What causes the light and dark bands in an annual tree ring?

• A. Variation in cell size due to growth conditions throughout the year
• B. The presence of resin canals
• C. Insect activity
• D. Uniform cell size across years

Answer: (A)

The light and dark bands come from seasonal changes in the size and density of the wood cells formed during the year. In spring, rapid growth produces earlywood with large-diameter, thin-walled cells that are lighter in color. Later in the season, growth slows and latewood forms with smaller cells and thicker walls, which are denser and darker. This combination creates the alternating light (earlywood) and dark (latewood) rings you can see in a tree core. Resin canals or insect activity aren’t what create the regular yearly banding, and uniform cell size year to year wouldn’t produce the distinct rings.