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Why can ring widths vary around different sides of the same tree trunk?

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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 width of growth rings can vary around different sides of a tree trunk because the growth increment produced by a tree in a given year is often unequal at different points around the bole.1

While radial growth is generally tied to the weather conditions of the current year, the specific width of an annual increment depends on several factors, including soil quality, the timing of the initiation and cessation of radial growth, the rate of cell division, and the magnitude of cell expansion. Because these physiological and environmental factors can fluctuate across different locations around the circumference of a tree, the resulting wood deposition is not always uniform.1

This phenomenon can lead to variations such as eccentric rings, where there is an overproduction of wood on one side of the trunk, or fluted rings, which involve overproduction at various sites around the circumference. These irregularities are a natural occurrence and can be observed in a given tree from the crown to the base.1