Part 1 - What is Dendrochronology?
A Brief History
In the late 1800s and early 1900s, Andrew. E. Douglass founded the science of dendrochronology— the technique of dating events, environmental change, and archaeological artifacts by using the characteristic patterns of annual growth rings in timber and tree trunks. As a young astronomer working at the Lowell Observatory in Arizona, Douglass had a particular interest in the sun, especially the cyclic behavior of sun spots and how the sun influences weather. He began looking at the annual growth rings of trees and noticed a relationship between the size of the growth rings and climate factors such as moisture and elevation. He plotted the width of tree rings and compiled the first chronologies to show how trees record climate changes through time. Noting the similarity in the response of trees across the region, he invented a technique that would prove to be a fundamental tool in tree-rings studies: cross-datingis a technique that ensures each individual tree ring is assigned its exact year of formation by matching patterns of wide and narrow rings between cores from the same tree, and between trees from different locations, or matching the patterns of tree rings from one tree to another. This allowed scientists to mark exact calendar dates for each ring. Today, tree-ring analysis is not only used to determine what the climate was like in the past, it can also be used to date works of art (wooden frames), violins and other wood instruments, and buildings.
To see the process of cross-dating in action, watch historical footage of A.E. Douglas at work.
Trees typically grow one ring per year. They start growing in the spring (the cells are light tan in color, known as early woodis the light colored portion of a tree ring produced in the spring) and, as the growing season ends in the fall, the cell walls thicken (the dark band or late woodis the darker part of an annual tree ring produced in the summer season) and eventually stop growing in the winter causing a very distinct ring. The ring pattern that forms over the entire life of the tree reveals the climatic conditions in which the tree grew. Abundant moisture and a long growing season result in a wide ring. A dry year may result in a very narrow ring (see diagram). At sites where trees are more sensitive to temperature (e.g. at high altitude on a mountain top or in the borealof the north or northern regions forests of northern Alaska and Canada) then a wide ring indicates a warm year and a narrow ring indicates a cold year.
In this activity, you will learn how tree-ring science is done. Where do tree-ring scientists travel to to find suitable trees? What tools are used? What techniques can be used to prepare and analyze tree core samples to reveal the nature of past climate?
1. Finding Climate Sensitive Trees
Conducting tree-ring science isn't as simple as it might first appear. The vast majority of trees we see on hikes or while driving will not have captured a good climate record over long periods of time. Dendrochronologists need to search for long-lived trees that grow in fairly harsh environments, making them very sensitive to the surrounding conditions, where their growth is slow— so slow that many years will be recorded in their lifetimes. One such tree is the bristlecone pine, which grows in Arizona, Colorado, New Mexico, Utah, Nevada and eastern California, sometimes under very cold and dry conditions. The oldest known specimen is named Methuselah and is a 4,765 years old! You'll have a chance to explore a bristlecone pine site in Part 2 of this lab.
What characteristics do dendrochronologists look for in research sites that will give them the best chance to reconstruct past climatic conditions? Watch the video below and answer the Stop and Think questions.
Stop and Think
1.1 What types of trees and tree-ring sites are the most useful for tree-ring research?
1.2 Why are trees growing in landscaped areas not useful to study past climate conditions?
1.3 What governs the rate of tree growth at high altitudes?
2. Coring Trees
Now let's learn how to obtain tree cores samples from a tree. People often ask if drilling into the tree to take a sample of its wood harms it. The core samples that scientists take from trees are actually quite small (less than the diameter of a pencil) and non-destructive to the tree. The tree will heal itself fairly quickly, similar to when a spout is removed after tapping a tree for maple syrup. If there is concern for a particular group of trees, scientists can dip their tree borers into alcohol to be sure they are not spreading any diseases from tree to tree. If forest managers are concerned for a particular forest, they can deny scientists permission to core trees when they apply for a permit.
Watch dendrochronologist Nicole Davi describe how to core a tree.
WATCH: Nicole Davi cores a tulip tree with assistant Augie. Credit: Jacob Tanenbaum (MP4 Video 268.2MB Mar28 16)
3. Processing the Cores
Scientists collect core samples from twenty or more trees at each of their sites. These samples need to be carefully contained and brought into a laboratory for analysis. Cores taken from a tree need to be mounted in a special wooden holder, or core mount, and then finely sanded to bring out the ring pattern clearly.
Tree-ring patterns are then studied with a number of different scientific instruments. The buttons below will take you to the Lamont-Doherty Earth Observatory's Tree-Ring Lab in Palisades, New York. These images are 360 °, so you can look around and zoom in and out. Zoom in to both the main work area and the core room so you can see what's on the tables and walls.
Stop and Think
1.4 Why is one part of each tree ring light in color and the other part dark in color?
1.5 What factors did Dr. Davi consider in choosing a good tree to core at her location in the woods?
1.6 What instruments did you see in your tour of the Lamont Tree-Ring Lab?