How Tree Rings Form

Trees have two types of growth: primary growth and secondary growth.

· Primary growth occurs at the tips of roots and stems and results in their growing taller or longer.

· Secondary growth takes place in the vascular cambium and the cork cambium and results in an increase in the diameter of the stem or trunk of the tree.

Cambium lies between the old wood and the bark of the tree. The vascular cambium is a thin layer cells that produces conducting cells – xylem and phloem.

· The phloem is the outer layer, and is sometimes referred to as the inner bark. It is a food conducting tissue. The xylem is located toward the inside of the cambium layer.

· The xylem is the vascular tissue through which most of the water and minerals of the tree are conducted.

More secondary xylem (added toward the inside of the cambium layer) than secondary phloem (added toward the outside of the cambium layer) is produced by the cambium. The definition of wood is secondary xylem, reflected by the origin of the term ‘xylem’, the Greek word xylon, for wood.

Another layer, the cork cambium, contributes to the expanding girth of a tree. The cork cambium is a thin layer cells that ultimately produces the bark of the tree. The bark is composed of several types of tissue produced (both toward the inside and the outside of the tree) by the cork cambium layer.

The development of a master tree ring dating system for any given site requires consideration of slope, temperature, age of the tree, precipitation, and other growth factors. Sometimes trees can produce ‘extra’ or ‘false rings’ in a year. Data from many individual trees are collected and standardized to eliminate variations that arise from isolated factors. By matching rings from older trees (dead or alive) with modern ones, researchers develop overlapping time spans extending farther back in time.