The rings are more visible in trees which have grown in temperate zones, where the seasons differ more markedly.The inner portion of a growth ring forms 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" Many trees in temperate zones produce one growth-ring each year, with the newest adjacent to the bark.A tree-ring history whose beginning- and end-dates are not known is called a floating chronology.
Hence, for the entire period of a tree's life, a year-by-year record or ring pattern builds up that reflects the age of the tree and the climatic conditions in which the tree grew.
Adequate moisture and a long growing season result in a wide ring, while a drought year may result in a very narrow one.
A fully anchored and cross-matched chronology for oak and pine in central Europe extends back 12,460 years, Timber core samples are sampled and used to measure the width of annual growth rings; by taking samples from different sites within a particular region, researchers can build a comprehensive historical sequence.
The techniques of dendrochronology are more consistent in areas where trees grew in marginal conditions such as aridity or semi-aridity where the ring growth is more sensitive to the environment, rather than in humid areas where tree-ring growth is more uniform (complacent).
Horizontal cross sections cut through the trunk of a tree can reveal growth rings, also referred to as tree rings or annual rings.
Growth rings result from new growth in the vascular cambium, a layer of cells near the bark that botanists classify as a lateral meristem; this growth in diameter is known as secondary growth.
Diagram of secondary growth in a tree showing idealised vertical and horizontal sections.
A new layer of wood is added in each growing season, thickening the stem, existing branches and roots, to form a growth ring.
In addition, particular tree-species may present "missing rings", and this influences the selection of trees for study of long time-spans.
For instance, missing rings are rare in oak and elm trees.
Currently, the maximum span for fully anchored chronology is a little over 11,000 years B. In 2004 a new radiocarbon calibration curve, INTCAL04, was internationally ratified to provide calibrated dates back to 26,000 B. Dendrochronology practice faces many obstacles, including the existence of species of ants that inhabit trees and extend their galleries into the wood, thus destroying ring structure.