Tree Biology

Trees are perennial plants that grow year after year after year. They have one stem (trunk), and are generally considered either hardwoods or conifer. Hardwoods trees experience seasonal loss of all of their leaves. Their leaves are usually broad and thin, which is why they are sometimes referred to as Broadleaf trees. Conifers - also called Evergreens - keep their either all or a part of their leaves year-round and usually grow in a conical or pyramid-like shape.

Tree shapes differ as well, frequently based on the sunlight and water conditions of the area in which they grow. Some trees grow quite tall and straight. Some smaller trees, referred to as understory trees, collect indirect or fragmented sunlight through openings in the overhead canopy of larger trees. Nearer to the equator, the noontime sun is almost directly overhead year-round, and so tall trees with flat treetops (or crowns) are very common. The flat shape helps expose more leaves to the direct, overhead sun. Nearer to the Artic circle, however, the sun lays close to the horizon. Trees in that region tend to be cone-shaped, in order to make the most of the available light. Artic trees also sport needles (rather than flat leaves), which have a higher capacity to retain water.

trunk/stems, which provide support, transport, and storage regulation.

roots, which provide support, transport and storage.

leaves, which provide food for living cells.

The trunk supports the tree crown, gives the tree its shape and strength, and produces the bulk of a tree's useful wood. The trunk consists of four layers of tissue, and each of these layers contains a network of tubes that runs between the roots and leaves. Some transport water and minerals from the roots to the leaves, and others distribute sugar (glucose) from the leaves to the branches, trunk and roots.

Heartwood - As a tree grows, older xylem cells in the center of the tree become inactive and die, forming heartwood. Because it is filled with stored sugar, dyes and oils, the heartwood is usually darker than the sapwood. The main function of the heartwood is to support the tree.

Xylem/Sapwood - The xylem, or sapwood, represents the youngest layers of wood. Its network of thick walled cells carry water and nutrients from the roots to leaves and other parts of the tree through a tubular network inside the trunk. With age, these cells decline and die, to become part of the heartwood. Sapwood is further divided into "early wood," the clear yellow area of rapid cell growth; and "late wood," as cell growth slows down and compresses to form the dark area of the ring (not necessarily on an annual basis).

Cambium - Cambium is a very thin layer of growing tissue that generates new cells, which will have the capacity to become either xylem, phloem, or additional cambium. Every growing season, a tree's cambium adds a new layer of xylem to its trunk, producing a visible growth ring in most trees. The cambium is what makes the trunk, branches and roots grow larger in diameter.

Phloem/Inner Bark - The phloem is located between the cambium and the outer bark, and acts as a food (sugar and nutrients dissolved in water) supply line from the leaves to the rest of the tree.

Bark - Bark is the visible outer covering of a tree's trunk, branches and twigs. It is actually a layer of dead phloem cells, shed outward to act as a shield against insects, disease, storms and extreme temperatures. In some species, the outer bark is effective as protection against fire.

A tree's roots absorb water and elements from the soil, store food and anchor the tree upright in the ground. All trees have lateral roots that branch into smaller and smaller roots, and usually extend horizontally beyond the branch tips. Some trees have a tap root that reaches down as far as 15 feet. Each root is covered with thousands of root hairs that facilitate water and mineral absorption from the soil. The majority of the root system is located in the upper 12-24 inches of soil, because the oxygen roots require to function property is most abundant and accessible at these depths. REMEMBER that, underground, a tree's root system normally spreads 2-3 times the height of the tree.

The crown, which consists of the leaves and branches at the top of a tree, plays an important role in filtering dust and other partricles from the air. It also helps cool the air by providing shade, intercepts rainfall, and reduces erosion.

Leaves function as a tree's nutritional factory through a process known as PHOTOSYNTHESIS