Inducing bark inclusions in branch junctions of aspen (Populus tremula L.) by bracing them with horticultural wire

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ABSTRACT Branch junctions are typically strengthened by the production of dense wood with a tortuous grain pattern in their axils (axillary wood). The strong association found between natural braces in trees and bark-included branch junctions set below them suggests there may be a causal relationship: that if a branch junction receives little mechanical stimulation due to being naturally braced, little or no axillary wood will be formed and bark is occluded instead. To investigate this relationship, sixty branch junctions in young specimens of aspen (Populus tremula L.) were braced with horticultural wire and their development contrasted with sixty “controls” where no bracing was applied. After three years, 95% of the braced branch junctions exhibited included bark, whereas only one control junction developed included bark and that was found to be natural braced. This is the first recorded instance where bark-included junctions have been knowingly created by artificial means. The results point to a causal relationship between natural bracing and the formation of bark-included junctions, which has implications for tree management. This study also provides evidence that axillary wood should be categorised as a reaction wood, as its formation requires specific mechanical stimulation, and due to its distinct anatomical and mechanical properties.