The increment borer removes a small cylinder or core of wood from the tree trunk.
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As soil water declines through the summer, the cells become thicker-walled and more dense.
Thus each annual ring consists of early (light) and late (dark) wood. Douglass, the 'father' of dendrochronology was interested in the affect of sunspots on the earths climate.
Often the borer does not reach the center of the trunk, so the total number of years must be extrapolated from the radius of the trunk.
The radius (r) can be determined from the circumference of the trunk (C=2πr), or from special tape measures that give the diameter directly.