Dendrochronology
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Dendrochronology, otherwise known as tree-ring dating, is a dating method which analyses the lateral growth patterns of tree rings. It is a valuable dating method, because it can determine the age of a tree to an exact calendar year.
[edit] Summary
Trees, especially in temperate zones where the seasons are more varied, lay down a growth ring each year, which can be seen by cutting a cross section of the tree. This ring results from the change in growing speed of the tree throughout the year. In the growing season (usually spring to early summer) the tree shows rapid growth, which appears on the cross-section as a ring of low density wood (called 'earlywood'), and between growing seasons (usually late summer to autumn) the tree grows more slowly, which appears on the cross section as high density wood (called 'latewood'). This difference in texture is visible - earlywood appears as a light, wide ring and latewood is very dark, thin ring - and so the age of the tree could theoretically be calculated by counting the rings.
However, variations between years and individual environments exist, which complicates the dating method. For example, a local drought will result in a thinner earlywood, and an opening in the forest canopy above an understory tree will widen subsequent growth rings. False rings can also appear, when harsh conditions occur for a short period of the growing season, causing the formation of latewood. False rings differ from true latewood in their cellular structure, but nonetheless can complicate dating by mere visual inspection. On the other hand, locally absent rings can appear during long periods of environmental stress, where disease, floods or insect defoliation can reduce growth through the entire growing season, causing the absence of earlywood on part or or all of the growth ring.
To eliminate individual variations, dendrochronologists crossdate ring-growth characteristics across many samples from area of similar climate and environment. Initially done by visual inspection, modern computers now compare the ring patterns. By comparing many over-lapping ring chronologies, a full ring history (called a master dendrochronology) can be determined for up to 10,000 years in the past.
These chronologies play a vital role in the calibration of radiocarbon dating. Tree ring variations can also determine climate histories, which can be checked against those determined by ice cores for further replication of data.
