Tree Care 101

Rough and Ready Guide to
Basic Tree Biology – Above Ground

What’s A Tree?.
Attempting to define a tree is as difficult as defining a party. Trees and parties can come in a variety of sizes and take on a staggering array of appearances. It’s safe to say that a tree is like a perennial that doesn’t dieback after its annual reproductive cycle. Unlike a perennial, trees have another way of increasing in size (girth). A tree usually achieves a height over 15 feet and usually has one trunk with a diameter at breast height (4 ½ feet) of at least 3 inches. Even a vague definition like this excludes a number of small-multistemmed trees and bonsai specimens, and even promotes some unusual shrubs into the ranks of trees. Trees and shrubs in urban settings are usually called “woody ornamentals.”

How Do Trees Grow?
There are two areas, called meristematic tissues, on trees where new growth can occur. The first area is found inside of buds (visible and latent) and at root tips. Following cell division new tissues emerges resulting in an increase in the length of a branch or root. The width of a tree increases by a thin layer of cells forming a ring called the cambium located just beneath the bark of a tree. When cells in this cambium layer divide they produce phloem cells on the outside and xylem cells on the inside. The phloem cells transport sugars throughout the tree. The xylem cells serve as a series of very narrow collection of pipes allowing water to climb to the leaves and evaporate. The living portion of a tree’s trunk consists of the thin layer of cells in the cambium and phloem layer. The xylem cells are dead but remain in place because of their strong cell walls so each year the cambium layer is pushed outward. The phloem cells will eventually undergo some chemical and physical changes and become the tree’s bark. This is why string trimmers and lawnmowers that repeatedly damage the base of a tree will eventually “girdle” the tree. Although water may still be traveling up to the foliage, sugar is unable to travel up or down the trunk.

Twigs (Branches)
Isn’t most of a tree just a collection of twigs and leaves? Twigs have a simple biology but one that is crucial to understanding how a tree grows.
When a twig increases in length from the terminal bud, a circular ring will remain marking the location of that particular terminal bud. These terminal bud scale scars can be used to determine the rate of growth for a tree. Usually a lateral bud will be located above a leaf scar on 1 and 2 year old portions of a twig. These buds are usually inhibited from growing by the terminal bud (apical dominance). The chances of these lateral buds sprouting improve with increased distance from the terminal bud. Keep this principle in mind when pruning. When you cut back a twig, try to make that cut just above a lateral bud. Before this new terminal bud can exert apical dominance several other lateral buds in this area may also begin to rapidly grow.

Think of twigs as miniature versions of trunks. Twigs also have a living layer (cambium and phloem) and a dead cylinder (xylem cells). Like us, the living non-photosynthetic cells in a twig need oxygen to live and need to get rid of their waste products (carbon dioxide).  This gas exchange is performed through lenticels, which are raised areas of loosely connected bark cells.

Leaves have evolved over millions of years to maximize the blade’s surface area while developing structures to minimize water loss. This balancing act has produced a wide variety of leaf shapes. All leaves have a protective cuticle or waxy skin on the outside of the blade to prevent desiccation of the leaf. In order for gas exchange to occur, leaves have stomata (“little mouths” in Greek) that can open shut to help the tree regulate water loss and gas exchange.