Module 2: How Trees Grow, Survive, and Thrive

Tree Response to Wounding
Unlike animals, trees have no wound healing process. Healing means to restore to a previous healthy state, to repair or replace injured tissues. Trees, with their rigid cell walls, are unable to heal injured or infected tissue. Trees seal off damaged tissue rather than heal it. When tree bark is damaged, as in this picture (left), microbes attack the plant tissue, and trees respond by creating walls around the tissue. This process is called “compartmentalization” and it occurs as the tree builds four walls around the injured area in order to preserve the rest of the tree.  How well the tree ultimately survives the wound will depend on how successful the tree is at compartmentalizing the damage.  You've probably seen trees with abnormal looking growth occurring around a wound.  That is called "callous" tissue, and it is a reaction that happens as the tree tries to seal (not heal) the wound.

The Nature of Tree Defects
Structural defect characteristics identified by tree experts include: poor branch attachment, ineffective compartmentalization response, lack of taper in stems or branches, stem defects such as cracks or deformities, reaction wood development, excessive or recent lean, and improper canopy development.  The photo at right shows what happens when one of these defects fails.

Human caused defects may be the result of improper arboricultural practices such as: topping or heading, excessive thinning, lack of branch spacing, poor cut placement or execution, crown raising or crown reduction. Other activities that may damage trees include construction, utility work, landscaping, grounds maintenance, or irrigation. Other living organisms may responsible for defects. These include small animals like birds and squirrels, plant diseases, insects, and even other plants such as mistletoe.

Individual Tree Characteristics
The hazardous nature of trees is dependent on certain characteristics that may be identified and even predicted. Tree species vary in their potential to be hazards. Weak wood or poor branching habit are characteristic of certain species but not others. The age of the tree (relative to the growth curve) is significant. Old senescent trees are more likely to fail than young vigorous trees. Size is an obvious factor. Large trees or large tree parts are more dangerous when they fail. Research is also beginning to describe failure patterns, determining which trees are more likely to fail in certain regions and which parts are likely to fail.