Determine age on dominant or co-dominant trees (Figure 2-9) of a species that is to be favored by management and is common on the property.
If trees were planted in your woodland, look for records showing the year of planting to determine tree age. If some plantation trees were recently harvested, count the annual growth rings on a stump or log (Figure 2-10).
Figure 2-9. Tree dominance. The dominance of a tree refers to the position of its crown relative to other trees in the canopy. Dominant trees have relatively large crowns and are taller than most other trees in the stand. Co-dominant trees make up the general canopy level. Intermediate trees are slightly lower than the general canopy and have relatively small crowns. Suppressed trees are below the general canopy level.
To measure the age of a living tree without cutting it down to count rings, you can take a core sample at 4.5 feet above the ground using an increment borer (Figure 2-11). An increment borer is similar to a hand drill, but has a hollow drill bit. It extracts a core of wood about the size and shape of a pencil. This core shows annual growth rings as bands of light and dark wood. By counting the rings in the core and adding the number of years it took the tree to grow to a height of 4.5 feet, your forester can determine tree age.
Figure 2-10. A tree grows a new ring of wood each year.
The age of conifers such as red (Norway) pine, white pine, and balsam fir that produce one whorl of new branches each year can be estimated by counting whorls (Figure 2-12). This approach works best when the trees are young (<25 years) because it is easier to see each whorl. False whorls can develop if more than one spurt of growth occurs in a year.
Site Index Curves
If you know the average tree age and total tree height of a tree species of primary interest, refer to a set of site index curves for that species to determine site quality. Site index curves for red pine are shown in Figure 2-13. These curves were developed for use in pure, even-aged stands of red pine. A pure stand is one in which at least 80 percent of the trees are of the same species. An even-aged stand is one where the age difference between the youngest and oldest tree in a stand does not exceed 20 percent of the projected rotation length. A rotation is the number of years required to establish and grow trees to a specified size, product, or condition of maturity at which they can be harvested.
Figure 2-11. Tree core sample removed by an increment borer.
Figure 2-12. Count the whorls of branches to determine the age of a conifer.
For example, if you measured several dominant and co-dominant red pines and determined their average age was 80 years and average total tree height was 90 feet, find 80 years on the bottom axis in Figure 2-13 above, then follow the vertical line upward from that point until it intersects the 90-foot level on the left vertical axis. Now follow the site index curve from the junction of those two lines to the right vertical axis, where you will find that the site index is 65. These trees would be expected to be 65 feet tall at age 50. The higher the site index, the better the site. Site index curves for common tree species are shown in Appendix B. Site index curves are reasonably accurate measures of site quality for stands older than 20 years. For younger stands, special equations are used to estimate site index.