When people saw off a tree trunk, many concentric rings of varying depths can be seen on the cut surface of the wood. These rings are related to the age of the tree, with one ring added each year, hence they are called annual rings. One notable feature of annual rings is that, regardless of their thickness or curvature, they are always arranged in alternating layers of deep and shallow rings. Why do annual rings vary in depth? We know that the growth and development of plants result from the continuous division of cells, and the thickening of the stem is no exception. However, only dicotyledonous perennial woody plants can continuously thicken their stems. This is because they have a special tissue between the bark (phloem) and the xylem, which is called the cambium in botany. Cells that divide outward form the bark (phloem), while cells that divide inward form the xylem. However, the number of cells produced by division differs greatly—much more cells divide inward than outward, so the xylem continuously thickens while the bark's thickness remains largely unchanged. Moreover, the division rate of cambium cells is affected by seasonal changes, resulting in periodic variations. In spring and summer, with warm weather and abundant rainfall, the cambium produces large and numerous new cells, so the wood formed during this period is porous and light in color. From late summer to autumn, as temperature and water conditions gradually become unfavorable for plant growth, the division rate of the cambium slows down, and the cells produced become smaller, resulting in denser and darker wood during this period. In winter, the trees enter a dormant state, and the cambium stops dividing, halting tree growth. It is precisely because of these recurring seasonal changes that the division rate of the cambium also fluctuates accordingly, giving rise to the annual rings' varying depths.
However, not all plants have cambium cells or can thicken their stems. In fact, only gymnosperms and dicotyledonous plants possess this cambium. For example, many monocotyledonous herbaceous plants, lacking cambium tissues, cannot thicken their stems.
Within the annual rings' varying depths, there lies information that only botanists can decipher. Each annual ring contains a layer of wider light-colored rings and a layer of narrower dark-colored rings, which act like "special writing" in nature, faithfully recording changes in the climate and environment of the tree's growth area. Under favorable climate conditions, trees grow well, with increased xylem formation, resulting in wider annual rings. Conversely, if a ring is relatively thin, it indicates poor climate conditions such as cold, drought, or pests and diseases. In addition, we can use annual rings to determine the age of deceased ancient trees. For example, if there are three consecutive years of drought, the trees will form three narrower annual rings, which are identical for trees of the same area and period, whether they are large or small. In this way, we can identify the exact year of the drought from a living tree and then locate the corresponding annual rings on the deceased old tree, allowing us to estimate the age of the old tree.