While walking in the Arboretum, you may have noticed the radial pattern of tree growth displayed on stumps. Have you ever given this pattern much thought? What does it mean? Why are some rings bigger than others? These are the questions pondered by dendrochronologists. “Dendro” means tree, and “chronology” means arranging events in their order of occurrence in time.
There are a few key principles of dendrochronology. “Uniformitarianism” is the idea that the present is the key to the past. Dendrochronologists tend to assume that the processes affecting tree growth today also affected tree growth in the past. “Cross-dating” refers to the idea that patterns in tree rings can be recognized across cores, allowing longer chronologies to be created. The principle of “limiting factors” is about tree growth being limited by particular environmental factors, meaning that ring width allows us to gain insight into these factors in a given year.
The work of a dendrochronologist begins in the field with an increment borer, drilling small holes into trees of interest and removing cores. After drying for a couple of weeks, the cores are mounted onto wooden bases and sanded down to clarify the rings. This allows the rings to be counted and the trees to be dated, because each year, one ring is grown. Cores from different trees can then be compared and cross-dated, allowing dendrochronologists to extend records back thousands of years.
Processed data can be used to gain insights into the factors affecting the growth of trees in the past. The focus can be on climatic factors, insect outbreaks, forest dynamics, landslides, glacial advances, and much more. Those pretty radial growth patterns might be more useful than you thought!
The Arboretum is an interesting dendrochronological site because it was historically used as farmland. This means that most of the trees are relatively young. Only minor investigations have been done in Introduction to Geology courses. We have a largely unexplored natural library in our backyard! What information might our trees have to share?
- Dexter Pakula ’26, for the Cole Student Naturalists
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