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Tree Tips for Construction

Can trees be damaged by construction?

A tree’s environment may be altered greatly during construction. Various tree species respond differently to construction. Trees, unlike animals, cannot move. As a consequence, a tree must adapt to the changes that occur during construction. Some trees are better at this than others. Construction damage to trees may not be apparent to the homeowner for three to eight years after the damage was inflicted. This makes it difficult for the homeowner to relate cause and effect.

What is a construction injury?

Construction injury often occurs from accumulation of many smaller injuries. Any root injury or disturbance is the most detrimental, but other types of injury can also compound the problem. Since a tree may have been healthy before construction, it can take anywhere from 2 to 8 years for a tree to die as it slowly uses up its energy reserves to survive from year to year.

Some of the more common causes of construction injury include:

  • soil compaction: Equipment used in construction (backhoes, bulldozers, payloaders, cranes, dump trucks, equipment trailers, utility vehicles, and skid-steers) reduces the capacity of soil to hold both oxygen and water for the tree roots. Water from rain and even irrigation is more likely to run-off the top of the soil instead of soaking in the soil, causing an altered drainage pattern.
  • altered drainage patterns: Placing sewers, streets, curbs, and gutters at a site will greatly alter drainage patterns. Water that once had a chance to soak in after a shower now is whisked into the storm sewer, bypassing the ground water system. Less ground water is available to tree roots. The soil dries out faster since less water is stored, and the effects of droughts are heightened. This effect is worse at the bottom of a hill where plants requiring moist, but well-drained soils are often found.
  • decreased grades: Reducing the grade around existing trees causes serious injury as the majority of fine roots are found in the top 15 to 30 cm (6-12 inches) of soil. Large portions of the fine roots thus are lost by soil removal, causing a reduced root zone.
  • reduced root zones: Trenching, cuts for roads and/or sidewalks, and decreased grades result in the mechanical severing of fine roots, decreasing the size of the root zone. Since most of the fine roots are near the surface, even a shallow trench or cut for a sidewalk can damage the tree. The tree now has a decreased ability to absorb water and nutrients and store carbohydrates.
  • disturbed soil profiles: Builders are often faced with a disposal problem with the soil removed from digging the basement. The lowest cost solution is to spread the spoil from the basement over the existing soil. The spoil is often finer textured than the original soil, resulting in a disturbed soil profile and increases the grade. Water does not move downward from a fine textured soil such as clay into a coarser textured soil such as a sand or gravel. Water remains in the fine textured soil at the interface of the soil types until the head pressure is sufficient to force the water into the coarser soil.
  • increased grades: Distributing soil from construction excavations can also bury tree roots. The result is decreased aeration to the fine feeder roots and a disturbed soil profile. Adding soil over the root zone of existing trees is problematic. As little as 10 cm (4 inches) of fill may kill a mature climax forest tree such as linden, sugar maple, and beech.
  • mechanical injury: Bark and limbs are often damaged by construction equipment. The tree is further weakened and subjected to invasion by decay organisms.
  • debris in the soil: Virtually anything and everything is buried at a construction site. Concrete spills, sheetrock, and plywood are examples of construction debris that cause disturbed soil profiles and alter soil moisture distribution patterns.
  • impounded water: Sometimes altered drainage patterns and/or disturbed soil profiles can cause water to be impounded, creating a boggy site from one that had been well drained. Many native trees will not tolerate poor drainage. Oxygen levels in the soil are reduced as water fills the pore spaces that formerly held oxygen.
  • increased competition: Often little thought is given to the effects of altering plant communities. Inappropriate combinations of plant species are left to compete for the limited resources. Ultimately one species will out compete the other, leaving one weakened and unhealthy.
  • increased light: Another alteration of the suburban landscape is increased light. Reduced numbers of trees and the resulting increase in reflected light contribute to a much higher light level. Reflected light alone can double the incident light levels to which a plant is exposed. Thin barked trees, such as beech, seem particularly sensitive to this problem.
  • increased temperatures: Soil and air temperatures both rise in the city. Plants may be more sensitive to soil temperature changes since air temperatures normally fluctuate more. Plants that are at the southern part of their natural range are most likely to be sensitive in this situation.
  • interrupted nutrient cycling: This is not a factor that is likely to have severe consequences itself, but it is one more stress with effects that are additive. Micronutrients are likely to be the ones that are found to be deficient. Removal of leaves and other organic debris takes the nutrients they contain out of the system.
  • modified insect and disease complexes: A number of insects and diseases attack weakened hosts. The increased stress levels associated with urban sites often predispose the plant to attack. Engraver beetles and metallic wood boring beetles are insects that are associated with increased stress levels. Nectria canker and cankerstain are diseases that are stress related.

How do I prevent injury?

The most important thing is to have your site evaluated by an arborist before construction begins. Arborists are often called upon to provide help for trees injured during construction. Many times the arborist can be helpful, but all too often the call is made after the damage has been done and the trees are in a declining condition. The best option for the homeowner is to prevent construction injury by having an arborist create a tree preservation plan.Should you feel that your tree has incurred construction injury, you should first have an arborist evaluate the health of the trees as well as the extent of construction injury. If the trees appear to be in rapid decline or the original construction injury occurred a long time ago, attempts at restoring tree health are unlikely to be successful. If, on the other hand, construction recently occurred and/or limited tree decline is observed, remedial treatments have a better chance of success. Your tree care company will have to evaluate the trees and the site and then make recommendations.

What is a tree preservation plan?

Large-scale construction projects, such as subdivisions, normally involve specially trained, consulting arborists working with landscape architects and building developers in creating a tree preservation plan. The plans usually catalog trees, rate their condition and value, indicate which trees will be protected, and outline how that will be accomplished. For smaller projects, such as building a house on a wooded lot, building a garage, paving a driveway, installing landscaping, building a retaining wall, adding an addition, etc., your local tree care company should be able to assist you.

Preparing for construction

Based on the tree, site, and impact evaluations, an arborist may choose to prepare the tree for construction by applying fertilizer, biostimulants, and/or mycorrhizal inoculations. If any pruning is needed, it should be done as soon as possible to give the tree time to adapt before construction.Will changing the grade on our property hurt our trees? Both soil fills and cuts will have an affect on the health of your trees. When fill is placed on the soil, tree roots initially remain at the original grade. They are subject to a condition of reduced soil aeration and increased moisture. Damage and death of some roots will occur as a result. Slowly the tree will attempt to grow new roots into the soil fill. Eventually these new roots will grow upwards into the fill to the point where they regain acceptable soil aeration and moisture conditions. If trees are to survive having fill placed over their roots, those roots must have adequate oxygen and drainage so that the tree can survive until the time that its own roots have satisfactorily grown into the new fill. Provisions must be made to allow air exchange between the roots and the atmosphere and be certain there is adequate drainage in the soil. Your tree care company may recommend the installation of a soil aeration system. Generally, this system will include a tree well. The goal of this system is to provide adequate aeration and drainage for the initial root system while new roots grow up through the fill soil to better aerated and drained soil.Soil cuts as a whole are probably more damaging than soil fills. The fine roots of most trees are in the top 15-30 cm(6-12 in) of the soil, with the majority of these in the shallower depth. Many are lost when even a small amount of the surface soil is removed. It should also be noted that the most fertile soil, and thus the largest supply of nutrients for the tree is removed when cuts are made. Loss of roots and fertile soil will tend to increase the probability of the tree suffering from drought and mineral deficiencies. Deep cuts will severe large, supporting roots and may cause the tree to be wind-thrown in moderate to strong winds. Where cuts are made only on one side of a tree, retaining walls, or terracing should be constructed to prevent excessive loss of soil from the remaining roots. Pre- and post-construction preparations, such as fertilization or inoculation, may be needed to stimulate new root growth. Irrigation may also play a key role, since much of the tree’s ability to absorb water is lost when the roots are cut.