What is an annual ring in a tree?

An annual ring in a tree is defined as the secondary growth that occurs in one year. This growth is primarily a result of the activity of the vascular cambium, which produces new layers of xylem (wood) and phloem (bark) each growing season. As trees grow, they add a layer of xylem each year, creating concentric rings when viewed in cross-section. The width of these rings can vary depending on factors such as climate conditions, soil fertility, and water availability during that particular year, allowing scientists to gain insights into historical growth patterns and environmental changes.

The other options refer to different aspects of plant anatomy or structure and do not describe annual rings. The outer layer of dead cork cells pertains to the bark of the tree rather than the growth rings themselves. A specialized stem used for support generally refers to structures like stilt roots or tendrils but does not relate specifically to growth rings. The horizontal underground stem is known as a rhizome, which is entirely different in function and location compared to the annual rings formed above ground in the tree's structure.