How can drought create a false growth ring in a conifer?

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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.

In conifers, a false growth ring occurs when the normal growing season is interrupted by an environmental stressor, such as a drought in the spring.1

As drought conditions intensify, the radial diameters of secondary tissue cells decrease and their walls thicken, causing the wood to temporarily resemble latewood. When the drought subsides and favorable conditions return, the tree resumes growth, producing cells with larger radial diameters. This transition creates the visual appearance of a new growth ring.1

However, this is identified as a false ring because it lacks the sharp disjunction characteristic of a true annual boundary. Instead, it features a gradient of increasing cell-wall thickness and decreasing cell diameter at the start of the false ring, followed by a gradient of decreasing cell-wall thickness and increasing cell diameter at its end. These variations present a challenge for dendrochronology, though they also serve as valuable indicators for tracing historical weather patterns.1