Trees and construction activities (new buildings, roads, and utility lines) often interact non-constructively, resulting in damage or death to trees. Injury to above ground parts of trees (trunk and crown) and its effect on the health and vigor of trees is easy to see and evaluate. It is easy to avoid damaging the above ground portion of a tree because of its high visibility. However, during construction, damage to the root system and/or the soil around the root system results in more loss and death than above-ground damage during construction. Root and soil damage is less obvious and may result in tree death or damage which may not be evident for as long as 8 to 10 years.
Soils and Tree Growth
Most of the non-infectious disorders of trees in Iowa are related to the roots and/or the soil. Soils have physical, chemical and biological characteristics or systems which 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-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 of 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 together, 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 top soil. Root growth will often be slowed by soil layers which are significantly 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 are made up of large permanent roots (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 are functional 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 will often extend out from the trunk in an irregular area which is 4 to 7 times larger than the area of their crown.
How to Minimize 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 least at the outer dripline 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 which will 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, removing ground covers, and many others.
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 and/or use fill material which is well drained. If possible build structures on posts rather than footings or extensive foundations. Strive to minimize activities which cut roots or adversely affect soil characteristics.
Trees vary significantly in their tolerances to construction damages. In addition to species differences, the root distribution pattern for each individual tree will affect its tolerance to construction. If most of a tree's roots are located away from the construction zone, it will suffer 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.