Trees are actually long-lived, woody, perennial plants. They are segmented organisms with rigid cell walls that restrict movement. The vertical orientation of tissues allows for movement of water and essential elements upward (kind of like a big straw!), while carrying carbohydrates down to the stem and roots. Each year a tree grows new living tissue on top of, and to the outside of, the previous year’s growth. This process can be described like a cone: a tree is simply a living layer of tissues on top of older layers, some of which are no longer living. Every year the tree adds another layer to the cone, both upward and outward. Trees have a genetically controlled life span influenced by the environment. This life span has been described as a growth curve, moving upwards as the tree captures energy and grows, peaking at maturity when it reproduces, then declining through old age and decay.
What Makes Trees Grow
Trees capture sunlight and convert it to chemical energy through the process of photosynthesis. This chemical energy is used or stored as carbohydrates such as sugars and cellulose. Energy is required to build structural tissues (wood) and to enable all living cells to respire (burn energy). Trees have growing living wood called sapwood, and more dense structural wood called heartwood (see left).
Compare young trees to mature trees- young trees are making and storing energy. The surplus energy enables them to combat stress and control decay. They typically have lower hazard potential and can overcome many stress-causing conditions. Mature trees must use their energy to protect the stored energy in their tissues. They typically approach the end of their life cycle in a spiral of decline- multiple stresses lead to further decline resulting in mechanical failure and/or death. Mature trees in decline frequently become hazardous due to their size and condition.
The Tree's Roots
Tree roots are shallow, they exist in the soil oxygen zone. Roots are extensive, spreading to several times the width of the canopy. Roots live in the soil with other organisms: other plants, fungi, bacteria. The graphic at right (courtesy of the International Society of Arboriculture) shows what tree roots really look like. Damage to roots is a major cause of decline, death, or physical failure. Roots are injured or destroyed by soil compaction, soil removal, severing roots, fill soil over roots, flooding or drought.
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.
When Trees have 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.