Several disks or tree cookies cut from the stems of trees are included in the Forest Discovery Box. These disks clearly show the annual growth rings. You can tell how old the tree was at the height the disk was cut simply by counting the rings. The rings result from a difference in cell width. During a single year a tree grows many cells in width. Cells are mostly water when growing conditions are good in the spring and early summer. In late summer and autumn growing conditions are poorer and so there is less water in the cells making them look darker (the cell walls are dark).The annual rings show up as a result of the transition from slow growth and dark cells of one year, to the larger lighter water-filled cells of the following spring. The amount of annual growth, or the width of the annual growth rings provides an indication of the tree's health and vigour. Trees that have been healthy and free to grow have wider growth rings than trees that come from crowded conditions or trees that suffered from disease or insect attacks. The four tree disks included in the Forest Discovery Box clearly show differences in growth rates as a result of different growing conditions. One sample from a balsam fir tree shows decreasing annual ring widths as it grew in increasingly crowded conditions. In contrast, a balsam fir from an area that was precommercially thinned, shows a dramatic increase after it was released from competition by the thinning. A disk from a natural mature white spruce tree shows changes in growth rates through the tree's life. These changes probably result from competition, insect attack, occasional poor growing years, and a general decline in growth as the tree gets much older. A disk from a young planted white spruce is also included. This tree's rings are easy to count due to the ideal growing conditions that the tree has been able to enjoy. As trees grow larger, the width of their annual rings naturally tends to get somewhat narrower simply as a result of the greater circumference that the annual rings must cover each year. When someone counts the annual rings in order to determine a tree's age, it is important to note the height at which the rings are being counted. The age of the tree below the height at which the rings were counted must be estimated and added in order to determine the tree's true age since germination. | |
