The roots of a tree are essential to its survival as a means of anchoring itself in the ground, and for the uptake of water and minerals from the soil. Clearly any obvious damage to the root system is a cause for concern. The vast majority of the tree’s structural support comes from the area of roots closest to the stem, known as the root plate. The root plate extends to approximately the same area as the crown (although there are more complicated methods of calculating it), and so any obvious death or damage to roots in this area, especially caused by trenching can seriously affect the stability of the tree.
Certain species, especially Cherries and their relatives (Prunus spp.), have very shallow roots which are easily damaged by lawn mower blades. Although the damage is unlikely to be terminal at first, repeated damage by mowing can allow the ingress of fungal or bacterial infection into the roots presenting a much more serious problem.
Like any other part of the tree, roots need to breathe. One of the most common causes of damage to roots in urban areas is caused by compaction of the soil. This is especially bad where a path or drive passes over the roots of a tree. Continued pressure exerted on the soil compresses any air spaces, preventing air from diffusing down to the roots. This may contribute to a lack of vigour in the canopy and can eventually lead to the death of the roots. Compaction can be alleviated by using an ‘air spade’ to fire compressed air into the soil, though a much lower impact solution is to apply a good layer of woodchip mulch, thus preventing the soil from being compressed so badly, and inviting earth worms into the area.
Flooding of the soil can create a similar (though more serious) effect to compaction, since air is unable to diffuse in great enough quantities through a body of water. Some species, for instance Willows (Salix spp.), Apples (Malus spp.) and Alders (Alnus spp.) are very tolerant of flooding; other species for instance Rowan (Sorbus aucuparia), Cherries (Prunus spp.) Hazel (Coryllus avellana) and Holly (Ilex aquifolium) may be seriously damaged very quickly once waterlogged.