http://en.wikipedia.org/wiki/Dendrology
Dendrochronology (the previous answer) is actually where they study the tree rings or is a method of scientific dating based on the analysis of tree-ring growth patterns. It's actually really interesting http://en.wikipedia.org/wiki/Dendrochronology. We are able to tell which years had droughts, which had floods, if there were any fires, or if there was a sudden change in the direction the tree was growing (like in a landslide). You learn this when you take a course of study in Forestry.