Protection of existing trees on construction sites

Wounds such as open branches and torn or nicked bark can damage a tree by depleting a tree’s energy and providing entry points for disease and insects (Johnson 1999). Many jurisdictions have their own tree preservation standards and/or guidelines. Toronto, for example, has detailed tree protection specifications for construction near trees.

Underground Damage to a Tree

Compaction within the tree’s root zone adversely impact the tree. Compaction can result from machinery/vehicular traffic, foot traffic, and from stockpiling materials over a tree’s root zone. Grading within the tree’s root zone, whether cutting or filling, also negatively impacts trees.

The part of the root system in which construction activities, including material storage and traffic, should be avoided is called the Protected Root Zone (PRZ). Only about half of the root system is under the tree canopy (University of Florida 2013). Many tree roots extend beyond the dripline a distance equal to two or more times the height of the tree (Johnson 1999). The PRZ should therefore extend beyond the tree canopy (see MnDOT specfications in Appendix A for recommended PRZ).

How much of a tree’s root zone can be disturbed before a tree’s health is compromised? Johnson (1999) writes:”Just how close an activity can come without seriously threatening the survival of a tree depends on the species, the extent of damage, and the plant’s health. Some healthy trees can survive after losing 50 percent of their roots. However, other species are extremely sensitive to root cutting, even outside the dripline. Table 1 shows the relative sensitivity of various tree species to root disturbance. If possible, disturb no more than 25 percent of the roots within the dripline for any tree, protect intermediate species to the dripline, and allow extra space beyond the dripline for sensitive species. For all trees, avoid needless or excessive damage. A qualified tree-care specialist can help you determine how much root interference a particular tree can tolerate.”

Older trees are more susceptible to damage from construction activities than younger trees (Cappiella et al 2006, Johnson 1999).

Negative impacts from construction activities within a tree’s root zone are often not visible in the tree until 3 to 7 years after the construction has ended (Johnson 1999).

Removing trees from a site can also expose the remaining trees to increased sun and wind exposure, which can shock the remaining trees (Johnson 1999). Saving groups of trees instead of individual trees can minimize sun and wind stress (Johnson 1999).

While it can generally be assumed that tree roots are found beyond the dripline a distance equal to two or more times the height of the tree, where the exact location of roots needs to be known for some reason, ground penetrating radar technology can be used to locate exactly where roots are located without digging, for example, when tree roots are under pavement (Bassuk et al 2011). Where tree roots are not under pavement, an airspade can be used to examine roots without damaging the tree.

Damage to Tree from Soil Chemical Changes

According to Johnson (1999) “Improper handling or disposal of materials used during construction also can harm roots. For example, wood products treated with pentachlorophenol and creosote can be deadly to tree roots; CCA-treated timber (greenish color) is a better alternative.”Changes in soil pH, for example, from concrete, alkaline clays, or limestone can also damage trees (Johnson 1999).

See the references and additional resources listed below for much more information on adverse effects of construction on trees.

Best practices for minimizing impacts from construction activities, such as installation of fences around trees

As described by (Cappiella et al 2006), minimizing impacts from construction activities is a five step process:

1. Inventory existing trees on project site
2. Identify trees to protect
3. Design project with tree conservation in mind
4. Protect trees and soils during construction
5. Protect trees after construction

Inventory existing trees on project site

Record the location, species, size, and health of each tree. The size and health of the tree will affect whether or not the tree should be preserved or cut. According to Johnson (1999), “Wilted leaves, broken or dead limbs, trunk rot, and thin tops are all symptoms of stress. Trees that are overmature, display poor form, lean heavily over future buildings, or have severe insect or disease problems should be marked for removal prior to construction.”

The extent of the inventory will depend on the needs of the specific project. Some jurisdictions have tree preservation ordinances that require an inventory and dictate the size and types of trees that must be inventoried. The following additional features are recommended in the tree inventory (Cappiella et al 2006)

• Property boundary
• Roads
• Utilities
• Easements and covenants
• Topography
• Streams and stream buffers
• Critical habitats
• Adjacent land uses
• Cultural and historical sites
• 100 year floodplain
• wetlands
• Soil types
• Steep slopes

Identify trees to protect

Healthy, vigorous trees with good structure are the most likely to survive through construction activities and should be the top priority for preservation (University of Florida 2013). Johnson (1999) provides the following additional tips for selecting which trees to protect from construction activities:

• “Save the best and chip the rest. Use those wood chips to provide a blanket of protection over the root systems of trees that can be saved.
• Understand the characteristics of your trees or get the advice of someone who does. If you know about your trees you can help insure their survival and improve the future site appearance of the site.
• Select tree species that fit the spatial constraints of the site (Table 1), remembering that trees grow throughout their lives. Be sure to consider overhead powerlines.
• Young, small trees tend to survive disturbance better than old, large trees.
• Large trees almost never survive within five feet of a new building and should not be kept.
• Healthy young trees that fall in the construction zone may be saved by transplanting.
• Don't put all your eggs in one basket! Save a mixture of tree species to safeguard your landscape against contagious diseases or insects.
• Improve tree survival by saving groups of trees rather than individuals.”

Design project with tree conservation in mind

Cappiella et al (2006), provides the following examples of better site design techniques to conserve forests:

• Minimize impervious cover
• Design structural elements such as roads and utilities to minimize soil disturbance and take advantage of natural drainage patterns.
• Where possible, place several utilities in one trench in order to minimize soil disturbance.
• Reduce building footprints by building up, not out.
• Use the minimum required street and right-of-way widths.
• Use alternative turnarounds instead of cul-de-sacs.
• Use efficient street layouts.
• Consider shared driveways for residential lots.
• Use the minimum required number of parking spaces instead of creating additional spaces.

Protect trees and soils during construction

Project plans shall clearly mark which trees are to be protected. Numbering and field tags can effectively help avoid cutting down the wrong trees.

Value of trees to be preserved should be appraised using “Guide for Plant Appraisal”, and the value should be noted on large scale labels on each tree to be preserved. Labels shall be attached in a manner that does not damage tree.

Prepare the trees for construction disturbance by making sure they are as healthy as possible before construction begins. Regularly water them before and during construction if rainfall is not adequate (i.e. whenever soil is dry 6 inches below the soil surface), and prune branches dead, diseased, and hazardous branches.

Install a layer of wood chips at least 12 inches deep over areas that will be used for traffic or material storage in areas that will be used for future planting, and over tree roots outside of the PRZ. Where dump trucks will be moving across such areas, increase wood chip depth to 18”.

Trees shall be protected in accordance with the most recent version of MnDOT specification 2572, Protection and Restoration of Vegetation. Protection measures per MnDOT specification 2572 shall remain in place throughout the duration of construction, and penalties for violation should be strictly enforced. Visit the site regularly to ensure tree protection requirements are not violated.

When possible, remove trees that are not to be preserved in winter after the leaves have fallen. When they are dormant, trees to be preserved will be less susceptible to damage from the removal of adjacent trees, and frozen ground helps protect roots.

Ensure that tree roots and soils are not exposed to adverse chemical changes during construction. Johnson (1999) recommends the following, for example, “ Ask the builder about the materials to be used on the site and read product labels. Chemical spill damage can be prevented by filling gas tanks, cleaning paintbrushes and tools, and repairing mechanical equipment well outside tree PRZs. Insist that all building debris and chemical wastes be hauled away for proper disposal, and not burned or buried on the site. Finally, avoid changes in soil pH (acidity). Increases in pH are particularly dangerous to many species [note from Kestrel team: see Task 3 species list for pH tolerances of various tree species]. Alkaline clays or limestones should not be used for fill or paving, and concrete should be mixed on a thick plastic tarp or outside the site. Mixing trucks should never be rinsed out on the site.”

If any construction damage occurs to trees, address problems as soon as possible, photograph the damage, and inform contractor immediately (Johnson 1999). Levy liquidated damages if applicable.

Irrigate trees during construction whenever soil is dry 6 inches below the soil surface.

Also irrigate thoroughly before and after trees receive any kind of direct damage (e.g. severed roots) (Johnson 1999).

If roots are cut, “cut cleanly to promote quick wound closure and regeneration. Vibratory plows, chain trenchers, and hand tools do a better job at this than bulldozers and backhoes. Minimize damage by avoiding excavation during hot, dry weather; keeping the plants well watered before and after digging; and covering exposed roots with soil, mulch, or damp burlap as soon as possible” (Johnson 1999).

If utilities must be installed under existing tree root zone, use installation techniques that will minimize root damage. According to Johnson (1999), “As much as 40 percent of a tree's root system could be cut during the installation of a nearby utility line. This reduces water and nutrient uptake, and may compromise the stability of the tree. If it is not possible to relocate the utility line outside the tree's PRZ, you can reduce root damage by as much as 25 percent by tunneling under the tree's root system. When digging a trench near a tree, begin tunneling when you encounter roots larger than one inch in diameter.” Another technique that can minimize root damage when installing utilities is to use an airspade to excavate the utility trench under tree root zone (University of Florida 2013). An airspade pushes air at a very high speed, removing soil without damaging roots.

Johnson (1999) recommends the following for pruning and wound repair:

“Prune broken or dead branches cleanly at the branch collar (Figure 6.1). To test whether a branch is dead, bend several twigs. Twigs on live branches tend to be pliable, while twigs on dead branches tend to break. Buds also can be used to evaluate branch condition. Live buds appear full and normal in color while dead ones appear shriveled or dry.

Pruning is commonly recommended for large trees that have suffered root damage. However, opinions differ over the merits of this practice. Assuming that the tree has adequate water and is not in severe decline, some experts believe that retaining maximum leaf cover is important for root regeneration and only dead limbs should be removed. Others argue that pruning selected live limbs is necessary to compensate for lost roots. Generally, it is best to follow the recommendation of your tree-care specialist experienced in construction damage to trees.

When properly done in moderation by a skilled professional, pruning may reduce wind resistance and limb failure and improve tree health and appearance. DO NOT let anyone cut off all of the top branches to the same height (" topping").

The treatment of trunk wounds depends on the extent of damage. If 50 percent or more of the bark has been removed around the entire trunk, the tree will not likely survive and should be removed. If only a patch of bark has been removed leaving a few splinters, use a sharp knife to cleanly cut off the loose bark to a place on the stem where it is firmly attached. DO NOT make the wound any larger than necessary.

You do not need to use pruning paint or dressing to cover exposed wounds or pruned limbs. Except for special cases involving disease control, these products do little more than improve appearance.”