Most trees can live 100 to 200 years, and some, like the giant
redwoods and bristlecone pines, can live thousands of years.
As trees age, they grow taller and wider. Each year, trees form
new cells, which are arranged in rings around the tree's center.
So when you look at a cross-section of a tree, the rings you see
will show you the amount of wood produced during one year. The study
of tree rings is called dendochronology.
Trees produce different amounts of wood in response to temperature,
rainfall, and other conditions. Lots of rain increases tree growth,
producing wider rings. Not enough rain or crowding from nearby trees
both cause slower growth with narrow rings.
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