Trees and Construction
by Ann Barklow, Consulting Arborist

TREES AND CONSTRUCTION

Construction damage is one of the most common causes of death and decline of trees in urban areas. Successful tree preservation begins with careful planning that must focus on preventing injury to trees. It is vital that trees be protected during construction because repair to damaged trees is limited, if not impossible in many cases. The most important step is to have a professional arborist involved early in the planning stages. An arborist can work with the general contractor to protect trees throughout each phase of construction. An arborist can also advise landscape professionals helping to integrate trees in a final design. Professional arborists understand the trees physiology and the components needed for tree health.

A key thing to remember is that a tree may take five to ten years to die after construction damage. Tree failure is often blamed on other factors when in reality previous construction was to blame. Homeowners often don’t feel that the construction was the cause since the damage did not kill the tree immediately! It is impossible to retain trees on a construction site without some damage occurring, however this damage needs to be kept to a minimum.

HOW TREES ARE DAMAGED DURING CONSTRUCTION

Construction impact to trees often includes some degree of root injury, removal of leaf mass from pruning, loss of root space, soil compaction, and changes to soil moisture and microbiology. In most cases, tree damage happens over time rather than all at once. Trees respond and adapt to each injury in various ways, from slower growth, poor foliage color, branch dieback and death.

A typical scenario of tree injury during construction may go something like this: Initially the roots are injured and the site microclimate is altered by clearing and shutting off water sources. Then more changes occur during grading and trenching to install utilities. During this time heavy equipment being used or building materials piled up against the tree may be compacting the soil around the tree or causing physical damage. The new construction itself causes another series of harm to the root zone or branch canopy. Finally, finish grading and hardscaping along with a new landscape may encroach into the root area and alter the tree's microsite environment.

	Figure 1. "...90% of the fine, absorbing roots critical to the tree's survival are in the upper few inches of soil..."

Most people believe a trees root system is many feet under the ground. In reality in most trees, 90% of the fine, absorbing roots critical to the tree's survival are in the upper few inches of soil, from a depth of 4-12". See Figure 1.

For most trees, absorptive roots start a few feet from the trunk, then spread way past the outermost branches (known as the drip-line) by 40%-50% or more. In fact, for most trees, research shows that 2/3 of absorptive roots for the entire tree are located past the drip-line. It is not uncommon to find the absorptive roots of large urban trees occupying an entire lawn or backyard and beyond if physical barriers are absent. Trees growing in sandy well-drained soils may have more extensive root systems than those in finer textured clays. See Figure 2.

Each tree needs to be examined and evaluated by a professional to determine the best tree protection zone for your particular location and tree species.

	"The impacts of construction are cumulative and can send a tree into a spiral of decline that won’t be recognized by the untrained eye until it is too late."

PHYSICAL INJURY

Construction equipment and debris can injure the trunk and crown of trees. Broken branches, torn bark and wounded trunks are permanent injuries. If these injuries are extensive and the tree is stressed, it may never recover.

AVOIDING TREE DAMAGE DURING CONSTRUCTION

Hire an arborist at the planning stage to work the entire length of the project. The impacts of construction are cumulative and can send a tree into a spiral of decline that won’t be recognized by the untrained eye until it is too late. This is why the need to involve a professional arborist early in the planning stages is essential. The value of mature trees is in the tens of thousands of dollars so the cost of an arborist is well worth the investment.

A professional arborist will most likely take the following actions to protect your trees during construction:

• Erect tree protection fencing to protect root zones, trunks, and branches. These should be placed as far out from the trunks of the trees as possible. As a general guideline, allow 1 foot diameter from the trunk for each inch of trunk diameter.
• Install signs to identify tree protection zones
• Meet with the general contractor to educate on tree preservation
• Avoid grade changes
• Restrict construction traffic to specific areas to avoid tree root zones
• Monitor the site to enforce tree protection rules
• Store/mound soil away from tree roots in drip line and beyond
• Encourage the use of boring equipment instead of trenching to route utilities under tree roots
• Designate a concrete washout site to prevent soil contamination
• Use geotextile fabric covered with wood chips in the root zone to prevent compaction
• Set up a temporary automated irrigation system for tree(s)
• Monitor root pruning to prevent jagged or torn edges
• Discuss alternatives to non-permeable surfaces to prevent compaction and encourage water penetration
• Discuss the use of discontinuous footings to reduce root injury
• Discuss offsite parking for workers preventing vehicles from compacting site
• Send memo's to general contractor, city (if tree ordinance requires), and owner if above guidelines are not followed
• Monitor final landscape and irrigation installation
  

Despite the best intentions and stringent tree protection measures adopted by some cities, trees may still be injured by construction practices. Having a plan to successfully protect your trees during construction will give your tree the best possible chance of survival and it starts with consulting with an arborist. The recovery rate for construction damaged trees is low - don't wait until its too late.

For more details on trees and construction, see our Trees & Construction Photo Gallery

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