Construction and major landscape renovations can have a negative impact on existing trees, but the damage is often not immediately visible. We commonly see trees, in close proximity to the construction zone, die within five to seven years of building a new home. In many cases, trees should be removed prior to a home being built if proper protection is not feasible.
Changing the soil grade around existing tree roots, even a few inches, can cause permanent damage to trees. Soil cuts and fills occur when soil is removed (cut) or added (fill). Cuts and fills impact the roots’ ability to exchange oxygen and transport water. Tree roots grow closer to the surface in clay soils than they do in sandy soils. Cuts greater than two inches in clays soils cause significant root damage. Likewise, fills exceeding one inch of clay soil initiate root damage. Root death occurs with the addition of three inches or more of clay soil.
Trenching for utilities may also be damaging to trees. Trenching severs the root system, killing roots from the severed point outward. If many of the large anchor type roots are severed, the tree may become unsafe and prone to uprooting. A tree’s survivability will be impacted when 20% to 30% of the critical rooting zone is damaged.
Physical damage occurs when heavy equipment (bobcats, bulldozers, etc.) bump into a tree and causes obvious injuries to the trunk. The cambium layer which transports food and water is only protected by a thin bark layer. If the cambium layer is damaged, food and water transport will be affected. The tree will die if a significant amount of the cambium layer is damaged. Physical wounds cause permanent injuries to the tree that could lead to future problems with disease, insects, and wood decay.
Soil compaction is a silent killer of urban trees since you can’t see the problem. Tree decline from soil compaction may take three to seven years to appear. Stockpiled building materials, heavy machinery, and excessive foot traffic over the root zone all damage soil structure. It seems innocent enough, you park your truck in the shade to keep cool. However, by the time you have driven over the same spot a few times, the soil may reach 90% compaction. Lacking good soil aeration, roots suffocate and tree health declines.
Most of these injuries and root damage will not show up immediately as far as the tree’s appearance. However, several years after the building contractor is long gone, the damage will start to catch up with these trees, usually in the form of branch dieback in the upper canopy. The decline of a tree’s health is a slow process, but eventually accelerates into a downward spiral that cannot be reversed.
There’s very little that can be done to remedy the damage from these types of injuries. Large wounds take many years for the tree to compartmentalize and grow new bark, which exposes the tree to secondary problems for as long as it remains open. More often than not, moisture and wood rotting fungi establish in open tree cavities and accelerate the decay process. Over time, this will continue to weaken the structural integrity of the tree, making it a greater hazard to breaking.
Of course, there’s no way to truly predict if and when a tree will fail. A warning sign of impending failure is finding mushrooms or conks growing out of the main stem or buttress roots. By the time you see mushrooms growing, that’s a sure sign that extensive decay has already occurred.
To determine the minimum area for a tree protection zone, measure the diameter of the tree trunk at 4.5 feet from the ground. Multiply this value by 2.5 to determine the diameter of the root protection zone in feet. This is also known as the critical rooting distance. For example, if an oak tree has a trunk diameter of 20 inches, the tree protection zone is 50 feet (20 x 2.5). Another way to think about this is to protect an area extending 25 feet in all directions from the trunk of the tree.
Temporary safety fencing (orange plastic mesh) is recommended to exclude people and equipment from entering the tree protection zone during construction projects or major landscape renovations. Equipment is now available that can tunnel utility lines under trees without cutting roots. Instead of sidewalks or roads, consider creating a raised walkway or bridge over the rooting zone. With bridging, the only points of root disturbance are where the support pilings are anchored into the soil.
Not every tree can or should be saved. If a tree protection zone cannot be maintained for a particular tree, then it may be more economically feasible to remove the tree. A certified arborist should be contracted to evaluate the areas to be protected for weak trees and undesirable plant species prior to construction.
Paul Pugliese is the Extension Coordinator and Agricultural & Natural Resources Agent for Bartow County Cooperative Extension, a partnership of The University of Georgia, The U.S. Department of Agriculture, and Bartow County. For more information and free farm, lawn, or garden publications, call (770) 387-5142 or visit our local website at extension.uga.edu/bartow.