Annual Rings

The vascular cambium's activity is not constant through the year - it is controlled by physiological and environmental factors, and in temperate regions where the climate itself varies sharply through the year, the cambium's output varies with it. In spring, the cambium is highly active and produces a large number of xylary elements with vessels/tracheids that have wide lumens and comparatively thin walls; this pale, low-density wood is called spring wood or early wood. As the year moves toward winter, the cambium becomes less active and produces fewer xylary elements, with narrower-lumened, thick-walled vessels/tracheids; this darker, denser wood is called autumn wood or late wood. Together, one band of spring wood and the adjoining band of autumn wood laid down within a single year make up one annual ring - visible to the eye specifically because of the higher density (and darker colour) of the autumn-wood component. Distinct annual rings form best where climatic variation is sharp and are most pronounced in temperate plants; they are least distinct in seashore and desert plants, where conditions stay comparatively uniform through the year. Occasionally, adverse events during a single growing season - drought, frost, defoliation, flood, mechanical injury or a biotic attack - trigger more than one growth ring within that one year; these extra rings are called pseudo- or false-annual rings, which is exactly why not every growth ring is necessarily an annual ring, even though annual rings themselves are a special, one-per-year case of growth rings. Counting genuine annual rings to determine a tree's age is called dendrochronol …