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Why does bark become crushed and shed as a woody stem grows?

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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.

As a woody stem grows in diameter, the tissues outside the vascular cambium—collectively known as bark—are subjected to significant mechanical pressure. Because the stem’s circumference is constantly expanding due to the production of new secondary xylem and phloem, the rigid, non-living outer layers cannot grow or divide to accommodate this increase in girth.123

As the cork cambium produces new cork cells and the vascular cambium adds new secondary vascular tissue, the older, outer tissues are pushed toward the periphery of the stem. These outer layers, including the non-functional secondary phloem and older cork, become stretched, crushed, and torn because they lack the flexibility and capacity for cell division required to match the stem's expanding diameter. Consequently, these dead tissues eventually lose their structural integrity and slough off the surface of the plant. This process is a natural result of the plant's secondary growth, ensuring that the protective outer covering is continually replaced as the stem matures.1243