morphologyext

What a tree looks like on the outside (external morphology) is a large part of its aesthetic appeal. The overall shape of each tree lends character to the landscape. The shape may be straight and upright like a redwood , spreading like an oak, or weeping like a weeping willow.

The primary shoot of the tree is the trunk; a tree can have one trunk or multiple trunks. A critical part of the trunk is the point where it emerges from the soil. This part of the trunk is called the crown. It is important that the soil level is not changed around the crown; many trees are also damaged if the crown is constantly wet by irrigation. Damage to this area results in the death of the tree.

Valley oak , Quercus lobata, crown damage. Note dead ivy on trunk.

The trunk can be straight with a single growing point on top called the leader ("excurrent" growth form), or it can branched with multiple leaders ("decurrent" growth form).

Douglas fir, Pseudotsuga menziesii, (left) has an excurrent growth form with a single leader. Ginkgo biloba (right) has a decurrent growth form with multiple or "codominant" leaders.

Young trees naturally start out with a single leader. As they age, they either retain a single leader or branch and grow with multiple leaders. With some exceptions, a young tree from the nursery should still have its leader which will naturally be retained or lost with age. Trees with the leader cut off may have poor branch spacing along the trunk resulting in damage as the tree ages. An exception to this would be fruit trees.

Branches that grow off of the trunk are called primary scaffold branches, and the branches growing off those are secondary scaffold branches. These branches give the tree its basic shape. Branches beyond the scaffold branches are called lateral branches, and these give the tree its outline. The resulting top of the tree is called the canopy, and the edge of the canopy is called the drip line.

The drip line can assist you in determining the underground extent of another important organ, the tree's root system. The tree is kept alive by small, fine feeder roots that absorb water and minerals. These roots are generally in the top 6 to 12 inches of the soil, and extend out 2 -3 times the drip line radius. These are the roots that keep the tree alive, so it is critical that activities resulting in compaction (by people or equipment) or excess water (irrigation systems) do not compromise this area.

STUDENT RESEARCH QUESTION:
23. What causes crown rot in California native oak trees and how can it be avoided?

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