Root Cuts

Urban environments often have trees planted in straight rows in order to make the space greener and smooth out its rough edges. And often, that landscape would have to grow by way of road widening, drain improvement, etc., and some trees would have to go. So what would go into the decision of which trees to be removed, and which could be retained?

Construction activities such as demolishing, building, excavation and heavy vehicles traversing the site would damage and cut roots, which diminishes tree health and/or structural stability.

When roots must be cut, they should be cut as far away from the trunk as possible to reduce impact to tree health. If the root space is permanently lost as a result of lowering the grade or construction of a structure, then the portion of root system cannot be replaced, and stress and stability issues may not be overcome.

Before any works, it is a good idea to locate roots by hand digging or excavation using compressed air tool. A new technology, ground-penetrating radar (GPR) can be used to detect roots as deep as 0.9 m and with a diameter as small as 1 cm. However, the size of roots cannot be determined, two roots close together cannot be distinguished, and the soil type is ideally in sandy soil. Resolution and depth of detection is reduced with soil with high water and clay content, which is the case in Singapore's soil.

In nature, root injuries are common and roots have evolved to have strong compartmentalisation. Just as flush-cutting branches is no longer an acceptable practice, pruning a root to its point of origin should not cut into the parent root. And preferably beyond sinker roots, and several smaller cuts instead of one large-diameter cut. Tools that are not sharp enough to cut roots such as excavators are used, they would result in torn and crushed. Such damaged roots should be pruned with a clean cut instead of being left as they are.

Trenching in a linear manner at a distance from the trunk that is less than three times the trunk diameter (or DBH x 0.5 x 5 from the center of trunk) can reduce stability. Adventitious roots may be formed by trees with root loss and are able to provide them with water and nutrients to maintain green foliage. However, those roots are too small to support the weight of the tree and may result in tree failure in wind conditions. Tree support systems and tree removal are possible remedies.

More research is needed to determine and recommended specific practices on the factors that affect decay development after root pruning: root diameter, distance from the trunk and presence of heartwood on the cut surface.