Plant Growth

What are tree rings?

ANSWER: Plants experience two kinds of growth: primary and secondary. Primary growth is growth in length—a tree getting taller. Trees and other plants also grow wider as well—this is secondary growth. Tree trunks increase in diameter because they add a new layer of xylem tissue—otherwise known as wood—each year. In temperate regions—as in most of the United States—xylem growth is dormant in the winter. In the spring, xylem growth starts again. The first xylem cells to grow in the spring are usually larger in diameter and thinner-walled than xylem cells produced later in the summer. The boundary between the smaller xylem cells from one summer and the larger xylem cells from the next spring appears as a ring in the cross section of a tree trunk. You can determine the age of a tree by counting the number of rings. Tree rings vary in width from year to year because differences in temperature and rainfall affect the amount of xylem tissue that is produced in any one season.