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Local Knowledge: Rings of time

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In the last article I introduced you all to the five common species of conifers that grow in the Big Sky area. I would like to build on this knowledge and introduce you to some new concepts that these trees can tell us about their ages, past climate, and how scientists know the type of water, from snowpack or summer rains, that is most important to our forests.

Since you have been observing trees now for two weeks, hopefully you’ve asked the question, “How old are these trees?” Let’s start with a visual estimate. When you look at a pine, fir, or spruce, there is usually a single “spike” of growth on the top. That is how much the tree grew last season. Then below that are layers of branches, each representing a year’s growth. In the photograph below you can see this subalpine fir has the top spike, plus nine layers of branches. This means that this section of tree represents nine years of growth. When you measure the average length of each section it is about four inches. Therefore, if we estimate the height of the tree to be 50 feet, the tree would be approximately 150 years old.

Top of a Subalpine Fir next to Iron Horse showing nine years of growth. PHOTO BY PAUL SWENSON

Some trees grow faster than this, some slower, so you can use approximately two years of growth for every foot of height. Therefore, if you estimate a tree’s height in feet, then multiply by two, it’s a good approximation for the age of the tree.

This technique works well for trees in the lower elevations, but as you approach tree line, about 9,000 feet, the growing season is short and the conditions harsh. This leads to stunted, twisted trees that grow very slowly. Some of the trees growing here are less than 10 feet tall, but in the case of a few whitebark pines in the Spanish Peaks, can exceed a thousand years of age.

A more accurate way to age a tree is by counting the rings in the trunk. There are boring tools that let you do this to living trees without harming them. But if you have one cut it’s easy to count the rings. See the photograph of a 7-inch radius lodgepole tree. Here I have marked every 10 rings representing 10 years of growth. The wide part of the ring represents the early part of the growing season, the very narrow, dark part of the ring, the late growing season, fall and winter. Counting from the center out, there are 194 rings, so this tree is 194-196 years old depending how far up the trunk this section came from.

Lodgepole pine cross-section showing growth rings and associated ages. Notice the difference in width of each decade. The smaller the width, the slower the growth. PHOTO BY PAUL SWENSON

There is an entire science dedicated to the study of tree rings: Dendrochronology. Using tree rings and their relative spacing, one can determine regional environmental changes, ages and lengths of major droughts, frequency of major avalanche cycles, and date archeological artifacts. Using a method called cross-dating, a reliable data set for historical climate can be found as far back as 14,000 years.

Another interesting use of dendrochronology is detecting forgeries of valuable wooden instruments, such as violins, cellos, and pianos. The wood grain in these instruments displays the ring spacing of the trees they were made from. Knowing this pattern from cross-dating determines when the tree was harvested. Antonio Stradivari, probably the most well-known luthier of violins, built his instruments in the early 1700s. Since they are worth millions of dollars there have been countless forgeries made over the centuries. To make sure the wood is at least old enough for the violin the appropriate ring pattern must be found in the spruce top.

The spruce top of 180-year-old violin displaying the grain pattern used to date the top. PHOTO BY PAUL SWENSON

Last question for this column: “Do trees use the water from the snowpack, or summer rains, for most of its growth?” For water to evaporate it takes a certain amount of thermal energy, and that depends on the temperature of the water. The warmer the water, the more energy it has, so the faster it can evaporate. It’s why summers are more humid.

Oxygen has two common isotopes, O-16 and O-18 (which has two more neutrons). They both behave the same chemically, but the added mass of O-18 in water molecules means it takes more energy to evaporate. So summer water has a higher O-18 proportion than winter water.

Photosynthesis uses energy from light to combine water and carbon dioxide to make glucose which plants then combine to make larger carbohydrate molecules, like wood.  Testing the oxygen from the carbohydrates found in the rings shows a depleted ratio of O-18, meaning that trees use more water from snowpack than from summer rains.

Paul Swenson has been living in and around the Big Sky area since 1966. He is a retired science teacher, fishing guide, Yellowstone guide and naturalist. Also an artist and photographer, Swenson focuses on the intricacies found in nature. 

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