An Accurate "Log-ing" of Time
Scientists use many tools to put a date on events of the past. A geologist can use relative dating, comparing rock layer ages against one another. Archeologists may use carbon 14 for an absolute date. Climatologists can access over a million years of climate data using ice cores. If you want to know when a historical log cabin in the mountains was built, you use dendrochronology.
The original logs tend to be larger in size and they tend to have over a hundred rings. Replacement logs tend to be smaller and they tend to have 30 to 40. We also look for the side of the log that’s not cut, and sometimes we even see bark, which is great because then we have an anchor in time, and we know absolutely this is when the tree was cut. Then we take as many cores as possible from the original logs in the structure, and then we match that with an existing tree ring record that’s anchored in time.
That’s Dr. Saskia van de Gevel, professor and researcher at the Department of Geography and Planning’s Appalachian Tree Ring Lab at Appalachian State University. Tree rings hold a powerful and accurate record of the past because a tree gains a new ring every year.
We look for those patterns of drought years or narrow rings vs. wide rings.
Worldwide events can act as markers to synch tree ring records globally. For example, any tree alive in 1815 has a very distinct marker.
The eruption of Tambora that shows up as the year without a summer. The amount of particulates and aerosols in the atmosphere that occurred basically caused photosynthesis to not occur because there wasn’t enough sunlight to reach the earth’s surface.
How far back do tree ring records go?
In the Southeastern US, the tree species that gives us the longest record are Bald Cyprus. They can be as old as twelve hundred years. In the Western United States we have bristlecone pines, and those trees can live to be 5,000 years old. And so we can look at the living and also the dead bristlecone pines and overlap them. So, in the United States we have a climate record for 10,000 years. In Europe we can go back using oaks in the bogs in Ireland, 12,000 years. When you think about the cathedrals of Europe, all of those can be authenticated with tree rings, and they have been.
Dr. van de Gevel’s main focus is forest ecosystems. Using tree ring data she is able to see how trees respond to stresses like drought, forest fires, or invasive species.
Carolina hemlock is a separate species from eastern Hemlock. It only exists in this part of the southern Appalachian mountains. And with the hemlock woolly adelgid, which is an insect that’s negatively affecting hemlock species in the Eastern United States, this species is also under threat. But because it’s a different species than eastern hemlock we were looking to see, is it as heavily impacted as eastern hemlock has been. Using tree rings to look at pre-infestation vs. post infestation it really gives us a sign of how stressed is this tree becoming because of this invasive species.
Data on tree rings collected worldwide is found in the International Tree Ring Data Bank. This allows research from small to global in scale.
I think it’s amazing looking at records of thousands of trees, and hundred of thousands of tree rings. You can take a very local question and expand it to have a much larger special context. For example, hemlock forests, especially Carolina hemlock, we can compare that to species that have a much wider distribution like eastern hemlock. Then we can compare that on a global scale with hemlocks around the world, and so are they responding to worldwide drought? Are they responding to local drought? Are they responding to fluctuations in temperature? It’s approachable, it’s tangible, and it’s really not abstract. That grabs the attention of so many children that I work with. They understand that there are years that vary between a lot of growth and a little growth, and that there’s a hurricane, that would impact the signature that year in the tree ring. If there was a fire it would leave a scar. How does that relate to changing climate, to the species that depend on that tree species?