Annual Rings

annual rings, the growth layers of wood that are produced each year in the stems and roots of trees and shrubs. In climates with well-marked alternations of seasons (either cold and warm or wet and dry), the wood cells produced when water is easily available and growth is rapid (generally corresponding to the spring or wet season) are often noticeably larger and have thinner walls than those produced later in the season when the supply of water has diminished and growth is slower. There is thus a sharp contrast between the small, thick-walled late-season wood cells produced one year, and the large, thin-walled cells of the spring wood of the following year results. Where the climate is uniform and growth continuous, as in wet, tropical forests, there is usually little or no gross visible contrast between the annual rings, although differences exist. When rings are conspicuous, they may be counted in order to obtain a reasonably accurate approximation of the age of the tree. They are also reflective (by their range of thickness) of the climatic and environmental factors that influence growth rates. The science of dendrochronology is based upon the phenomenon of variability in the thickness of annual rings.

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Annual Rings 

(1) In plants—the growth zones of wood caused by seasonal variations in cambium action as a result of the alternation of warm and cold seasons. Annual rings are readily distinguishable in cross sections of the trunk, branches, and roots of woody plants as not strictly concentric rings. Annual rings are most pronounced in plants of the temperate and cold latitudes and correspond to growth during one growing season. The wood formed by the cambium in the spring or early summer differs in structure, color, brilliance, hardness, and other mechanical features from the wood formed during the second part of the growing season. The first (inner) part of the annual ring is more porous and lighter in color, while the second (outer) is firmer and darker colored. The cells forming the earlier growth have thinner walls and wider interiors, while the wood cells that were formed later have thicker walls and narrow interiors. The transition from early wood to the later, as a rule, is gradual, while the transition from later growth of one growing season to the early wood of the next is clearly defined.

It is possible to determine the age of a plant by the number of annual rings at the neck of a root that has been cut in cross section. The width of the annual rings varies in relation to the growing conditions, the part of the trunk and the age of the tree. The width of the annual rings increases in proportion to the growth of the tree, then when the tree reaches maturity the width stablizes, and as the tree ages the width again decreases. In freestanding trees, annual rings become smaller toward the top, while in trees growing in thick stands they become smaller toward the base. Sometimes so-called false rings form: this occurs when annual rings double as a result of spring frosts that cause leaf kill, or if caterpillars eat the leaves and the dormant buds develop subsequently. Annual rings can be used, because of certain regularities of their formation, to reconstruct the climate of the past and to predict it in the future (dendroclimatology), to determine the time of growth of this or that tree, for example, in archaeological finds (dendrochronology).

O. N. CHISTIAKOVA

(2) In animals—formations in some tissues that form every year and persist for a long time; they make it possible to determine the age of a specimen. Annual rings are found in skeletal structures that maintain long-term growth and are not subject to cardinal transformation during the specimen’s lifetime. They are formed as a result of seasonal changes in the rate of growth of tissue, reflecting seasonal changes in the organism’s metabolism. Each annual ring is composed of relatively wide and narrow layers of varying structure and optical density, in proportion to the concentration of mineral salts in mineralized tissues. Annual rings appear in direct examination under penetrating or reflected light (as in fish scales, mollusk shells, the horny beaks of squids, flat bones in the skulls of fish, amphibians and reptiles, and the claws of mammals), in cuts and sections (as in the otoliths and fin rays of fishes and in certain bones of fishes, amphibians, and reptiles), or in specially dyed microscopic sections of the teeth and bones of mammals. The number of annual rings can be used to determine age, and their relative width can be used to judge the specimen’s rate of growth. Annual rings are preserved in fossil remains and can be used to establish the growth rates of extinct animals.

REFERENCES

Chugunova, N. I. Rukovodstvo po izucheniiu vozrasta i rosta ryb. Moscow, 1959.
Klevezal’, G. A., and S. E. Kleinenberg. Opredenlenie vozrasta mlekopitaiushchikh po sloistym strukturam zubov i kosti. Moscow, 1967.
Physiology of Mollusca, vols. 1–2. Edited by K. M. Wilbur and C. M. Yonge. New York-London, 1964.
Peabody, F. E. “Annual Growth Zones in Living and Fossil Vertebrates.” Journal of Morphology, 1961, vol. 108, no. 1.

G. A. KEVEZAL’