How do tree rings work?

The concept of determining a tree’s age by counting its rings is only half right; most trees in temperate regions produce a single visible ring every year. Wood is made of dead xylum cells – long, tubular, vertically oriented cells that transport nutrients from the roots to the rest of the plant. Early in the tree’s annual growth season, xylum cells have thinner walls, producing the light-coloured section of tree rings called earlywood. By the end of the growing season, the cell walls have become thick and dense, leaving the telltale dark stripe (or latewood) that separates one tree ring from the next. But dendrochronologists – people who study and date tree rings – say that the problem with ring-counting is that false rings and missing rings are common. Plus, counting individual rings will only give you the age of the tree when it was chopped down or died.
How do you place the true age of that tree in the context of geological time? The solution is called cross-dating. The cross-dating process starts with a young tree with a precisely known age. Maybe we know its exact planting date or its rings contain evidence of a well-documented drought or volcano.  Dendrochronologists bore out a cross-section of the tree and measure the widths of its individual rings. This sequence of measurements can then be matched –by eye or with the aid of computer software – with the cross-sections of slightly older trees from the same geographical zone. Working backwards through time, scientists can create a master chronology of the region, an overlapping record of tree ring widths from today back through hundreds or thousands of years.