On a lark, when I was a college student I took a class in field biology. It sounded romantic and I was young, so even though it didn’t really make any sense for a geology student to take the senior level class in another discipline, I was there bright and early on the first day of the semester.
One week everyone in the course walked to a grove of old hardwood trees near the edge of campus. We had “boring tools” — drills with long, hollow bits — with us. The idea was to sink the bit into the trunk of the tree. When the bit reached the middle of the tree, we used a narrow spatula to extract a thin “dowel” of wood from within the hollow bit.
These elegant little samples give you access to the life history of the tree as recorded in its growth rings. As every school child knows, counting the rings tells you how old the tree is.
But some samples of trees can tell you much more than age. That’s because some trees live in difficult environments. They grow best only when there is a good year in terms of precipitation, temperature, and the like, so they have growth rings that are quite uneven. Some rings are thick, representing good years for growth, while others are quite thin from when times were tough for the tree. This means that tree rings can tell us about variations in past weather and climate.
In the southwest U.S., a lot of work has been done with tree rings. Indeed, the whole science of what’s called dendrochronology was worked out in that region in the early and mid 20th century. But since then, scientists around the world have also used basic ideas about tree rings to do several different things.