Trees in a forest grow as a community, protecting each other from the elements. The trees grow tall, with long, straight trunks and high arching canopies. Removal of neighboring trees, or opening the shared canopies of trees, exposes the remaining trees to sunlight and wind. The higher levels of sunlight may cause sun- scald on the trunks and branches of trees with thin bark. Most forest trees have long, thin trunks, with little taper. If neighboring trees that helped provide protection for many years are removed, the remaining trees will be more prone to breaking from wind and uprooting in heavy rains.
Roots of trees growing in wooded areas spread much greater than the height of the tree. Fine absorbing roots are concentrated in the upper twelve inches of soil. These roots can easily be damaged or killed by construction equipment. When tree roots are removed or damaged due to construction activity, the trees may show decline symptoms within a few months, or the symptoms may not appear for a few years. Symptoms may include small or yellow leaves, premature fall color, extensive water-sprout development on the trunk and main limbs, dead twigs, and eventually; major branches may die.
To fully understand the effects of construction on trees requires comprehension of the biological needs and growth processes of trees, both as individual specimens and in groups or forests. Every aspect of tree health, growth, structure, and development can be affected. The impacts of construction are cumulative and can send a tree into a spiral of decline that might not even be recognized by the untrained eye until it is far too late. This fact underscores the need to involve a professional arborist early in the planning.
Typical construction damage to trees normally involve the following items:
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, even if they are compartmentalized. If the injuries are extensive and the tree is stressed, it may never recover.
Construction impacts to trees can be devastating. Unless damage is extreme – trees may not die immediately, but decline over the next several years. With this type of delayed effect most homeowners never associate tree death and decline with construction activities. Construction equipment operating near trees can cause branch and trunk injuries that can cause serious wounds that lead to long-term decay. Although, the most serious injury to trees is not always immediately visible – and is located underground!
Cutting of Roots – The digging and trenching necessary to construct a building and install underground utilities will likely sever a portion of the roots of many trees in the area. If the grade is lowered near a tree, a large percentage of the root system may be removed. With an understanding of where roots grow, it is easy to appreciate the potential for damage. The roots of a mature tree can extend far from the trunk of the tree. In fact, roots typically will be found growing a distance of one to three times the spread of the branches.
The roots of these trees have been exposed, just how shallow they are. Knowing this helps us understand why compaction or grade changes can be so root destructive.
Even the construction of a path close to showing trees can sever a large percentage of the trees roots.
The Live Oak on the right suffered significant root lass from construction. Efforts were made to preserve the tree, and arborists have removed some of the dead branches that resulted from construction. The root area has been aerated, and the health of the tree is being monitored.
Significant loss of root mass can lead to total tree failure in weather events. This occurs frequently in this situation as trees no longer have the structural roots that help keep them anchored. If a large tree such as this were to fail in an environment with numerous cars and people – it could be a catastrophic event.
Design professionals need to take these “risk factors” into consideration as they make decisions on preserving large mature trees.
Grade Changes – If the grade is raised, the roots may be suffocated. As little as 4 inches of soil placed over the root system can kill some species. Increases in grade may require major steps to protect the tree. Sometimes aeration systems are installed to preserve trees, and designs of complex aeration systems and tree wells are described in various texts. There is little research, however, to confirm the value of these systems. Very large tree wells may keep soil fill far from tree trunks and minimize the percentage of the root system covered. Small-diameter wells built around the trunks of trees are rarely adequate to provide enough oxygen to keep trees alive.
Some specifications call for use of gravel or stone below the fill dirt to increase water and oxygen penetration. However, if soil is placed over gravel, water will not drain out of the soil layer until the saturation point is reached. If fill soil is placed in direct contact with the original grade soil, water penetration will be improved and conditions will be more favorable for root penetration and development in the added fill soil.
Tree wells can be very expensive and involve many details – and do not guarantee success. Realistic considerations must be given to tree size, species, drainage patterns, soil conditions, depth of fill, and future maintenance – before wells are designed and constructed. Trees should be thoroughly evaluated for current health and construction impact to determine expectation of survival.
Grade Changes – If the grade must be lowered completely around a tree, a tree island can be constructed. Obviously; the greater the percentage of root system that remains at the original grade, the greater the chance of tree survival. The techniques used for creating a tree island are similar to those used in terracing.
Avoiding Root Damage During Construction
Erecting Barriers – The single most important action that can be taken at the start of the construction process 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 1 foot of diameter from the trunk for each inch of trunk diameter. The intent is not merely to protect the aboveground portions of the trees but also the root systems. Remember that root systems can extend much farther out than the branches of trees. Instruct construction personnel to keep the fenced area clear of building materials, waste, and excess soil. No digging, trenching, compaction, or other soil disturbance should be allowed in the fenced area.
Homes are often constructed on wooded sites and lots can be valued at thousands of dollars more than sites without trees. However, unless protective measures are put in place before lot clearing begins – this investment will be lost.
The installation of utilities in this trench, followed by site grading and sidewalk construction, lead to accumulated injuries that will soon be present as the tree begins a spiral decline.
Limiting Access – If at all possible, it is best to allow only one access route on and off the property. All contractors must be instructed as to where they are permitted to drive and park their vehicles. Often this same access drive will later serve as the route for utility wires, waterlines, the driveway; or other paved surfaces. Limit storage areas for equipment, soil, and construction materials. Specify areas for burning (if permitted), cement washout pits, and construction work zones. These areas should be away from protected trees.
Reducing Compaction – One technique to reduce compaction on construction sites is to spread a thick layer of mulch, such as wood chips readily available to arborists. The mulch should be 6 to 12 inches deep. This has the effect of dispersing the weight of construction equipment. Additional weight dispersal can be obtained by placing large plywood sheets over the mulch. Construction mulching is a temporary measure, and the mulch cannot be left in place at that thickness for a prolonged period of time. The mulch must be removed carefully so as not to damage the trees in the process. An alternative is to spread the mulch to a layer 4 inches in depth.
Root Pruning – Anytime roots can be precut prior to digging activities – will reduce root damage beyond construction limits. This precutting allows a cleaner cut that increases the opportunity for re-growth with less adverse affects. This reduces roots being torn back to the trunk – limiting injury to the tree trunk and root collar area. All roots damaged and exposed during excavating that were not precut – will need to be pruned (cleanly cut) afterwards. All roots pruned in this manner should be treated with a systemic fungicide to inhibit decay pathogens. Trenches and cut roots should be covered immediately. Backfill material should include good organic material to stimulate new root growth.
Irrigation and Drainage – One of the most important tree maintenance procedures following construction damage is to maintain an adequate, but not excessive, supply of water to the root zone. If there is a drainage problem, the trees will decline rapidly. This problem must be corrected if the trees are to be saved. If soil drainage is good, be sure to keep the trees well watered, especially during the dry summer months. A long, slow soak over the entire root zone is the preferred method of watering.
Mulching – One of the simplest and least expensive things that can be done for trees may also be one of the most effective. Applying a 2- to 4-inch layer of organic mulch such as wood chips, shredded bark, or pine needles over the root system of a tree can enhance root growth. The mulch helps condition the soil, moderates soil temperatures, maintains moisture, and reduces competition from weeds and grass. The mulch should extend as far out from the tree as is practical for the landscape site. When it comes to mulch, deeper is not better, and piling it up against the trunk can lead to disease problems.
Aeration of the Root Zone – Compaction of the soil and increases in grade both deplete the oxygen supply to tree roots. If soil aeration can be improved, root growth and water uptake can be enhanced. Vertical mulching and radial trenching are techniques that may improve conditions for root growth. If construction damaged trees are to survive the injuries and stresses they have suffered, they must replace the roots that have been lost. Research to date on improving aeration has been limited, and there are still some question how successful attempts at improving aeration have been.
Drilling Holes / Vertical Mulching – The most common method of aeration of the root zone involves drilling holes in the ground. Holes are usually 2 to 4 inches in diameter and are made about 1 to 3 feet on center, throughout the root zone of the tree. The depth should be at least 12 inches but may need to be deeper if the soil grade has been raised. Sometimes the holes are filled with organic material such as compost, peat moss, or other materials that maintain aeration and support root growth. This is called vertical mulching.
Radial Trenching – Another method of aeration is radial trenching. Trenches are dug in a radial pattern throughout the root zone. The trenches appear similar to the spokes of a wagon wheel. If a mechanical trencher is used, it is important to begin the trenches 4 to 8 feet from the trunk of the tree to avoid cutting any major support roots. The trenches should extend at least as far as the drip line of the tree.
Use of an air excavator has proved very effective for this purpose. While mechanical trench digging sacrifices some roots, air excavation devices do little damage to roots, even if trenching is not radial. If the primary goal is to reduce compaction, the trenches should be about 1 foot in depth. They may need to be deeper if the soil grade has been raised. The narrow trenches are backfilled with native soil and compost. Root growth will be greater in the trenched area than in the surrounding soil. This can give the tree the added boost it needs to adapt to the compacted soil or new grade.
Repairing Damaged Bark and Root Wounds
Often the bark may be damaged along the trunk, limbs, root collar or roots. If this happens, the loose bark should be carefully removed. Jagged edges can be cut away with a sharp knife, taking care not to cut into living tissues. This procedure is called bark tracing.
Wound Dressings – Wound dressings were once thought to accelerate wound closure and reduce decay. Research has not substantiated this, however. Some studies have shown beneficial effects in specific cases in reducing borer attack, oak wilt infection, or control of sprout production or mistletoe. However, wound dressings are primarily used for cosmetic purposes, and are neither required nor recommended in most cases. If a dressing must be applied, only a light coating of a nonphytotoxic material should be used.
Monitoring for Decline and Hazards
Despite everybody’s best efforts, some trees may be lost as a result of land development and construction activities. If a tree dies as a result of root damage, it may be an immediate hazard and may require immediate removal. Trees may appear to recover from construction damage but may have extensive root rot. Arborists must inspect and monitor trees that have suffered construction stresses for signs of possible hazards. Look for cracks in the trunk, split or broken branches, and dead limbs. Watch for indications of internal decay such as cavities, carpenter ants, soft wood, and mushroom-like structures growing on the trunk, root collar, or along the major roots. Trees should also be inspected for signs of insects or diseases. Stressed trees are more prone to attack by certain pest. Additional tree mitigation services may be required depending on tree impacts and environmental factors.
Conclusion: As discussed earlier, preventing tree injuries, root damage and soil compaction, is critical. One of the most effective ways to accomplish this is by establishing penalties for damage and non-compliance as part of contract documents (in both specifications and on plans). Contractors and their subs, are more diligent when they have ownership of a problem.
OTHER RESOURCES FOR “TREE PRESERVATION FOR CONSTRUCTION”
National Arbor Day Foundation – Bulletins #7 and #20 www.arborday.org / programs / Tree City USA / bulletins
Protection and maintenance efforts are critical to long-term tree health and safety.
If tree protection is not performed with proper planning and execution – trees die, become unsafe, and become and unnecessary removal expense.
PRE-CONSTRUCTION CARE AND PREPARATION
“Special Focus on Protecting Roots!”
- Define and mark root protection zones
- Set up tree protection barriers and signs
- Define and mark access corridors
- Mulch root protection zone
- Remove undesirable trees
- Prune deadwood
- Fertilize and irrigate as needed
- Aeration systems / root pruning
Critical Protection Zone
The more permanent and rigid the fencing, the less likely it is to be removed or knocked down.
- Remove signs and fences
- Root invigoration
- Irrigation as needed
- Prune damaged trees if needed
- Inspect on a regular basis for signs of stress
- Treat pests