HOW TO: Protect trees from construction damage

It is not a matter of luck that some trees survive construction damage and continue to live long, useful lives. It's usually a combination of events, types of damage, site conditions and tree species that determine the fate of a tree or forest. When trees are treated as living systems — and not just disjointed collections of roots, stems and leaves — and are selected for preservation because they are worth saving, success rates justify the efforts and costs involved with preservation tactics. But preservation is more than just not cutting a tree down. Let's look at five steps you can take to ensure the trees you want to keep will actually survive.

  1. Selection

    “Save the best, chip the rest.” Trees must be evaluated for their ability to tolerate construction activities before extensive preservation tactics are employed. Tree vigor (of the species) and vitality (the individual tree's relative health) play the most important roles in tolerance and survival potential in most cases.

    Some trees have the genetic potential to tolerate more damage and changes to their system than others. Flood plain trees, such as American elm and green ash, are usually more tolerant of soil compaction, filling of soil over roots and poor drainage as a result of construction activities than upland forest trees. White oaks and sugar maples normally exhibit very little tolerance. They also recover from damaging wounds better than lindens and hackberries, which are more likely to develop extensive decay.

    Healthy trees generally are more tolerant of damage and changes and are much better able to recover after a construction project is completed. In general, younger trees are healthier than older trees, trees with normal root systems are healthier than those with already compromised root systems (for instance, boulevard trees), and those with characteristically dense canopies are healthier than those with abnormally sparse canopies. If a tree is old, full of dead wood and already in an obvious state of stress or decline, don't waste time on it with preservation tactics. Remove it, chip it up and use the wood chips to protect another more valuable tree.

    Tree health can be improved before the project starts, however, if enough planning time has gone into the project. This is when supplemental additions of nitrogen, irrigation, mulching, corrective and dead wood pruning, disease and insect control will improve the tree's vitality and, therefore, its potential for surviving construction damage. Ideally, these treatments should occur at least one full growing season before the construction activities begin.

    The final factor in tree preservation selection relates to the tree's condition and stability. If the tree has a large column of decay in its trunk, significant bark loss, stem girdling roots, greatly reduced live crown ratio or a greatly reduced root system relative to its size, it's more likely to end up as a liability in the landscape. The question isn't whether or not the tree can survive the construction damage, it's whether it deserves to survive.

  2. Predict the damage

    Not all construction damage is severe enough to maim or kill trees. The worst construction damage is damage to the root systems and the soil surrounding the roots. Any activities that remove or kill roots, compact or change the soil chemistry surrounding the roots will have the longest and greatest effects on tree health. When roots are removed or killed, consequences can range from a relatively quick death to several years of defensive dieback of branches and leaders. Trees become more vulnerable to insect attacks and disease pathogens, and less stable in windstorms.

    Trees do not need 100 percent of their root systems to survive. Research has indicated that healthy, younger trees can take root severance on one side as close as 3 to 4 feet from the trunk and regain their health — even though they may lose their stability.

    When the root loss is two-sided, the older and less healthy trees will become unstable and less likely to recover. If the root loss is on three or more sides, mortality rates dramatically increase and stability in windstorms is greatly decreased.

    To make certain that trees are not impacted at all during construction or trenching, protect a “critical root area.” You can calculate this area by measuring the stem diameter of the tree (in inches). For each inch of stem diameter (take the measurement at 4.5 feet above ground), allow 1 to 1.5 feet of radius from the tree trunk as the critical root area. For instance, a tree with a 10-inch stem diameter would have a critical root radius of 10 to 15 feet, measured out from the tree trunk.

    Changes in soil pH, compaction and drainage are insidious forms of construction damage and usually have long-term, highly negative effects on trees. These are all considered predisposing agents that weaken a tree's vitality so much that it becomes more vulnerable to weather extremes, insects and diseases. Compaction of clayey soils and anything else that decreases soil drainage will reduce soil oxygen levels. Without sufficient oxygen, roots cannot grow normally and gradually die off.

    Buried concrete or concrete truck clean-out areas can elevate soil pH to a level that some of the essential minerals are no longer available to the trees. Often, these trees become more chlorotic (leaves are pale green to yellow), suffer more winter dieback and other damage than normal, and become more vulnerable to other problems.

    Wounding of stems and branches is usually the least harmful type of construction damage. If it occurs, trees usually can recover from the wounds if they are kept healthy. In certain instances, wounding can lead to severe decay in decay-prone or weakened trees, or provide an opening for disease pathogens, most notably, oak wilt. If oaks are wounded during the high-risk season for infection (the time of the year when both the fungal spores and the sap beetle vectors are both present, usually spring through mid-summer), those wounds should be covered immediately with a water-based paint or shellac.

  3. Avoidance

    Once the lucky trees have been selected for survival, avoidance is the most cost-effective and survival-effective tactic available. The simplest avoidance technique is to give the trees a wide berth, avoiding the critical root area. In landscapes, the most common type of construction damage is root loss. Some species, such as red maples, bur oaks and many pine species, are more tolerant of root loss and easily recover. Younger and healthier trees are also more tolerant of this damage. However, trees like black walnut, ironwood and white oak are much more sensitive, as are older and less healthy trees. These trees need a relatively undisturbed root system to stay healthy and in good condition.

    If possible, avoid all damaging activities within the critical root area: no trenching, no grade changes, no storing or cleaning of equipment or vehicles, no compaction. Designate parking, clean-out and staging areas that are separate from the critical root area. If possible, redesign sidewalks, driveways and patios to avoid this critical root area. If this isn't possible, instead of hardscapes that require excavations for bases of gravel or stone dust, redesign the patios or sidewalks as elevated boardwalks or hardscapes and bases that are above grade and do not require extensive excavation.

    Rather than dig in a utility trench within this protected area, trench around it. It doesn't matter if the trench is 4 inches wide or 4 feet wide, it still severs all of the roots at that point. Unfortunately, the most important and efficient roots for absorbing water and minerals from the soil are normally located in the top 12 to 36 inches of the soil, so even shallow trenches can be harmful.

    If the utility line absolutely must interrupt the critical root area, tunnel under the critical roots with a boring machine or create the trench with an air excavation tool. Air excavation equipment is becoming commonplace and can be used to create trenches without damaging the root system. After the soil is displaced with this equipment, the utilities can be laid in the trench under the critical root mass and backfilled with little to no damage to the roots.

  4. Protection

    Damaging soil compaction can be avoided if the soil is protected, and the simplest and most effective method of protection is a thick layer of mulch. Depending on the amount of traffic and the weight of the equipment used, 6 to 12 inches of coarse wood chips (or a mineral mulch) is effective at minimizing soil compaction. If you plan on removing the mulch after the construction activities, use a layer of synthetic weed-barrier fabric under the mulch. This will make its removal much easier.

    Brightly colored fencing can be an effective protectant for the critical root area, stems and branches. Protective fencing should be accompanied by signage that explains the purpose of the fencing and why it's important to the success of the project. Additionally, “tail gate” training sessions emphasizing the importance of tree preservation and protective fencing should be a regular event with all contractors and sub-contractors on the site.

    Prior to construction, all branches that could conflict with construction activities should be pruned out correctly. A well-placed pruning cut is much less damaging to a tree than a ripped or broken branch.

  5. Treatment and monitoring

    Treatment of damaged trees should begin when the damage occurs. If a significant portion of the root system is destroyed, then the remaining root system should be pampered. Mulch it to hold soil moisture, moderate temperature extremes and remove competition from turfgrasses and weeds.

Of all treatment tactics, regular irrigation is the most effective. Trees that do not become moisture stressed are much more likely to survive construction activities and recover. Tensiometers (soil moisture content sensors) can be inserted into the soil where the critical roots are located and monitored daily. If soil moisture levels drop below optimal levels, apply irrigation water. The use of tensiometers eliminates the guess work of irrigation schedules and soil-percolation rates. It also eliminates the use of mindless irrigation timers that can end up drowning the preserved trees.

Some defensive dieback of branches will occur on trees that have suffered significant root loss, especially older or more sensitive trees. Expect this to begin 1 to 2 years after construction and continue for a few years. Removal of these dead branches should be an annual event, more for the safety of the landscape users than the health of the tree.

Construction-damaged trees will also be more vulnerable to insect infestations and diseases. In particular, the wood-boring insects are attracted to weakened trees as are root-rotting fungi and pathogenic cankers. Maintaining the vitality of the trees, regular monitoring for these problems and timely controls are critical for successful preservation.

In certain instances, vertical mulching or soil aeration on a regular basis (not a one-shot treatment) can be effective at eventually reducing the impact of soil compaction. If fertilization with nitrogen is warranted, a slow-release form may be applied after a period of recovery. Most of the time, this is beneficial when the construction damage was primarily loss of roots. Therefore, the trees need a period of recovery to produce the fine roots that actually absorb the nitrogen. Usually this takes about 1 to 2 years after the damage, and that is when the nitrogen will be most effective.

Treatments and monitoring for problems is an on-going process and should continue for the life of the trees, literally and figuratively. As anyone who has ever lived in a new, treeless development can attest, a mature shade tree is well worth the effort to preserve it.

Dr. Gary R. Johnson is associate professor of urban and community forestry at the University of Minnesota (St. Paul).

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