Confined Space for Root Systems

The root system of a tree is often depicted as the mirror image of the crown but in reality it can extend beyond the drip line as much as two to three times the diameter of the crown. (The drip line is an imaginary line on the ground under the outer edge of the crown of the trees.

Trees growing in urban spaces such as streets, parking lots, commercial and other areas usually do not have enough space for their roots. Building foundations, curbs, retention walls, containers and paving can all restrict the full growth of a tree's roots.

Trees exist in the spaces left over after other infrastructure is in place. Often they are planted in containers and median strips. Trees growing in such spaces are more subject to girdling roots, drought effect, and other secondary problems such us pests and disease.

Exposed/Surface Roots

Exposed or surface roots are often seen on urban trees. Often this problem is related to compacted soil condition or soil erosion or is a result of heavy use of the space close to the tree. Since the major portion of the absorbing root system of a mature tree is within the top 90 cm of soil, and most of the fine roots that are active in water and nutrient absorption are in the top 30 cm (Morgan, R., 1993 the roots most susceptible to exposure are those most critical to the health of the tree.

Girdling Roots The normal pattern of tree root growth is horizontal to the ground surface and radially away from the trunk. The pattern of girdling roots is to grow tangentially to the trunk, and in many cases, upwardly. This abnormal root growth causes physiological stress on the expanding tissues as the trunk and roots grow in diameter (Figure 1). Eventually the root collar and roots may become constricted causing decline in the condition of the tree and even resulting in the death of the tree (d'Ambrosio, 1990). Figure 1 Girdling root symptoms are found more often on park, street, yard, and trees in nursery trees than on forest trees. Some tree species such as transplanted Norway Maples seem more prone to the problem than others (d'Ambrosio, R.P. 1990). Pirone (1972) indicates that Sugar Maple, Oaks, Elms and Pines are also prone to girdling roots. Many symptoms can indicate girdling roots. These include weak foliage and defoliation. However, the focus here will be on symptoms such as abnormal swelling of the trunk (expanding trunk restricted by a girdling root), and the lack of normal butt flare at least part way around the base of the tree. Trees with a root that girdles the lower trunk usually have little or no flare on one side of the tree at the ground line. You should keep in mind that if soil is spread around a tree, the butt flare may also be buried. In this case, the effect will be the same all around the tree. In some cases, girdling roots can be easily seen on the surface (girdling roots below the root collar). Tree ProblemsRoot Trenching