Dendrochronology is a scientific method that uses the annual growth rings on trees to find out the exact year the tree was formed, which helps scientists date events, environmental change, and archaeological artifacts. A tree’s rings start from the middle, with the oldest rings at the center of the tree and new growth occurring in a layer of cells near the bark. The rate at which the tree grows changes in a predictable pattern throughout the year due to seasonal climate changes, which causes visible growth rings. Each ring on a tree represents a full year in the life of the tree.
Not only can these rings tell us how old a tree is, but each ring can show what the climate was like during that year. In temperate climates, a tree will grow one ring each year. In the spring, there is more moisture, so the cells of a tree expand quickly. In the summer, however, it becomes very dry and the tree cells start to shrink. When looking at tree rings, this change is cell size is visible and results in different sized widths of rings. The rainier the year, the wider the ring and vice versa.
Not all trees are datable, due to factors such as natural tree variation and too much water, but about 40% of trees can be dated. Simply counting the rings on a tree sample tells us how old the tree was when it was cut down, but to find out what time period the tree is from requires a little more work. Scientists must look at the pattern of the rings, not just how many there are to find out the time period the tree is from. All trees in the same climate or region will have rings forming the same pattern, since a rainy year or a drought will affect all trees the same way. To know if trees are from the same climate or region scientists must simply match the tree rings. This is possible, since tree ring patterns never repeat themselves, so they are specific to a time and place.
The process of identifying a pattern is not as complicated as it would seem. Dendrochronologists lay a strip of graph paper on a sample of tree and mark where the narrow rings are. This process is called Skeleton Plotting, since it is only marking the seasons of drought. It is easier to identify patterns by hand rather than by computer, because humans are so good at recognizing patterns. Dendrochronologists will repeat this process for thousands of trees from the same region or climate to create a master pattern, which is called a master chronology.
Labeled Tree Sample. NASA, 25 Jan. 2017, climate.nasa.gov/news/2540/tree-rings-provide-snapshots-of-earths-past-climate/.
Mason, Matthew. “Dendrochronology: What Tree Rings Tell Us About Past and Present.” EnvironmentalScience, www.environmentalscience.org/dendrochronology-tree-rings-tell-us.
Skeleton Plotting. PBS, 30 Jan. 2013, www.pbs.org/time-team/experience-archaeology/dendrochronology/.
“Tree Ring Dating Dendrochronology.” PBS, 30 Jan. 2013, www.pbs.org/time-team/experience-archaeology/dendrochronology/.
Tree Sample. PBS, 30 Jan. 2013, www.pbs.org/time-team/experience-archaeology/dendrochronology/.