Larger trees growing in a forest present the greatest challenge. As noted above, it is very difficult to estimate tree age simply from size. So much depends on the tree’s microenvironment (access to light, water, space and nutrients), its unique species-dependent growth habits, and the events that alter the tree’s environment or health over the course of its life. The frequency and intensity of disturbances such as fire, insect attacks, or windstorms profoundly influence tree growth over time.
Trees growing in managed forests, particularly evenaged “second growth” or “third growth” forests, were likely planted. Foresters record the year of planting and seedling age at time of planting. Most companies will have year of establishment printed on company forest maps or indicated on company aerial photos for ease of use. In these cases, simply researching office records before one goes out to the field will provide stand age.
Trees growing in naturally regenerated stands, unmanaged stands, or stands managed for an unevenaged structure are harder to evaluate. In these cases, individual tree age can vary greatly from tree to tree. Knowledge of tree silvics can help with ballpark estimates. For example, a young (< 30 yrs.) true fir will have smooth bark with resin blisters. This gradually develops into plates or fissures as the tree ages. A tree over 100 years will have regular, geometric shapes in the bark patterns. The crown of a very old tree will also have a rounded top, different than the tiered leader of a young tree. On Douglas-fir, the smooth bark gives way to thick fissures in the bark. But these type of physical characteristics, without some site history clues, may only get a person to within about 30 years of the actual age.
Annual Ring Counts
The most direct way of determining tree age is to count the annual rings on a tree’s stump or a round “cookie” cut from the tree. In the Pacific Northwest, trees produce one “ring” of diameter growth each year, so the number of rings present on a cross-section of the tree’s trunk represents the tree’s age at that height. Counting rings on a stump will result in a pretty accurate estimate of the former tree’s age. Counting rings from a cookie cut at a height of ten feet or twenty feet will tell you how many years the tree grew after it reached that particular height (Figure 4.5). In fact, researchers examine cookies cut from regular intervals along fallen trees to derive information about species’ growth rates, and sometimes to investigate evidence of historical events such as fires, droughts, insect outbreaks, etc. in a science called dendrochronology.