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Trees & Construction, can they co-exist?

The value of trees in residential and urban settings is well documented. Economic, environmental and aesthetic facts abound as reasons to make sure trees are part of the “green infrastructure.” At times, this necessitates establishing new landscapes, at other times, it means protecting what is already there.

Whether a new development is planned in an existing forest, a sidewalk is being replaced or a building addition is planned at home or work, trees are most likely part of the existing environment. All too often, these trees are not factored into the planning phases of the development or construction.

While certain trees may be recognized as valuable assets, a lack of understanding of tree physiology, coupled with a lack of planning, often ends in tree death. Construction damage to trees can result in sudden death or surface years later as a weakened tree, which finally succumbs to an insect or disease pest. Either outcome can result in hazardous situations, lost value and a cost to remedy the situation.

Through decent planning and proper tree protection measures, the story can have a much happier ending.

Protecting trees in or near construction areas begins before the first piece of equipment goes to work. The initial planning should include trees in the drawings; even better is to include an arborist or forester in the planning phase.

The questions that need to be asked and addressed include: Where is the planned construction to take place? Can that be modified? What specific trees are particularly valuable (reasons may include historical significance, size, uniqueness, location, beauty, energy conservation, etc.) and what is the proximity to the construction site. How much space is needed for storage of equipment & materials, where does the access to the construction site fit into everything?

Perhaps not everything will fit in the limited space. It is better to make the tough calls now, than when liability and expense become bigger issues later. Perhaps modifications to the construction location(s) can be made or specific measures can be put into place early on to protect certain trees. These components should be built into the plan and the budget.

Tree protection measures are put into place to prevent or minimize damage to all parts of the tree to include leafs, branches, trunk and roots. Protecting the above ground parts is rather straightforward. Do not run into or over a tree, do not build large burn piles under a tree, do not cut branches without regard to proper pruning techniques, etc. For the most part, these follow common sense.

The most ignored measure related to tree protection is often the most important. It is the “ROOT” of the matter. A big part of the tree is underground. Being that it is out-of-sight, it unfortunately is also out-of-mind in most cases. This is exactly where most damage occurs.

Before we go any further, let us review a bit of biology. Roots function mechanically to keep plants upright and physiologically by taking up water and nutrients from the soil to transport throughout the plant. To accomplish their physiologic functions, roots need to “breath.” Roots uptake oxygen and discharge carbon dioxide (just the opposite of leaves) to enable cellular respiration.

There you have it, roots need soil but not just any soil. It needs to be soil that has nutrients and structure. Soil with good structure has space for air and water. When soil structure is destroyed, a root’s ability to take up nutrients, air and water is diminished. Soil structure is most commonly destroyed by compaction. While excessive tilling can also harm structure, compaction is the most common culprit everywhere except gardens and agricultural crops.

Depending on texture, soil is easily compacted. This is good news when it comes to building roads and pouring foundations, but bad when roots are trying to survive. Sandy soils are least prone to compaction while clay compacts easily, resulting in potentially high bulk densities (surpassing concrete in some cases).

Compaction results from any number of above ground activities, most notably vehicle traffic and foot traffic. An amazing mechanic of soil compaction is that most of the compaction actually occurs on the first pass. Therefore, the notion of “just driving under this tree once” doesn’t hold water.

Speaking of water, compaction also limits infiltration of rainwater, thereby increasing runoff. This results in even less water for plant growth, adding insult to injury.

So what measures can be taken to protect a tree? A tree protection zone (TPZ) is the most basic and effective measure. It purposefully protects the above AND below ground portions of a tree or group of trees. The size of this

zone will vary depending on tree size, species, and vigor. Larger trees need larger zones as do species more sensitive to disturbance and those with low vigor (perhaps because of old age).

A good rule of thumb when fencing off the TPZ is to place it underneath the drip line for broad canopied trees. Understand, however, that most trees’ roots extend well beyond the drip line. A variety of other methods exists to establish TPZ. One other way is to determine a minimum TPZ size based on trunk diameter. First, measure the diameter of the tree (at 4.5 feet above ground) in inches. Then define a circle around the tree with a diameter in feet equal to the number of inches in diameter the trunk measured.

The TPZ is a zone where nothing happens. This means no equipment or material storage, no vehicle parking, no refuge dumping, or anything else that would result in compaction or soil pollution. The TPZ needs to be well fenced to prevent these damaging agents. If possible, a sign should be posted explaining the purpose and consequences of tampering (for those communities with relevant tree ordinances).

In cases where not enough space is available for an effective TPZ, other options exist such as temporary ground covers like mulch and geotextile fabric. These and other strategies are part of the bag of tricks certified arborists can employ to help you save trees during construction. Find a certified arborists near you by searching on-line at http://www.treesaregood.com/ . Call 1-866-VATREES to learn more about proper tree care and other urban forestry issues.

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Last modified: 9/5/10