Girdling, also called ring barking or ring-barking, is the complete removal of a strip of bark (consisting of cork cambium, phloem, cambium and sometimes going into the xylem) from around the entire circumference of either a branch or trunk of a woody plant. Girdling results in the death of wood tissues above the damage. A branch completely girdled will fail and when the main trunk of a tree is girdled, the entire tree will die, if it cannot regrow from above to bridge the wound.
Among the causes of girdling are human practices, including forestry, horticulture, and vandalism. Foresters use the practice of girdling to thin forests and orchardists use it as a cultural technique to yield larger fruit. Girdling can also be caused by herbivorous mammals feeding on plant bark and by birds and insects, both of which can effectively girdle a tree by boring rows of adjacent holes.
Forestry and horticulture
Like all vascular plants, trees use two vascular tissues for transportation of water and nutrients: the xylem (also known as the wood), and the phloem (the innermost layer of the bark). Girdling results in the removal of the phloem, and death occurs from the inability of the leaves to transport sugars (primarily sucrose) to the roots. In this process, the xylem is left untouched, and the tree can usually still temporarily transport water and minerals from the roots to the leaves. Trees normally sprout shoots below the wound, if not the roots die. Death occurs when the roots can no longer produce ATP and transport nutrients upwards through the xylem.
Ring barking techniques have been developed to disrupt or impede sugar transport in phloem, stimulating early flower production and increasing fruiting, and for controlling plant size, reducing the need for pruning.
Girdling is a slow process compared to felling and is often used only when necessary, such as in the removal of an individual tree from an ecologically protected area without damaging surrounding growth.
Accidental girdling is also possible and some activities must be performed with care. Saplings which are tied to a supporting stake may be girdled as they grow, due to friction caused by contact with the tie. If ropes are tied frequently to a tree (e.g. to tether an animal or moor a boat), the friction of the rope can also lead to the removal of bark.
The practice of girdling has been known in Europe for some time. Another example is the girdling of selective Douglas-fir trees in some Northern California Oak Woodlands, such as Annadel State Park, in order to prevent that fir from massive invasion of the mixed oak woodland.
Girdling is also used as a technique to force a fruit-bearing plant to bear larger fruit. A farmer would place a girdle at base of a large branch, and remove all but one fruit from that branch. Thus, all sugars manufactured by leaves on that branch have no sinks but the single fruit, which thus grows to many times normal size.
Both herbivores and birds can effectively girdle trees in the process of normal feeding.
In North America, voles in particular are prone to damaging trees by girdling both their roots and trunk. Among North American birds, the sapsuckers are the most common girdlers of trees. While sapsuckers will bore holes in tree trunks to feed upon insects, they also make parallel rings of holes in order to eat sap that collects in the openings or to feed it to their young. They most frequently attack pine, birch, maple, spruce and fruit trees and do the most damage during breeding season and territory establishment between February and June.
Many kinds of deer inadvertently girdle trees by rubbing their antlers on saplings and mature trees.
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