Module 2: How Trees Grow, Survive, and Thrive
If we are to understand how trees may become hazardous, we must first understand some general concepts: how trees grow and function, the root system model, response of trees to wounding, the nature of tree defects, and the significance of individual tree characteristics.
How Trees Grow
Trees are 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.
Trees capture solar energy (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 graphic at right).
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.
Understanding 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.
