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What causes eccentric growth rings in leaning trunks and branches?
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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.
Eccentric growth rings in leaning trunks and branches are caused by the formation of reaction wood, an adaptive morphogenetic response to physical displacement or gravity. This specialized tissue develops to restore the tree's intrinsic growth direction or to maintain the angle between a branch and the main axis.1
- In conifers: Reaction wood, known as compression wood, forms on the lower side of the stem. It exerts an upward pushing force, resulting in increased growth on that side and an eccentric, oval cross-section.1
- In hardwoods: Reaction wood, called tension wood, typically forms on the upper side of the lower trunk. It exerts a contractive force to pull the tree toward an upright position. While this can lead to wider annual rings, eccentricity is generally less pronounced in hardwoods than in conifers.1
This process is a permanent anatomical reaction that creates in-built stresses within the wood, which can affect the quality and structural utility of the timber.1