In the horticultural sense, a tree ring is an area around the trunk of the tree that is maintained as a barrier between the surrounding lawn and the tree's base to avoid unintentional damage from mowing equipment. This area is generally kept mulched or planted with a ground cover or other ornamental plants. "Tree ring" is also a colloquial term for the annual growth rings found in a tree's wood.
Tree rings provide an important purpose in protecting the tree, since being bumped by mowers or hit by rotary weed trimmers can cause severe damage to the base of the tree by either crushing or removing the cambium layer (thus causing a dead area), or by simply exposing the cambium which makes the tree susceptible to infection or insect damage since the protective bark is removed. Eventually this can cause the parts of the tree above the damaged area to die, or in extreme cases the whole tree can die due to disease or unintentional girdling.
Tree rings should at a minimum be at least 1 foot (30 cm) beyond the flare of the tree's base, though wider is better. If there are surface roots beyond the tree ring that are being hit by mowing equipment, the ring should be expanded to include those roots, since scarfing the roots also created open wounds and thus make the tree susceptible to infection.
While tree rings are often maintained by simply maintaining a layer of mulch, the use of ground covers is a much better option in most cases, since the presence of a ground cover will discourage people from walking on the soil within the ring, and so helping to avoid soil compaction. Dense ground covers such as Pachysandra terminalis, Epimedium, and so on are the best choices, since they require less weeding and so less foot traffic within the ring.
In cases where surface roots continue to appear outside of an existing tree ring, vertical mulching can be used to provide better air, water, and nutrient circulation within the deeper soil, which will encourage the tree to grow deep roots rather than surface roots.