Trees by Bob Watson

Author:Bob Watson
Language: eng
Format: epub
ISBN: 9781847977335
Publisher: Crowood
Published: 2014-01-29T16:00:00+00:00

Longitudinal wound with seam of woundwood on Chorisia speciosa (Kapok tree).

A fully occluded wound.

An occluding wound showing woundwood progression.

Close-up of woundwood around a lateral branch cavity showing its site of production is beneath the old existing bark.

Seams and woundwood.

A tree developed from a bundle planting.

Ongoing years of woundwood production will eventually occlude the wound unless the damaged/ wounded area is much too large to occlude fully, or the vigour of the tree is low. Break-out (tear-out) cavities are formed when a branch union or co-dominant stem union collapses, and they always cause extensive tissue damage and expose large surface areas of both old and young tissues, and because of the extent of the damage (especially the depth of tissue damage), they rarely occlude. Callus production often commences very vigorously in these cases, but even the relatively high volume of callus and woundwood produced cannot occlude the deep cavity created. Wound occlusion is also prevented if the callus cells die due to infection, or the bark and underlying tissues around the wound are already dead.

When a branch is removed from a tree, the vascular cambium (the tissue between the xylem and phloem responsible for new cell production) is exposed. The sudden lack of containment by the outer tissues, and the change in environment created by the sudden exposure of the internal tissues, triggers the vascular cambium into producing callus. Callus soon changes in nature, and the process of change (differentiation) involves both the inclusion of lignin (a tough, waxy material) being laid down on the inside of some of the cell walls, and drastic changes in cell shape. Ultimately the callus cells will differentiate into woundwood having all of the tissues associated with a woody stem (including bark, the periderm).

Wounds around old branch unions will often produce epicormic shoots (sprouts), as some of the cells of the original callus may differentiate into complete shoot systems if the environment and physical factors are favourable – particularly light.

Bacterial Wet-Wood and Slime Fluxes

Small cracks in the side of a main trunk or lateral limb occlude readily if the cambium is healthy and vigorous. In some instances, however, small cracks may be reluctant to occlude, remain open and exude a watery fluid, even on an otherwise healthy tree. These cracks are associated with bacterial activity in the tissues of the tree, and although the low-oxygen environment deep in the tissues suppresses bacterial activity, it does not preclude some degree of bacterial action. The infection causes the surrounding tissues to become wet and exude a fluid known as a slime flux, which at times runs down the bark of the tree and is typified by a dark wet line down the tree. The slime flux is toxic to green tissue, so kills all of the epiphytes in its path, and methane gas can sometimes emanate from the crack (which in extreme cases can build up and may be ignited with a match). Bacterial wet-wood is not normally a major problem – it may be aggravating to the tree but is not life-threatening, and it is not usually treated.

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