survey can pinpoint the locations and show the base elevation of the trunks. The engineer should project to the center line of the tree when plotting locations. An indication of the dripline for each tree would also be helpful. On 20-scale or larger maps, plot the circumference of the trunk for larger trees. Be sure that tree locations show on all building site plans. Number the trees on the map so that all discussions can address specific trees. It is helpful to have a corresponding numbered tag attached to the tree, as well. Assessing Potential Impacts to Trees Evaluate the development plans around the trees to estimate the potential impact. This requires skill in reading plans and an understanding of construction procedures. The following plans and specifications should be reviewed:

—	Topography and tree survey plan;
—	Grading plan and specifications;
—	Plot or development plan;
—	Utilities placement and depth;
—	Drainage plan; and
—	Geotechnical survey.

The plans will provide information on the depth of cuts, fills, utilities, subdrains, and other excavations, as well as their distance from trees. The geotechnical survey describes the soil profile. It also specifies the compaction and over-excavation required under structures, pavement, and fills. To better visualize the changes that will occur next to trees, draw cross-sectional views showing existing and future grades, and all improvements (Fig. 15). Based on the geotechnical requirements, the amount of soil work could also be estimated and recorded on the drawing.

Soil Alterations During Construction Soil favorable for root growth is a mixture of mineral particles, organic matter, air and water. About half of the volume of the soil is pore space containing varying amounts of water and air. To soil engineers pore spaces are known as voids. Although an asset to plant growth, voids reduce a soil's value as structural material. To stabilize the soil, engineers compact it to remove as many of the voids as possible. Compacting soils for construction involves removing the soil to a specified depth, mixing the soil with water, and replacing the soil in thin lifts (usually six to eight inches thick). Heavy equipment, designed to maximize the compaction, drives repeatedly over each lift. This activity hurts tree roots by first cutting them, and then by breaking down the soil structure. Location of Tree Roots To protect tree roots from damage, it is important to know where to look for them. Although root systems are often depicted as mirror images of the tops, they usually cover a much larger area. Roots can extend far beyond the dripline, as much as two to three times the diameter of the crown. The major portion of the absorbing roots system of a mature tree is within the top three feet of soil, and most of the fine roots active in water and nutrient absorption are in the top twelve inches (Fig. 16). Many trees form vertical sinker roots that arise from larger horizontal roots near the trunk of the tree. Sinker roots aid in water and nutrient absorption from deeper layers of aerated soil during times of drought. Root patterns also are affected by topography and characteristics of the soil or substrate. Trees on slopes tend to have more roots on the downhill side. Roots of trees along streams will parallel the bank. Developing a site is seldom possible without hurting tree roots to some extent. Even preliminary grading, stripping the site of debris and organic laden topsoil, can cause significant root damage. It is commonly thought that a healthy tree tolerates removal of one-third of its roots. Most guidelines for tree preservation advise holding construction and grading outside of the dripline. However, based on a typical root structure, even that restriction could lead to removal of over half the tree's roots. As land values rise, there is more pressure to encroach within the dripline to gain usable space.

Several variables affect a tree's ability to tolerate encroachment, including the health, species, root structure and environmental factors. The type of construction that will occur and how it will be executed are equally important factors. For example, construction of a concrete sidewalk on natural grade requires about six inches of excavation. This would cause less injury than an asphalt road requiring at least twelve inches of excavation. A road with water and sewer improvements along the curb-usually in trenches four to six feet deep- must be held farther away from trees than one with no improvements.

If work must be done close to trees, extra care can minimize the damage. For example, lowering the grade two feet at a distance fifteen feet from the trunk of a tree that forms sinker roots may be possible. But it would require that the excavation be done by hand and the roots cut cleanly with a saw, rather than with equipment that rips and shatters roots. Evaluating the potential of trees to survive and adapt to the new environment must be made on a tree-by-tree basis. The evaluation involves estimating the percentage of root loss, the potential of the tree to tolerate immediate water stress, and the ability of the tree to produce new roots.

Figure 16

Tree Preservation Techniques Major construction impacts and techniques to minimize them are described in Table 1. Following is a discussion of the major points to consider in design and construction. Preserving Trees in Groups -Whenever possible, preserve trees in groups so that relatively large areas can be set aside for native vegetation. These areas serve as green belts or open space. In forest situations the amount of thinning the trees will tolerate depends on the size, age and species present. Mixed forest stands, which contain conifers and hardwoods of different ages and sizes, should be preserved in large blocks (at least 60 feet wide).

Figure 17.

On the other hand, single oaks growing in the open can often be preserved effectively. If only a small amount of land can be dedicated to tree preservation, it may be better to allot a larger area around the few best specimens than to attempt saving a greater number of trees that would sustain heavy damage.

Figure 18.

Site Preparation and Clearing Specifications - Clearly identify the trees to be preserved on all plans. Flag and fence them in the field. When next to trees that will remain, trees to be removed should be cut rather than pushed over with equipment. This avoids possible root damage to remaining trees. Specify no stripping of top soil or grubbing of understory in tree preservation zones. Locate storage for equipment and materials, access roads to the site, and traffic patterns within the site well away from trees to avoid unnecessary root injury and soil compaction. These areas should be identified on the site plans (Fig. 18). Protecting Trees from Fill - Few trees can tolerate having fill soil placed over their trunks. Furthermore, the soil work involved in placing a fill causes root damage and creates an environment unfavorable for new root development (see Table 1).

The following treatments can minimize the adverse effects of fill:

—	Hold fill away from the tree with a retaining wall designed with a discontinuous footing (Fig. 19 & 20).
—	Unless the fill will support a structure, ask the soil engineer to specify the minimum soil compaction (usually about 85 percent).
—	If fill will be imported, provide specifications for the chemical characteristics of the soil, so that trees will not be harmed by toxic conditions, such as high salts. Engineering specifications usually cover only the physical properties of the soil
—	For fill covering large portions of the roots, consider installing an aeration system on natural grade prior to placing the fill. If the potential for soil subsidence is a problem, air vents can be installed through the fill into natural grade after construction is complete (Fig. 17).

Lowering the Grade - Obviously, root damage occurs when soil is stripped away to lower the grade. Here are a few ways to minimize the damage that occurs:

—	Keep cuts as far from trees as possible by installing retaining walls. If the cut is greater than three feet, a continuous footing can be used because few roots are encountered below that depth. For shallow cuts, use discontinuous footings to minimize root injury (Fig. 21 & 22).
—	Excavate by hand at the cut face, cutting the exposed roots cleanly with a saw. Once the trench has been dug to the depth of the finish grade a backhoe can be used to pull away the soil. The backhoe should sit outside the dripline and remove the soil to finish grade.

Figure 21.		Figure 22.

Paving - Paving inflicts more extensive root damage than might be expected. Because it is often considered a structure, pavement requires soil compaction similar to buildings. Excavation must be deep enough to accommodate the compacted sub grade, the base material, and the pavement itself. To minimize damage caused by paving:

—	Maintain an area of several feet around the base of the trunk free of all pavement. Mulch the soil surface.
—	Use paving materials that allow water to penetrate, such as inter locking bricks on sand, where possible over the root zone of trees.
—	Avoid excavation into the root zone by placing base material and pavement on natural grade, making the level of the pavement higher than the tree flair (Fig. 23). Make sure water will not collect in the well surrounding the tree.
—	Install aeration vents in impervious pavement (Fig. 24).

Figure 23.		Figure 24.

Utilities and Street Improvements - Underground utilities are often overlooked as a possible cause of root injury. Water and sewer main lines are usually placed just inside the curb and gutter during road construction. Depth requirements vary, but four to six feet deep is typical. Contractors connect buildings to utility lines and take the shortest and straightest path, unless otherwise instructed. These connections are usually placed in trenches three feet deep. Consider the following methods to minimize damage from installation of utilities:

—	Place notices on tree protection fences informing workers to prevent encroachment by routing all trenches around trees.
—	Trench by hand when digging close to trees, so the woody roots can be bridged, or tunnel under tree roots.
—	Curve trenches for street improvements towards the middle of the road or move them to the other side of the street.

Post-Construction Site Re-Assessment Once construction is complete and the site clean, the tree care professional begins to direct maintenance. The remaining trees were chosen by others in the development process, and the reasons for selecting them may not be known. If protection measures were applied, they may be hidden. So, before post-construction maintenance can begin, the tree care professional must re-assess the condition of the site. Begin by determining the condition of the trees and sites. Safety, value to the property, and the chances of remaining vigorous are all factors with considering. The maintenance prescription should compensate fore construction damage, including: disruptions to drainage, restricting surfaces, impacts of fill, and interference with the trees. Trees stressed by construction must be carefully maintained to avoid loss of vigor or attack by harmful insects and disease. They require special attention to irrigation, fertilization, pruning, and pest control. In extreme cases, improper preservation techniques or severe construction damage could prompt heavy pruning—or even removal of whole tree. To determine the need for and the practicality of restoring a site severely changed by construction, the tree manager should draw on information from building plans as well as the designers and contractors involved in construction Damage or modification to the above-ground portion of the trees is easy to detect. Impacts to the roots and soil are more common, but generally difficult to uncover. These impacts are too often overlooked. Be sure to consider all factors affecting tree health, before developing a maintenance plan.