Assessment of the Load-Bearing Capacity of Bark-Included Junctions in Crataegus monogyna Jacq. in the Presence and Absence of Natural Braces

A comparison of the distinct morphologies of branch junctions: normally formed; cup union; wide-mouthed bark inclusion; cracked.

The occurrence of a natural brace (top arrow) formed directly above a BI junction (bottom arrow). Here, the natural brace is a fusion of two adjacent limbs, which is acting to arrest the movement of the junction below, substantially reducing the mechanical perturbations experienced across the BI junction.

An illustration of the typical process where a natural brace is present and then subsequently lost through self-shading (its loss may otherwise be attributed to pruning). The BI junction begins to repair by forming compensatory tissue proximal to the bark inclusion where the vascular cambium is responding to mechanical perturbations. The formation of a sufficiently strong branch junction is likely if the continuation of compensatory secondary growth is achieved.

Illustration of four different forms of natural brace: A: Fused limbs; B: Entwining limbs; C: Crossing limbs; D: Lateral branch crossing to be in physical contact with an adjacent stem.

Diagram of a specimen bolted to two U-shaped brackets in the testing machine, illustrating the measurements of branch lengths that facilitated the calculation of the internal angle of each branch junction and other parameters needed to calculate the peak relative load (P_r) from each rupture test.

Measurements of the branch junctions taken in this study to calculate peak relative load (P_r). The larger-diameter branch was defined as “Branch A” and the smaller-diameter branch as “Branch B.” The diameter of the parent stem was measured in line with the bifurcation (PS₁) and perpendicular to it (PS₂) at the base of the visible branch bark ridge. The diameters of both arising branches were also measured in line with the bifurcation (A₁; B₁) and perpendicular to it (A₂; B₂) above any swelling proximal to the branch junction and away from any swelling associated with it. The external height of the branch bark ridge (H₂) was also recorded for each specimen.

Mean P_r of the three groups of branch junctions that were rupture tested: control (n = 37), natural brace removed (n = 19), and natural brace retained (n = 16). Error bars represent standard error of the mean. Letters denote heterogeneity between groups. Groups sharing the same letter were not significantly different from each other.

Load/displacement graphs of typical specimens in each category. A: The testing of a control branch junction that failed by cracking at its apex (red), contrasted with a branch failure (green), where the plastic deformation phase is much longer and peak load higher. B: The testing of three BI junctions, one with no natural brace present (blue), one with crossing branches bracing the junction (grey), and one with entwining branches (orange). For specimens with natural braces left in situ, the plotlines juddered as the natural brace provided resistance to displacement.

Specimen that exhibited fusion of branches between the two limbs (indicated by an arrow), which we found to be uncommon in hawthorn.

Box and whisker plot of wood density sampled from four different locations (n = 15 × 4). The whiskers denote the range, limits of the boxes show the upper and lower quartiles, asterisks indicate outliers, and the midline is the median value. Letters above groups identify heterogeneity as identified using a post hoc Mood’s median test.