Growth Rings
Growth rings (or "tree rings" or "annular rings") can be seen in a horizontal cross section cut through the trunk of a tree. Visible rings result from the change in growth speed through the seasons of the year, thus one ring usually marks the passage of one year in the life of the tree. The rings are more visible in temperate zones, where the seasons differ more markedly. The growth rings are responsible for the grain and knots visible in wood.
The inner portion of a growth ring is formed early in the growing season, when growth is comparatively rapid (hence the wood is less dense) and is known as "early wood" or "spring wood" or "late-spring wood". The outer portion is the "late wood" (and has sometimes been termed "summer wood", often being produced in the summer, though sometimes in the autumn) and is more dense. "Early wood" is used in preference to "spring wood", as the latter term may not correspond to that time of year in climates where early wood is formed in the early summer.
Adequate moisture and a long growing season results in a wide ring. A drought year may result in a very narrow ring. Trees from the same region will tend to develop the same patterns of width for a given period. For the entire period of a tree's life, a year-by-year record or ring pattern is formed that in some way reflects the climatic conditions in which the tree grew.
Outer Bark - The area of the tree trunk composed of dead cells. It insulates and protects inner tissues from disease infections and drying.
Inner Bark (phloem) - Phloem conducts usable food from the leaves to the cambium - to nourish it - or to storage areas in the wood.
Cambium layer - A thin-walled layer of cells beneath the inner bark of a tree, made of living cells that continually divide and account for the tree's growth. When this is left in tact and the bark is removed, it will dry to a shiny dark finish.
Sapwood - Comparatively, new wood comprising living cells in the growing tree. All wood in a tree is first formed as sapwood. Its principal functions are to conduct water from the roots to the leaves and to store up and give back according to the season the food prepared in the leaves. The more leaves a tree bears and the more vigorous its growth, the larger the volume of sapwood required. Hence trees making rapid growth in the open have thicker sapwood for their size than trees of the same species growing in dense forests. As a tree increases in age and diameter an inner portion of the sapwood becomes inactive and finally ceases to function, as the cells die. This inert or dead portion is called heartwood.
Heartwood (Duramen) - The portion of the tree contained within the sapwood. The heartwood is dormant and unnecessary for the tree's continued life; the living part of the tree is contained in its outer parts. Usually darker in color, in some instances this distinction in color is very marked; in others, the contrast is slight. Heartwood is more resistant to decay than sapwood.
Pith - The soft tissue about which the first wood growth in a tree takes place; the central core.