beech, hawthorn, hornbeam and oak can all undergo trunk remnant rejuvenation of this type. Sometimes two trees can form from a single discontinuous parent trunk. Lateral layering arises from limbs which subside to ground level, give rise to root systems and form new trunks. Basal regeneration involves the formation of a new tree from the base of an old stem following partial or complete trunk collapse. Perhaps most remarkable is phoenix crown regeneration, which has been observed in yew, sweet chestnut, lime and oak. Here the collapsed crown roots and forms new radiating trunks. Natural crown restoration occurs when a tree's original canopy fragments, only to be replaced by a rejuvenated trunk and crown growth.
Now that the value of veteran trees has been more fully recognized, methods of enhancing their continued existence are being actively explored, notably by the ATF. Restoration pruning has been effective in promoting recovery in many species, especially where crown or root stability has been impaired. The aim is to increase the amount of light reaching particular regions of the crown by means of targeted pruning, with careful monitoring and further adjustment at intervals of 3-10 years. Inactive outer limbs are often trimmed to reduce stress on weak trunks using jagged 'coronet' cuts which simulate natural fracture and which seem to produce better stimulation of new growth than a clean saw cut (see Section 7.7.2). Natural fracture techniques are also used to shorten branches and thus maintain live stubs in suitable parts of the canopy. Attempts have also been made to promote adventitious bud formation by scoring branches with a timber scribe; it is too early for the value of this technique to have been proven. Many ancient trees in Britain have previously been pollarded (Section 5.7) and attempts to re-pollard after many years of neglect have been made to reduce the weight ofthe canopy and stimulate new growth. This is not always easy since complete re-cutting can kill the physiologically weak trees. Normally the canopy has to be cut in stages over several years.
In Sweden, which has the largest forest area in Europe and where it is the main exporter of forest products, a woodsman will test the internal condition of a tree by striking it with a hammer. By this process of 'sounding' it is possible to detect the presence of decay, a very necessary test since it is estimated that in southern Sweden 15% of the tree population is lost from fungal decay. It does not, however, provide information regarding the degree or stage of the decay. This information is vitally important in the case of ancient trees in public places, where a limited degree of decay may be permissible but safety is essential. Detection of trunk decay is a very complex subject and a wide range of methods and equipment have been used during the
150 years that this topic has been mentioned in forest literature. Ouis (2003) reviews the range of non-destructive techniques which can be used to detect decay in standing trees. Some employ vibro-acoustical techniques using either vibrations at frequencies within the acoustical bandwidth or sound waves at acoustical or ultrasonic frequencies. The second class of techniques uses methods based on electromagnetic radiation such as radar. Modern destructive testing methods in which only a small sample is extracted from the tree trunk are also of value.
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