Trees are generation organisms, generating new tissue on the outside of the old tissue with each growth increment – Growth Rings.
The tree trunk, branch or twigs – the woody bits, are called Xylem tissue. All of these are built up around a pith, normally central. Each growth cycle lays down new wood on the outside of the previous years wood. At growth flush the cell division is rapid, producing large open vessels. The light rings in the picture. The slower growing tissue is the darker rings. The length of the growing season determines the width between each growth flush. The environmental conditions determine the length of the growth period – the better the conditions the longer and wider the ring.
When wounding occurs the tissue that has already been laid down reacts to the wounding. The Cambium – the layer of meristematic tissue between the inner bark and the wood, also responds by producing a different type of tissue, Barrier Zone. The Barrier Zone isolates the tissue present from tissue produced after the wounding event. It also produces a different type of wood at the wound site called Wound Wood. Wound wood is the tissue that grows over and may finally close the wound site.
The Barrier Zones shown in the picture of the Ginkgo isolates each tree from the next. Once isolated by the barrier zone the starch storage sites are no longer accessible to the current tree. Thus if the Ginkgo tree in the picture had no wounding history all of the trunk would be available for storage and retrieval of starch. But in this case only the tissue on the outside of the large arrow would have been available.
The picture to the right shows the same barrier zone formation in Eucalyptus. The black arrows show a full circular barrier zone followed about 5 growth flushes later with another, Red Arrows. The green arrows show a crack that has formed from the end of the second barrier zone out, note the surface swelling. The wound wood has formed Rams horns, which also have internal decay.
The Rams Horns and a cavity are the end result of the total decay of the tissue present at the last Barrier Zone formation. The other “Wall” of Compartmentalisation, 1: Vessels, 2: Dense growth ring and 3: the Ray tissues have all fallen to decay in this area.
If trees did not have walls and barrier zones decay would move at will throughout the tree. We know this is not the case as decay follows predictable paths and over extended time frames. Fighting against decay takes energy. Compartmentalising tissue leaves energy behind for pests and pathogens to use as their food supply. White ants only attack wound altered wood, other ants use basal decay of roots and tree trunks as nest sites.
