As cities and suburbs expand, wooded lands are being developed into commercial and residential sites. Homes are constructed in the midst of trees to take advantage of the aesthetic and environmental value of the wooded lots. Wooded properties can be worth as much as twenty percent more than those without trees, and people value the opportunity to live among trees.

Unfortunately, the processes involved with construction can be deadly to the nearby trees. Further, unless the damage is extreme, the trees may not die immediately, but could decline over several years. With this delay in symptom development, you may not associate the loss of the tree with the construction.

It is possible to preserve trees on building sites if the right measures are taken. The most important step is to hire a professional arborist during the planning stage. An arborist can help you decide which trees can be saved, and can work with the builder to protect the trees throughout each construction phase.


How trees are damaged during construction

Physical injury to the trunk and crown

Construction equipment can injure the above-ground portion of a tree by breaking branches, tearing the bark and wounding the trunk. These injuries are permanent, and if extensive, can be fatal.

Cutting of roots

The digging and trenching that are necessary to construct a house and install underground utilities will likely sever a portion of the roots of many trees in the area. It is easy to appreciate the potential for damage if you understand where roots grow. The roots of a mature tree extend far from the trunk of the tree. In fact, roots typically will be found growing a distance of 1-3 times the height of the tree. The amount of damage a tree can suffer from root loss depends, in part, upon how close to the tree the cut is made. Severing one major root can cause the loss of 5 to 20 percent of the root system.
The roots of a tree will extend far from the trunk and will be found mostly in the upper 6 to 10 inches of soil.

Another problem that may result from root loss due to digging and trenching is that the potential for the trees to fall over is increased. The roots play a critical role in anchoring a tree. If the major support roots are cut on one side of a tree, the tree may fall or blow over.
 
Less damage is done to tree roots if utilities are tunneled under a tree rather than across the roots.

Soil compaction

An ideal soil for root growth and development is about fifty percent pore space. These pores, the spaces between soil particles, are filled with water and air. The heavy equipment used in construction compacts the soil, and can dramatically reduce the amount of pore space. This not only inhibits root growth and penetration, but also decreases oxygen in the soil that is essential to the growth and function of the roots.

Smothering roots by adding soil

Most people are surprised to learn that 90 percent of the fine roots that absorb water and minerals are in the upper 6-12 inches of soil. Roots require space, air and water. Roots will grow best where these requirements are met, which is usually very near the soil surface. Piling soil over the root system or increasing the grade will smother the roots. It only takes a few inches of added soil to kill a sensitive, mature tree.

Exposure to the elements

Trees in a forest situation grow as a community, protecting each other from the elements. The trees grow tall, with long, straight trunks and high canopies. Removal of neighboring trees, or opening the shared canopies of trees will expose the remaining trees to sunlight and wind. The higher levels of sunlight may cause sunscald on the trunks and branches. Also, the remaining trees will be more prone to breaking from wind or ice loading.

Getting advice

Hire a professional arborist in the early planning stage. Many of the trees on your property may be saved if the proper steps are taken. Allow the arborist to meet with you and your building contractor. Your arborist can assess the trees on your property, determine which are healthy and structurally sound, and suggest measures to preserve and protect them.
 
One of the first decisions is determining which trees are to be preserved, and which should be removed. You must consider the species, size and maturity, location and the condition of each tree. The largest, most mature trees are not always the best choices to preserve. Younger, more vigorous trees can usually survive and adapt to the stresses of construction better. Try to maintain diversity of species and ages. Your arborist can advise you about which trees are more sensitive to compaction, grade changes and root damage.

Planning

Your arborist and builder should work together in planning the construction. The builder may need to be educated regarding the value of the trees on your property and the importance of saving them. Few builders are aware of the way trees‚ roots grow, and what is needed to protect them.

Sometimes small changes in the placement or design of your house can make a great difference in whether a critical tree will survive. An alternative plan may be more friendly to the root system. For example, bridging over the roots may substitute for a conventional walkway. Or, instead of trenching beside a tree for utility installation, tunneling under the root system is much less damaging.

Erecting barriers

Because our ability to repair construction damage to trees is limited, it is vital that the trees be protected from injury. The single most important action you can take is to set up construction fences around all of the trees that are to remain. The fences should be placed as far out from the trunks of the trees as possible. As a general guideline, allow one foot of space from the trunk for each inch of trunk diameter. The intent is not merely to protect the above-ground portions of the trees, but also the root systems. Remember that the root systems extend much farther than the driplines of the trees.

Instruct the construction personnel to keep the fenced area clear of building materials, waste, and excess soil. No digging, trenching or other soil disturbance should be allowed in the fenced area.
 

Protective fences should be erected as far out from the trunks as possible in order to protect the root systems.

Limited access

If at all possible it is best to allow only one access route on and off the property. All contractors must be instructed where they are permitted to drive and park their vehicles. Often this same access drive will later serve as the route for utility wires, water lines or the driveway.

Specify storage areas for equipment, soil and construction materials. Limit areas for burning (if permitted), cement wash-out pits and construction work zones. These areas should be away from protected trees.

Specifications

Get it in writing. All of the measures intended to protect your trees must be written into the construction specifications. The written specifications should detail exactly what can and cannot be done to and around the trees. Each sub-contractor has to be made aware of the barriers, limitations and specified work zones. It is a good idea to post signs as a reminder.

Fines and penalties for violations should be built into the specifications. Not too surprisingly, sub-contractors are much more likely to adhere to the tree preservation clauses if their profit is at stake. The severity of the fines should be proportional to the potential damage to the trees, and should increase for multiple infractions.

Maintaining good communications

It is important to work together as a team. You may share clear objectives with your arborist and your builder, but one sub-contractor can destroy your prudent efforts. Construction damage to trees is often irreversible.

Visit the site at least once a day if possible. Your vigilance will pay off as workers learn to take your wishes seriously. Take photos at every stage of construction. If any infraction of the specifications does occur, it will be important to prove liability.

Final stages

It is not unusual to go to great lengths to preserve trees during construction, only to have them injured during landscaping. Installing irrigation systems and rototilling planting beds are two ways the root systems of trees can be damaged. Remember also that small increases in grade, as little as 2-6", which place additional soil over the roots can be devastating to your trees. Careful planning and communicating with landscape designers and contractors is just as important as avoiding tree damage during construction.

Post-construction tree maintenance

Your trees will require several years to adjust to the injury and environmental changes that occur during construction. Stressed trees are more prone to health problems such as disease and insect infestations. Talk to your arborist about continued maintenance for your trees. Continue to monitor your trees, and have them periodically evaluated for declining health or safety hazards.

Despite the best intentions and most stringent tree preservation measures, your trees may still be injured from the construction process. There are remedial treatments that your arborist can suggest to help reduce stress and improve the growing conditions around your trees. In addition, the International Society of Arboriculture offers a companion to this brochure titled, „Treatment of Trees Damaged by Construction

This brochure is one in a series published by the International Society of Arboriculture as part of its Consumer Information Program. You may have additional interest in the following titles currently in the series: Insect and Disease Problems; Mature Tree Care; New Tree Planting; Trees and Turf; Benefits of Trees; Tree Selection; Plant Health Care; Avoiding Tree and Utility Conflicts; Recognizing Tree Hazards; Why Hire an Arborist; Buying High-Quality Trees; Tree Values; Pruning Young Trees; Pruning Mature Trees; Why Topping Hurts Trees; Pruning Young Trees; Pruning Mature Trees; Avoiding Tree Damage During Construction; Treatment of Trees Damaged by Construction.
 

Developed by the International Society of Arboriculture (ISA), a non-profit organization supporting tree care research around the world and dedicated to the care and preservation of shade and ornamental trees. For further information, contact: ISA, P.O. Box 3129, Champaign, IL 61826-3129, USA.

© 1998 International Society of Arboriculture.
UPDATED FEBRUARY 2000