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Why is the boundary between one growth ring and the next often easy to see?

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temperate softwoods and hardwoods selected to show variations — wood Temperate softwoods (left column) and hardwoods (right column), selected to highlight natural variations in colour and figure: (A) Douglas fir, (B) sugar pine, (C) redwood, (D) white oak, (E) American sycamore, and (F) black cherry. Each image shows (from left to right) transverse, radial, and tangential surfaces. Click on an individual image for an enlarged view.

The boundary between one growth ring and the next is often clearly visible due to macroscopic differences in the structure, density, and cell characteristics of the wood produced at different times during the growing season.12

Growth rings are formed by the transition between earlywood and latewood. The end of a growth ring is typically marked by the formation of cells with the thickest walls. This creates a sharp disjunction because the subsequent cells formed at the start of the next growing season are large-diameter, thin-walled cells.1

In coniferous species, this boundary is further highlighted because the latewood is generally darker in color and denser than the earlywood. In broad-leaved species, the boundary is often distinguished by the size and distribution of pores; for example, in ring-porous woods, the large pores of the earlywood contrast sharply with the smaller pores of the latewood. These structural variations allow for the clear delineation of growth increments in many temperate trees.231