Ecology and significance of ageing trees

This topic is considered in detail by Fay (2002), who also considers the management of veteran trees and outlines the valuable work done in recent years by the Ancient Tree Forum (ATF) of the UK. Trees undergo complex changes during their very long lives, during which they play a series of different roles in the ecosystems they largely dominate. Though many foresters are mainly concerned with timber production and therefore in comparatively young trees, dead wood, whose decomposition is considered in Chapter 7, is essential at some stage in the life histories of many associated organisms and it is important to retain this saproxylic habitat. Humphrey (2005) suggests that the many spruce plantations in the British uplands (a mixture of Norway and Sitka spruce) develop features of old growth, including appreciable dead wood, after 80-100 years, not much longer than the normal stand rotation for wood production. At Windsor Great Park, southern England, an important site for insects of dead wood, felled trunks have been re-erected to increase the volume of habitat available (thus replacing the standing dead hulks left in truly natural forests where safety is not a consideration). This is important since wood in contact with the ground is often wetter and cooler than standing hulks, and so a habitat for different organisms. Wildlife here has also been enhanced by creating simple cavities for birds and bats. Additional nesting sites have also been produced by tying cut branches with suitable cavities into crown limbs. The leading figure behind this work considers that ancient trees could well be designated as Sites of Special Scientific Interest (Green, 2001). Some conservationists go further and suggest that healthy trees be deliberately injured by drilling or cutting to encourage pockets of dead wood and collection of water to create habitats for invertebrates and fungi (see Section 7.7.2).

Apical dominance is a major feature in the life of a tree as its branch and twig structure ramifies and becomes more complex in developing from a seedling to early maturity. The root system also develops rapidly at this stage, exploiting the available moisture and nutrients in the soil. The crown of the tree loses some peripheral vitality and becomes increasingly rounded during full to late maturity. A degree of branch loss is associated with dysfunction of conductive tissue. In the inner crown, however, there is often reiterative growth in which new branches grow vigorously towards the sky and are served by compart-mented vascular channels linked to the root system. Basifugal mortality, in which the outer crown starts to die back, is accompanied by incipient root death with increased fungal colonization leading to internal trunk decay from below at full maturity.

It is the final stages, during which the living crown becomes denser and lower, that are of particular interest when considering the physiology and ecology of veteran trees and their influence on the ecosystems they inhabit. The outer crown continues to degenerate so that the outer and uppermost branches become leafless and skeletal; often falling during storms. New shoots develop from dormant or adventitious buds on the stem and main branch system, so primary production remains adequate, although the bark circumference becomes discontinuous as areas of vascular dysfunction in the main trunk develop further. In this strange last stage much of the ancient tree is dead or dying, while other regions grow vigorously. The continued life of the tree is now largely dependent on the mechanical stability of the crown and root anchorage; the two being united by the vascular columns of the trunk that serve the recently formed reiterative shoots that may effectively act as mini-trees borne on the remains of the ancient trunk. Box 3.3 gives more detail on how veteran trees can hang onto life.

Box 3.3 Phoenix regeneration: new trees from old

Beautiful Pencil Drawings Landscape

Figure 3.24 Regeneration from the trunk of an English oak blown down in the storm that struck south-east England in October 1987. When photographed in late May 4 years later there were vigorous shoots along the trunk and the production of a number of new trees from the old trunk was about to commence by this means. (Photograph by Rodney Helliwell.)

Figure 3.24 Regeneration from the trunk of an English oak blown down in the storm that struck south-east England in October 1987. When photographed in late May 4 years later there were vigorous shoots along the trunk and the production of a number of new trees from the old trunk was about to commence by this means. (Photograph by Rodney Helliwell.)

Veteran trees frequently reproduce by means of seeds; under certain circumstances the cambial net may also enable them to give rise to one or more 'new' individuals vegetatively. The several different types of 'phoenix' regeneration involve tree survival strategies in which active new trees develop from the remains of damaged trees or disintegrating ancient individuals, often as a result of the effects of increased light following frequently natural clearance. Under such circumstances dormant meristems may be activated with the result that reiterative growth occurs and a new crown is produced. Extensive trunk layering can occur as, for example, when beech blew over in the Forest of Fontainebleau, France; existing lateral branches developed into main trunks that acquired effective root systems of their own. Occasional examples of trunk regeneration, in which adventitious roots develop inside a disintegrating ancient stem, finally giving rise to a new succession trunk, occur in most broadleaved tree species. Sometimes almost the entire trunk of an ancient tree will die and decompose, but small active columns remain and are sufficient to form a small diameter trunk supporting a rejuvenated crown. Ash,

Was this article helpful?

0 0
Worm Farming

Worm Farming

Do You Want To Learn More About Green Living That Can Save You Money? Discover How To Create A Worm Farm From Scratch! Recycling has caught on with a more people as the years go by. Well, now theres another way to recycle that may seem unconventional at first, but it can save you money down the road.

Get My Free Ebook


Post a comment