Protecting Trees During Construction

Why Preserve Existing Trees?

Trees add to the value of residential and commercial properties. During construction, established, healthy trees can be preserved with minimal effort and expense. Many trees are valuable enough to justify the extra concern.

Tree Preservation Planning Process

Many towns and cities in Colorado have tree protection specifications detailed in their land use code; property owners must adhere to the tree protection specifications spelled out in land use code of their town or city. Prior to beginning a construction project, the property owner should have a city forester, licensed arborist, or certified tree appraiser evaluate every tree in the proposed construction area to determine if preservation is worthwhile. For each existing tree, consider location, overall tree health, present size, future size, species, vigor, cost of preservation, and cost of removal. 

While evaluating trees for preservation, consider the future look of the landscape. If plans call for preservation of trees near buildings or in landscapes, the trees should be sufficiently healthy to justify preservation. Some large, mature trees may not be structurally sound, desirable species, or esthetically appealing to warrant preservation. Older trees do not adapt to environmental changes as well as younger trees, thus it may be more practical to protect a 1- to 8-inch diameter tree than a larger, more mature tree. Some existing trees (particularly smaller trees) could be relocated with a tree spade. Tree species differ in their ability to adapt to environmental change (Table 1). It may be impossible to preserve every tree. 

To avoid short- and long-term problems, consult all parties involved in the project early in the planning process, including the property owner, contractor, architect, engineer, and arborist. 

Once trees are selected for preservation, prepare a preservation plan that contains specific tree preservation methods. Establish contractor guidelines for tree protection within contracts. Such guidelines include: prominently marked protected areas around the tree(s); barricades around designated trees; avoidance of vehicular traffic or parking in restricted areas; and prohibiting material storage, grading, and dumping of chemicals and other materials in restricted areas. To ensure compliance, contractors should have tree preservation bonds to cover potential noncompliance fines. Noncompliance fines are based on trees species, tree value, replacement costs (if required), and the amount and type of damage done. Tree preservation bonds create an additional incentive for compliance. All parties should be aware of and agree to the consequences for noncompliance. 

Before construction begins, conduct an on-site meeting with all parties, with special emphasis on educating the project contractor. Provide a handout with specific guidelines that outlines the preservation plans to everyone working on the project. 

Table 1. Adaptability to environmental change. 

The Importance of Protecting a Tree’s Root System

Tree root systems contain large, woody roots, and small, short-lived absorbing roots (i.e. fine roots). Large, woody roots are perennial and grow horizontally and are mostly in the top 6 to 24 inches of the soil (Figure 1). Their main functions include anchorage, water and mineral transport, and nutrient and water storage.

Smaller absorbing roots, averaging 1/16 inch in diameter, constitute the majority of the root system’s surface area. These roots grow outward and upward from larger roots, near the soil surface where nutrients, minerals, water, and oxygen are more abundant. Their major function is to absorb water, nutrients, and minerals. 

The root zone extends horizontally from the tree for a distance at least equal to the tree’s height. Aim to preserve at least 50 percent of the root system to maintain a healthy tree. During summer construction, trees require adequate water, enough to saturate the soil, every one to two weeks. Following construction, it is crucial to provide water to the tree on a consistent schedule (every one to two weeks) to support root regeneration. 

Preventing Injury to Existing Trees

Preventing damage is less costly than correcting it. Post highly-visible and strong barricades and signs around the trees and areas to be protected (Figure 2.) The optimal size of barricaded areas varies by tree species, size, and construction project. For recently planted trees (one to four years), the area under the branches (dripline) should be adequate. For minimal protection of trees older than four years, barricades should extend beyond the dripline; for each 1 inch of trunk diameter, extend the protection area an additional 1 ft. For example, a 15-inch diameter tree should have the protection zone extended 15 feet in all directions from the tree’s dripline. A layer of wood chips 4 to 6 inches deep can be placed around each tree prior to placement of the barricades. Examine trees and barricades at least once a week during construction. 

Avoid Direct Injury to Existing Trees

There are four general forms of direct tree injury caused by mechanized equipment: bark removal, branch breakage, surface grading, and trenching injury (Figures 3 and 4). 

Bark removal or “skinning” of the trunk can be caused by any type of equipment. This can easily kill the tree, because it cannot survive without bark, which protects the cambium and vascular tissues in trees. Breakage of lower branches may make the tree unsightly or remove too many leaves, causing stress. Any broken limbs should be properly pruned by a tree care professional. Surface grading removes surface vegetation and topsoil that contains many absorbing roots. Trenching for utilities can also cause substantial root damage and should be done far away from existing trees.  If the trench must pass under or near a tree, avoid root injury by using a power auger to bore a tunnel under the roots. If trenching is unavoidable, place the trench as far from the trunk as possible (minimum 8 feet), cutting as few roots as possible. Cleanly prune cut roots and refill trenches as soon as possible to prevent excessive moisture loss. Wounds make the tree highly susceptible to drought stress, root pathogens, and decay fungi. Decline and death can result if more than 40 percent of the stem or roots are damaged or killed. Stressed trees are also more susceptible to insects such as bark beetles and borers.

Avoid Soil Compaction Problems

After a tree is established, any activity that changes the soil condition and structure is detrimental to tree health. Construction traffic compacts soil most severely near the surface, an area where the majority of tree roots are located (Figure 5). Soil compaction decreases soil permeability, reduces soil oxygen levels, and interferes with drainage. When root growth is restricted by compacted soils, fewer nutrient and water uptake are decreased. These factors limit root growth, reduce tree vigor, and can cause tree death. Trees may be slow to respond to stress and decline and dieback may gradually appear over a period of years. 

It is much easier to avoid soil compaction than to correct it. Keep construction traffic and material storage away from tree root areas, outside of the barricades and fencing. Apply a 4- to 6-inch layer of wood chips around all protected trees to help reduce compaction from vehicles that inadvertently cross the barricades. 

Avoid Soil Fills

When excessive amounts of soil are added around a tree base (fills), the additional soil interferes with normal air and moisture circulation to the roots. Minor fills with topsoil of less than 3 inches will not harm most trees. The topsoil should have greater pore space, be high in organic matter, and have good drainage properties—it should not be clay. Major changes in grading require an air supply to the roots. This can be accomplished with a drywell.

Early symptoms of decline from excessive fill can be smaller leaf size, leaf scorch, and premature fall coloration. Dieback of twigs and progressive dying of larger stems in the upper crown also occurs. This dieback may not be noticed for several years, depending on tree species and initial tree health.

Avoid Soil Cuts

Lowering the grade usually is less complicated than fills, but can be equally harmful. Where the grade has been changed near a tree, the most common damage is the complete severing of major roots in that area. This can cause decline, death or decreased stability to high winds. To protect the tree, terrace the grade or build a retaining wall. Walls should encompass an area extending at least to the tree’s dripline.

Pruning Injured Trees

Trees with injured roots may show branch dieback quickly or within a few months after the initial injury. Prune dying branches to reduce insect and disease damage to the rest of the tree. Also prune to reduce any hazardous conditions on preserved trees. It is better to wait until the tree exhibits branch dieback to see how much to prune, rather than preventively remove parts of the crown because you assume the root system was damaged. Wildfire regulations may require pruning to a height of 10 feet of all trees around your home and outbuildings.

Care of Trees Following Construction

Following construction, focus on preventing additional stress to the tree and provide water on a regular basis – at least twice per month during the growing season and once per month in fall and winter. Water helps support root growth.

Reference: CSU Fact Sheet 7.420