By Bob Mills (retired) and 
Hallie Dozier

Are you are planning to construct a house or other building on a lot with existing trees? If you are, many native trees usually can be saved and protected for valuable shade trees. Act now, not after your house is built. Be sure to check the following points carefully before construction begins.  

Before Construction

Shade trees not only provide beauty and energy savings, they can add thousands of dollars to the value of residential property as well. Unfortunately, construction damage to trees often doesn't show up until years after the builders have departed, so it  is well worth taking the effort up front to protect your trees. The best way to do this is to hire a consulting arborist to help make certain that you select the best trees for preservation and that the trees you want to save are well protected throughout the construction process.  Someone  who is specially trained in tree preservation during construction will be able to guide you through the process and help you protect your trees from beginning to end.

The first step to take, of course, is to decide which trees to protect and save and which ones to remove. It is much cheaper to remove trees before the building begins, and it may cost less to build with fewer trees. Decide which trees to save by evaluating each one carefully. Evaluate trees over a season or longer, if possible. Note growth patterns, location, color, shape, condition, species, shading, etc.

Location 
 
When selecting candidate trees to preserve during construction, first consider the location of each tree with respect to where you are going to put your house. Some people select and purchase properties for the trees themselves, so if a tree is especially valuable, it is worth taking that tree into consideration when locating the building site. It is difficult to build a house, driveway, walk or patio closer than about 15 feet (20 feet for larger species) from trees without harming or killing them. If you have too many trees, it will be difficult to have a quality lawn or most shrubs because of roots and shading. Ask yourself, if a tree were not growing in this spot, would I plant one here?

Species 
 
Consider the species of each tree on your lot. Some species have special problems. For example, some produce an undesirable fruit or nut. Sweetgum balls can be a problem in a manicured lawn. Other species—elm, maple and willows, for instance—are notorious for invading small cracks in sewer lines and eventually blocking the lines with their roots. Soil pH can be critical to survival of some species. Tight, clay soils may cause shallow, protruding roots and summer drought problems.  

Insects and diseases infest some species more than others. Pines, for example, may be killed by bark beetles. This is especially true when pines are damaged by construction, compaction or stress. Native slash and loblolly pines are susceptible to Cronarium (fusiform) rust and they are a poor choice for salvaging during construction unless the trees came from a genetically improved seed source. Silver maple in Louisiana is prone to disease, as is Lombardy poplar.

Some species, even with careful and expensive protection, will not survive the change in environment after clearing. When a forested area is changed by removing some trees, the light and moisture conditions are modified. Also, epicormic (profuse) sprouting along the exposed trunks and branches may detract from a tree’s appearance. Over time, epicormic sprouts can also become hazards as they grow in size because they tend to be poorly attached to the tree. Check with an arborist, horticulturist, Extension agent or nursery as to what species may be best for your site. In general, a native species that is suited to the site will have fewer problems over the long run.

Size, Age and Vigor

Larger trees that have been growing in a forest environment need more protection and often do not make good yard trees. Trees that grow in a dense forested situation often have tall, straight trunks with small tops. This type of tree will provide little shade and can be more easily broken by high winds than a stout, more-open tree. They are also more likely to develop epicormic branches. Also, because they are so large, their root systems are extensive, and they are much more likely to be damaged during construction—even by activities that take place away from the parts of the tree that are visible above ground.

Old trees don’t adapt as well to changes in environment as young trees of the same species, and their large root systems are much more difficult to protect. Young, newly planted trees grow rapidly in Louisiana. They may grow even faster with proper fertilization, mulch and water. It is often a better choice to preserve younger trees because they can recover from construction better and will develop a more-desirable mature form.

During Construction

Remove unwanted trees before construction begins. Do this carefully so the remaining trees will not be damaged. Unwanted trees and shrubs that are close to trees you wish to retain should be removed by hand.

During construction, protect the remaining trees from:

• Construction equipment and supplies (especially chemicals)
• Soil compaction
• Grade changes—either raising or lowering or excavation for underground utilities

Mechanical Injury 
 
Protecting the root system may be the most important step you take. Heavy equipment will damage tree roots by merely passing over the ground next to your trees—and the damage occurs with the first few passes. Even a careful operator can wound tree branches and trunks, giving you later problems with rot or insects. Construct a sturdy  fence or barrier around the tree to prevent equipment and vehicle injury to  roots, trunk and low-hanging branches. Most tree root systems extend far beyond the drip line of the tree's canopy, so it is critical to enclose as much of the area around the tree as possible. Mulch the area well (4-8 inches) before you fence, and be sure to mulch all areas where there will be foot and equipment traffic (up to 12 inches thick). Keep all foot and vehicular (including wheelbarrow) traffic away from the trees you are preserving.

Grade Changes:
  
Trees depend on the feeder roots in the upper 6 to 12 inches of soil for air, water and minerals. Most feeder roots are in an area under the tree crown in open-grown trees. If you extend perpendicular lines from the widest portions of the tree crown to the ground and connect these points, you’ll enclose an area that includes most of the root system.  

Because most of the roots are in the upper 6 to 12 inches of soil, lowering the grade and consequently removing those roots under a tree crown can be harmful. Generally, protection is achieved by terracing the grade to keep soil disturbance as far away as possible from the trunk. If space is available, the tree may be unharmed if you let it remain on a gently sloping mound. Another way to protect trees from root damage caused by lowering the grade is to build a retaining wall between them and the lower grade. This is an effective way to save trees if the grade difference is less than 2 feet. Don’t forget to leave drain holes in the wall.

On the other hand, if you raise the grade by adding soil, the tree may slowly decline in vigor or die because of difficulty in obtaining essential air, water and nutrients. It’s often possible to add up to 2 inches of a light sandy-loamy soil without killing your trees. A tree well with perforated drainage tiles is necessary to allow drainage if you plan deeper fills. Contact a professional consulting arborist with help with lowering or raising the grade around trees during construction.

Trees with damaged or buried roots often do not die immediately; in fact, they may live several years before declining. Once damage becomes evident, it is probably too late to correct the problem.  

Excavations

Trees can be protected when you excavate for water, gas and sewer lines. Start by considering the location of trenches. If you cannot route the trenches far enough around the trees, the next best thing is to tunnel directly under the tree trunk of species without taproots. Pine and cypress have taproots and may be severely damaged this way. Power-driven soil augers are often used for this purpose.