Protecting the Trees and Immediate Environment During Sitework

Protecting trees and the local ecosystem when building on previously unbuilt land is a vitally important—yet often overlooked—part of environmentally responsible construction. Healthy trees and shrubs can reduce a home’s environmental impact and directly benefit homeowners in a number of ways:

•Enhancing comfort by shielding the house from noise.

•Reducing home energy consumption by blocking winter winds and shading the house from sunlight during the summer.

•Providing attractive, relaxing back-yard space.

•Minimizing the need for lawns. Lawn mowing consumes energy and releases pollutants into the atmosphere, particularly when two-cycle lawnmowers are used. In many parts of the country, lawns are also watered regularly and treated with environmentally harmful pesticides and herbicides. From an environmental standpoint, wildflowers, perennials, and native woodlands are far preferable and should be encouraged whenever possible.

•Boosting value of a home or building lot. The July 1991 issue of

Builder Magazine reported on a Gallup Poll indicating that trees can increase the value of a home by up to 15%. The same article quoted experts as estimating that trees can generate lot premiums of $3,000 to $15,000.

•Saving money required for landscaping after the house is built. If the existing vegetation is not destroyed, fewer trees and shrubs will need to be planted.

Given these benefits, it pays to put a lot of effort into protecting what’s already there. During excavation and grading, the site receives the worst treatment it will ever get. Trees are regularly injured or killed by altering the soil grade, severing roots during grading or trenching, skinning tree trunks with heavy equipment, and compacting soil. By simply using some forethought during design and construction, getting advice from the right experts, and carefully overseeing sitework, you can make a tremendous difference in the outcome of the project.

To better understand how to protect trees on a building site, each of the common causes of tree damage during construction will be addressed, followed by a brief discussion of corrective measures and a checklist for builders and designers.

It is the root system of trees that is most frequently injured during construction, so that's where we will focus most of our attention. The depth and spread of a tree's root system is very dependent on the underlying soil. Trees growing on sites with lots of topsoil or in sandy soil usually have fairly deep roots that are less susceptible to damage during construction than trees that are growing on sites with heavy clay soil, or where ledge is close to the surface.

 

Raising the soil grade

 

Roots absorb oxygen and release carbon dioxide through the process of respiration. Raising the grade around a tree can impede this gas exchange, resulting in root suffocation. Even temporarily raising the grade around a tree during construction can result in injury. The degree of damage depends on the type and depth of fill. A porous fill of gravel or sand will be less damaging than a more impervious clay fill. If the grade is not changed but an impervious layer is added, such as asphalt or concrete, the effect will be the same. (Note that paving around an existing tree is very different from planting a young tree in a predominantly paved area, such as in cities where the growing tree’s roots will adapt to the conditions as necessary.)

Tree injury resulting from raising the grade is not distinctive, so the problem is often hard to diagnose. Effects can include reduced leaf size, premature changing of leaf color in the fall, dieback of twigs and progressively larger branches, susceptibility to parasites, and general decline of the tree—usually from the top down. On a site that has already been graded, you can often determine if the grade was raised around trees by looking at the trunk at ground level. If there is no buttressing, or widening, of the trunk near the ground (in other words, if the tree trunk looks like a telephone pole sticking out of the ground), the grade has probably been raised. Digging around the tree in a few places with a hand trowel will provide a more accurate determination of whether overfilling has occurred.

Many people attempt to prevent tree injury when raising the grade by building small wells around the trees, but this strategy may not succeed. Most of the root system will still be prone to suffocation by the elevated grade, since the roots extend well beyond the trunk—and usually even beyond the “drip line” (the outermost edge of the crown). To be successful—at least in clay soil—such a well would need to extend to include the entire drip line. If the grade must be changed and important trees need to be protected, a skilled arborist might be able to design a system of gravel fill, landscape fabric (geotextiles), and drainage tile to aerate the root system, but such measures should only be considered as a last resort.

 

Lowering the Grade

Woody plants, including trees and shrubs, have most of their roots in the top 18" of soil, and a large number of feeder roots are located within the top 6". Roots adapt to the ground they are growing in; on hilly ground, for example, a tree’s roots will grow along just beneath the surface, rising and falling with the topography, with feeder roots extending nearly to the surface. Removal of

any soil around trees, therefore, can cause considerable damage. Severing large roots close to the trunk is more damaging than severing smaller roots near the drip line. Symptoms of root damage from lowering grade are hard to distinguish from other tree health problems and will often be very similar to the symptoms of raising the grade discussed above.

 

Mechanical Injury to Trees from Surface Grading

Grading a site with a bulldozer can injure both roots and tree trunks. Any scraping of topsoil around trees will sever vitally important feeder roots, as described in the paragraph above, seriously injuring trees. This practice should be strongly discouraged. While scraping off a layer of topsoil from the immediate building site and piling it for use in landscaping around the finished house makes a lot of sense, do not extend this practice away from the immediate excavation site. The damage from bulldozers nicking or skinning the trunks of trees is obvious and highly visible, though not necessarily more serious than the hidden damage to roots caused by the same grading. Even though the symptoms of damage from grading are hard to distinguish from symptoms of other problems, you can pretty much count on

any bulldozer grading around trees causing damage.

Interestingly, the planting of lawns around trees after grading can compound the injury to trees. Grasses are more successful competitors for nutrients than trees, according to Dr. Terry Tattar, a professor of plant pathology at the University of Massachusetts and author of

Diseases of Shade Trees, especially in poor soils. The roots of grass may even extend deeper than the feeder roots of trees. If you scrape off the existing topsoil—including feeder roots of the trees—replace it, and then seed the area for lawn, the grass roots will outcompete the tree roots for nutrient and water uptake.

 

Trenching

Rather than trenching for buried pipes or cables, consider tunneling underneath major roots of trees.

Rather than trenching for buried pipes or cables, consider tunneling underneath major roots of trees.

Digging a trench by a tree can be the most damaging action of all, since it will sever all those roots intersecting the trench. The amount of injury from trenching is relative to the distance of the trench from the tree, as well as the depth of the trench. Obviously, a trench right next to the trunk will do a lot more damage than a trench out near the drip line. To avoid damage from trenching, there are two options: relocating the trench, or tunneling under roots. For a buried electric or gas line, your usual approach might be to keep the run as short as possible to save money in materials and trenching, but keep in mind that trees are valuable. The extra cost of digging the trench around valuable trees will usually be more than recovered through the direct economic value of the trees. Tunneling under roots can be accomplished with a power soil auger. Again, this extra effort may be expensive, but it is usually justified economically. When trenching is done and roots are severed, cut the roots cleanly with a saw or clippers, and refill the trench as soon as possible to keep the surrounding soil from drying out, further damaging the remaining roots.

 

Soil Compaction

While generally less damaging than grade changes and direct mechanical injury to roots and trunks, soil compaction can also cause serious tree injury. Both vehicle and foot traffic can compact soil, reducing its ability to absorb surface water and provide for gas exchange. When growing tree roots encounter compacted soil, they often turn away just as they will from barriers such as boulders or a foundation wall. Compaction is likely to be most severe in heavy clay soils, because they are poorly drained and poorly aerated. Symptoms of tree injury from soil compaction cannot be distinguished from the other types of damage discussed above.

 

What to Do if Tree Damage Has Occurred

As mentioned above, determining the exact cause of tree damage is often extremely difficult. The symptoms from different types of injury are the same, so you can usually only determine the cause by figuring out what type of sitework was done. If the cause of injury can be determined, some corrective measures can often be taken to improve the chances of survival of the injured trees.

When roots are injured or severed, estimates should be made of the extent of damage. If 20% of the root system is damaged, for example, the crown should be thinned to compensate— otherwise the crown will require more water and nutrients than the roots can provide (termed a “root:shoot imbalance”) and dieback can result. Phosphate fertilizers aid in root development and should be applied by a professional arborist. Avoid high- nitrogen fertilizers until a few years after root damage has occurred to prevent unbalanced shoot development. Injured trees should be watered regularly during dry periods to compensate for reduced water uptake by the roots. Finally, mulch the area under the drip line to minimize competition from grasses in nutrient and water uptake and to permit optimal gas exchange. If compacting has occurred, the soil can be aerated to a depth of 6-8” with a crowbar or needle-type soil aerator. For best results with all such measures, call in the services of a professional arborist. Your investment in those services will be returned many times over through enhanced property value and the other benefits of healthy trees.

 

– Alex Wilson and

Dan MacArthur

For additional information on avoiding damage to trees during construction and what to do about it, consult

Diseases of Shade Trees by Dr. Terry A. Tatter, revised 1989, 391 pages, Academic Press, Inc., San Diego, CA 92101. Dr. Tatter is a professor of plant pathology at the University of Massachusetts in Amherst and a private consultant on tree health. In addition to a chapter on construction injury and soil compaction,

Diseases of Shade Trees covers tree diseases and decay, insect damage, temperature and moisture stress, chemical and air pollution injury, and general strategies for maintaining healthy trees. Though expensive ($60), an investment in this book will be returned very quickly if you learn enough to save just a tree or two.

Diseases of Shade Trees can be ordered by calling 800/321-5068.