One of the most common questions that foresters get is, “How old is this tree?” Often, the response is less than satisfactory, as the forester carries on about site characteristics, species-specific silvics, land-use history, succession, and competition. This mini-forestry lesson, if nothing else, highlights the important point that tree age and size are not always positively correlated. For my students, this lesson comes in the form of counting annual growth rings from cross-sections of both big and small trees from different forests, and then plotting them on a graph. When the students get to an aspen cross-section that’s 16 inches in diameter and only 45 years old, followed by a mountainside eastern red cedar that’s only 2 inches in diameter and more than 200 years old, they really understand the variability.
However, there is a technique to determine tree age, short of cutting through the trunk and counting rings. Instead we’ll use an increment borer to extract a pencil-size sample, which will allow us to count the growth rings and to note major events in the tree’s history such as drought, fire, competition, and release from competition. Coring a tree is classified as “nondestructive sampling,” but does create a wound that serves as a potential entry point for insects and disease. To reduce the risk of disease transmission, disinfect the auger and extractor between each coring.
Increment borers are available from forestry supply stores. This tool comprises three parts: auger, handle, and extractor (sometimes referred to as the spoon). Be sure to purchase one that is long enough to reach the center of the tree that you want to investigate. Here are the steps to using it:
Step 1: The extractor and auger are nested inside the handle; unscrew the extractor and put it in a safe place (not shoved into the ground and not where you’ll step on it). A bit of ribbon tied to the end will help you keep track of it. Insert the auger in the center of the handle and close the handle latch.
Step 2: The standard sampling height is diameter at breast height (DBH), or 4½ feet above the ground. Square yourself directly in front of the tree so that the tip of the auger is aimed at the middle of the tree’s trunk and the auger is parallel to the ground. If the tree has thick bark, it’s generally easier to start in a bark furrow (crevice). Push hard and screw the borer into the tree, turning its handles in a clockwise direction.
Step 3: For accuracy, it is essential that you reach the center of the tree. If you’re not sure that you’ve reached that point, hold the extractor on the side of the tree to help you measure (the auger and extractor are the same length).
Step 4: At this point you’re ready to insert the extractor into the auger. Orient the auger handles horizontally and push the extractor into the auger slowly and carefully; the extractor is thin and it can break easily if you apply too much pressure. Once the extractor is all the way in, reverse the borer one complete revolution in order to cut off the front end of the core from the tree’s center.
Step 5: Carefully remove the extractor and confirm that your core sample extends to the center of the tree.
In some cases, it may be necessary to use a hand lens to count the rings. This 10-inch-diameter white pine had 52 annual rings. By comparing the number of growth rings of cut stumps of white pine trees in this same stand to the DBH corings, we know it took about five years for these trees to reach DBH. Therefore, we can conclude that the total age of this tree is approximately 57 years old.