Wooded properties can be worth as much as 20 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 nearby trees. Unless the damage is extreme, the trees may not die immediately, but could decline over several years. With this delay in symptom development, clients 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 steps must happen during the planning stage. A knowledgeable arborist can help the client decide which trees can be saved, and can work with the planner to consider design modifications that can help preserve tress. The arborist can also work with the construction manager to protect trees throughout each phase of construction.
How trees are damaged during construction
Construction equipment can injure the above ground portion of a tree by breaking branches, tearing the bark and wounding the trunk. In addition, the digging and trenching that are necessary to construct buildings and install underground utilities will likely sever a portion of the roots of many trees in the area. The amount of damage a tree can suffer from root loss depends, in part, on 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.
Root loss due to digging and trenching can also increase the potential for the tree to fall over. 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 could fall or blow over. Even if the roots are not severed so close as to make a tree unstable, the root loss can still have a devastating effect on the tree’s health.
Most people are surprised to learn that 90 percent of the fine roots that absorb water and minerals are in the upper 6 to 12 inches of soil. 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. Also, the heavy equipment used in construction compacts the soil, which 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.
Arborists should be part of the development team
Arborists are often not called in until long after the construction is completed and the trees are visibly declining or failing. If the opportunity arises, it is best to be involved in the early planning stages. The arborist can meet with the designer/planner and assess the trees on the property to determine which ones are healthy and structurally sound and have the potential for preservation.
The arborist should be an integral member of the development team and work with the designer and construction team in planning the construction. Few developers or builders are aware of the way tree roots grow and what is needed to protect them. Sometimes small changes in the placement or design of the development can make a big difference in whether a critical tree will survive. An alternative plan may be friendlier 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. Trees increase both the value and the aesthetics of the property, so many developers are becoming more open to the idea of modifying plans to preserve trees.
Erecting barriers and limiting access
Because our ability to repair construction damage to trees is limited, it is vital that trees be protected from injury. The single most important action to take once the plans are finalized is to set up construction fences around all of the trees that are to remain on the property. The fences should be placed as far out from the trunks of the trees as possible. As a general guideline, allow 1 foot of space from the trunk for every inch of trunk diameter. The intent is not merely to protect the above ground portion of the trees, but also the root systems. Remember that the root systems extend much farther than the drip line.
The area within the fence is called the tree protection zone (TPZ), and no grading, excavation or construction activity should occur in that zone. Construction personnel should be notified 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.
If possible, it is best to limit access on/off the property to minimize compaction to the soil around trees. Contractors should be instructed where it is permissible to park vehicles. Areas should be designated for cement washout pits, storage and stockpiling.
Maintain good communication
It is important for all those involved in the development project to work together as a team. The best-laid plans between arborist and builder can be destroyed by one uninformed subcontractor. If possible, the consulting arborist should arrange a preconstruction meeting at the site to ensure that all subcontractors understand the importance of the TPZ. During the construction project, the arborist should visit the site as frequently as possible-at least once a week. Vigilance will pay off as workers learn to take the arborist’s recommendations seriously. It is a good idea to take photos at every stage of construction. If any infraction of the specifications does occur, it may be important to link cause and effect.
The trees on a construction site may 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. Arborists should plan for continued maintenance and monitoring of the trees’ health, structure and overall vitality.
Resources for more information
The International Society of Arboriculture has several great references for arborists who are involved in construction projects. “Trees and Development: A Technical Guide to Preservation of Trees During Land Development,” by Matheny and Clark is the most comprehensive text and includes many photos, drawings and tables for guidance. ISA’s “Best management Practices – Managing Trees During Construction” provides a more concise overview of basic tree preservation methods. The industry standard for managing trees during construction is the American National Standards Institute (ANSI) A300 Part 5 – Management. All are available through ISA’s Web store or by calling 1-888-ISATREE.