How old is that tree?

Trees grow by adding new wood on top of old wood. In regions with distinct seasons, like here with our winters, this growth stops when conditions aren't right. Trees then form annual growth rings, which can be counted to tell how old they are.

If you know what you're looking for, growth rings contain all kinds of information, including which years were good and which ones were rainy or cold. And since two trees of the same species growing in the same region produce similar growth rings, you can compare the rings of trees that died at different times in the past to establish a chronology of a forest. This is called a dendrochronology (Study of growth rings of trees to trace their history.). The longest one created to date, in the United Kingdom, spans 7,000 years.

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