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Why are ring-porous hardwoods especially common in northern temperate regions?

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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.

Ring-porous hardwoods are particularly common in northern temperate regions because of the way they adapt to seasonal growth cycles. In these areas, trees experience a distinct "awakening" of the cambium at the start of the growing season.1

Ring-porous trees produce large vessels at the beginning of this season to facilitate rapid water conduction, which is essential for the tree's development. Because the rate of water conduction varies with the fourth power of the vessel lumen's radius, these larger vessels allow for much faster transport than the smaller vessels produced later in the season. This structural adaptation is highly effective in temperate climates where trees must maximize growth during a limited, favorable period.12

In contrast, diffuse-porous trees produce vessels of roughly the same diameter throughout the entire growing season. While both types of wood exhibit growth rings, the clear delineation between the large-diameter, thin-walled vessels of earlywood and the smaller vessels of latewood makes the ring-porous structure a defining feature of many temperate hardwood species.12