Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
10/21/02
Contacts:
Paul Wray, Extension Forester, (515) 294-1168, phw@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Yard and Garden Column for the Week Beginning Oct. 25, 2002
Reducing Damage to Trees during Construction
By Paul Wray
Extension Forester
Iowa State University
Trees and construction activities often interact nonconstructively resulting in damage and death to trees. Injury to aboveground parts of trees and its effect on the health of trees is easy to see, evaluate and avoid because of its high visibility. However, damage to the root system or the soil around the root system results in more loss and death than aboveground damage during construction. Root and soil damage is less obvious and may result in tree death or damage that is not evident for as long as 8 to 10 years.
Soils and Tree Growth
Most of the noninfectious disorders of trees in Iowa are related to the
roots and the soil. Soils have physical, chemical and biological characteristics
or systems that can be changed by construction activities. Ideal soils
for tree growth consist of about half of its volume composed of solid
matter, including mineral particles and some organic matter. The other
half consists of pore spaces filled in varying proportions with air and
water.
Soil texture refers to the size of individual mineral particles of clay, silt or sand. Soils with high clay content, will hold more water for plant growth, yet may be more susceptible to soil compaction. Sandy soils hold less water and are less susceptible to soil compaction. Water and air movement and the ease of root growth through soils are determined by pore size, not necessarily the size of individual mineral particles. This soil structure is an indication of the degree or level to which soil particles are aggregated, creating more pore space. In general, the more structure a soil has, the more desirable it is for tree growth. Bulk density of a soil is the weight of a given volume of soil; soil compaction increases the soil bulk density, resulting in less pore space and greater difficulty for tree roots to grow and expand.
Many of our urban soils have been disturbed. These soils are often shallow with little soil depth and often consist of subsoil material covered with less than 6 inches of topsoil. Root growth is often slowed by soil layers that are substantially different than the soil above or below. For example, a sandy surface soil on top of a compacted clay layer may be excessively wet at times because of poor drainage and droughty during dry periods because of poor root penetration.
Tree Roots
Tree roots are made up of large permanent roots that provide mainly anchorage
and transport and many small, temporary feeder roots and root hairs. These
small roots are the primary water and nutrient absorbers. They function
for only 1 to 3 years and then either die or become part of the large
root system. Most trees replace at least one-third of their feeder roots
each year.
Tree roots do not penetrate soils to great depths. Most roots will concentrate in the top 6 to 18 inches of topsoil. Tree roots occupy a larger area than previously believed. They often extend out from the trunk in an irregular area that is 4 to 7 times larger than the area of their crown.
Minimizing Tree Damage
The surest method of minimizing damage to a root system or soil during
construction is to do nothing around, in or on top of a tree's root system.
Construct a sturdy fence at the outer drip line of the tree or trees to
be saved and allow zero activity within this area. The larger the area
of zero activity around the tree, the less damage. The more this zone
is violated by construction activities, the more the tree may be damaged.
Activities that damage tree roots are cutting or stripping surface soils,
lowering grades, trenching, parking or operating any machinery in this
area, storing supplies, filling or storing soil or excavation materials,
building sidewalks and streets, creating sand or gravel piles over the
roots and removing ground covers.
In most cases, zero activity in the root zone is not possible. If construction activity cannot be avoided, strive to minimize their affect on both the root system and the soil. For sidewalks minimize any grade changes and excavation. When trenching for utilities, bore under the root system instead of cutting through roots and combine utilities in the same trench where possible. Use retaining walls as much as possible instead of complete site grading. Minimize compaction by working when soils are dry and using hand equipment instead of large mechanical equipment. When filling around a tree, make provisions for both air and water drainage and supplies or use fill material that is well drained. If possible build structures on posts rather than footings or extensive foundations. Strive to minimize activities that cut roots or adversely affect soil characteristics.
Trees vary in their tolerances to construction damages. The ranges presented in the chart are based on my observations. In addition to species differences, the root distribution pattern for each individual tree affects its tolerance to construction. If most of a tree's roots are located away from the construction zone, it suffers minimal damage.
Tree saving practices during construction can be both time-consuming and expensive. First, make sure that the tree is worth saving; it should be a desirable species, in reasonably good health, and have a projected life expectancy of at least 20 years. In some cases, removal and replacement after construction may be the most viable alternative.
| Tree Tolerance to Root Damage | ||
| Very Sensitive | Moderately Sensitive | Fairly Tolerant |
| Oaks | Sugar maple | Silver maple |
| Hickories | Black maple | American linden |
| Kentucky coffee tree | Ashes | Cottonwood |
| Larch | Walnut | White poplar |
| Pines | Sycamore | Willows |
| Serviceberry | Red maple | River birch |
| Redbud | Cherry | |
| Spruces | Hawthorn | |
| Firs | Ironwood | |
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