Article Figures & Data
Figures
- Figure 1.
A) Interlocking wood grain pattern at the apex of a junction of common ash (Fraxinus excelsior L.), as exposed by debarking. B) Wood grain pattern at the apex of a bifurcation of common oak (Quercus robur L.) incorporating whorled grain. C) Diagrammatic representation of interlocking wood grain in a normally-formed bifurcation in a woody plant, based on the anatomical model of Slater et al. (2014) with inset displaying a basic interlocking pattern of wood grain incorporating whorled grain.
- Figure 2.
Artificially-modified bifurcations left to grow in situ for two to four years. A) Diagram of rod-bracing created in 25 hazel bifurcations. B) Diagram of drill hole created in 50 hazel bifurcations. C) Diagram of split created in 25 hazel bifurcations.
- Figure 3.
Measurements taken on each bifurcation in order to calculate its breaking stress: Distances between the two drill holes and between each drill hole and the apex of the bifurcation (a, b, and c) measured using a metal rule, diameters of the two branches just above the bifurcation apex, both in-line with the plane of the bifurcation (A1 and B1) and perpendicular to the plane of the bifurcation (A2 and B2 (not shown), and the diameter of the parent stem (PS) just below the branch bark ridge, both in the plane and perpendicular to the plane of the bifurcation measured using digital calipers.
- Figure 4.
Diagram of the means of attachment of the bifurcations to the universal testing machine during the rupture tests.
- Figure 5.
Diagrams and images defining three morphological types of bark-included junctions in hazel, based on observations of the fracture surfaces of bifurcations. Embedded bark is surrounded entirely by xylem, the bark having been occluded into the junction. A cup-shaped bark inclusion has sapwood formed around included bark that lies at the center of the join; there is sapwood at the apex of the bifurcation rather than bark. A wide-mouthed bark inclusion has a substantial width of included bark at the apex of the bifurcation, situated above any connecting sapwood.
- Figure 6.
A) Fracture surface of a pre-drilled bifurcation after two growing seasons, showing the silicon inserted into the initial drill hole, dysfunction induced in the sapwood around the drill hole (discolored area) and the remodeling of the sapwood to form a cup-shaped union. B) Typical deformation of a pre-split bifurcation, where the crack had subsequently propagated to a knot in the parent stem and then been arrested. C) Typical deformation of the branches of a braced bifurcation around the implanted steel rod, after three years of growth, showing adverse taper in the smaller branch.
- Figure 7.
Mean breaking stresses for the main bifurcation types tested (excluding branch failures) and the mean yield stress of the smaller branches as found by three-point bending. The pre-split type is not included as its replicate number was too small to be statistically analyzed (n = 2). Error bars represent standard error. Letters above bars identify significant differences between groups by using a GLM ANOVA and post hoc Tukey test at a 5% confidence limit.
- Figure 8.
Mean breaking stresses of the three types of pre-drilled bifurcation tested. Error bars represent standard error. Letters above bars identify significant differences between groups through using a GLM ANOVA and post hoc Tukey test at a 5% confidence limit (F3,88 = 5.70; R2 = 42.34%; P = 0.001). The diameter ratio was a significant covariate (P < 0.001), and the parent stem diameter was not significant (P = 0.909)
- Figure 9.
Mean breaking stresses of the three types of bark-included bifurcation tested. Error bars represent standard error. Letters above bars identify significant differences between groups by using a GLM ANOVA and post hoc Tukey test at a 5% confidence limit.
- Figure 10.
Mean breaking stresses of the normally formed bifurcations and the two estimates of the breaking stresses of the braced bifurcations, taking into account the section modulus of the smaller branch either below or above the brace rod. Error bars represent standard error. Letters above bars identify significant differences between groups by using a GLM ANOVA and post hoc Dunnett test at a 5% confidence limit (F3,77 = 19.32; R2 = 35.59%; P < 0.001). The diameter ratio was a significant covariate (P = 0.017), with an increasing diameter ratio resulting in a lowering of breaking stress; the parent stem diameter was not found to be a significant factor (P = 0.631).
- Figure 11.
Mean wood density of samples excised from the apices and sides of normally formed and braced bifurcations. Letters above bars identify differences between groups by using a GLM ANOVA and post hoc Tukey test at a 5% confidence limit.
Tables
- Table 1.
Determination of drill size for hazel bifurcations modified by drilling, based on the diameter of the parent stem, measured just below the termination of the branch bark ridge and perpendicular to the bifurcation.
Diameter of parent stem (mm) perpendicular to bifurcation Drill size used upon bifurcation Up to 22.5 4 mm 22.5–27.49 5 mm 27.5–32.49 6 mm 32.5–37.49 7 mm 37.5–42.49 8 mm 42.5–47.49 9 mm 47.5+ 10 mm - Table 2.
Bifurcation types tested, research related to each type, numbers of replicates for each type, year of modification, and associated growing seasons prior to mechanical testing.
Name of bifurcation type Description Factor assessed No. of replicates Year of artificial modification Growing seasons between modification and testing Bark-included Naturally-occurring bifurcations with bark found to be incorporated within the apex of the bifurcation (Figure 6). Effect of bark obstructing the normal anatomical connection at a bifurcation. 70 Not modified N/A Braced Normally-formed bifurcations modified by the conjoining of the two branches above the bifurcation with a 3 mm steel rod fitted through both branches, with a 7 mm washer and nut fitted at each end of the rod. These were left to grow within the tree’s crown for three years prior to testing (Figure 1A). Effect upon remodeling by completely preventing mechanical perturbation at the apex of the bifurcation. 25 2011 3 Newly drilled Normally-formed bifurcations drilled at their apices using a drill-size as defined in Table 1, immediately prior to mechanical testing (Figure 1B). Effect of removing centrally placed interlocking xylem at the apex of the bifurcation. 60 2015 0 Normally formed Naturally occurring bifurcations with no flaws observed in morphology. To act as a benchmark for all other modifications. 60 Not modified N/A Pre-drilled Normally-formed bifurcations modified by drilling at their apices using a drill-size as defined in Table 1, and left to grow within the tree’s crown for two or four years prior to testing (Figure 1B). Effect of remodeling after the removal of the centrally placed interlocking xylem at the apex of the bifurcation. 50 2010 2 and 4 Pre-split Normally-formed bifurcations modified by carefully splitting the apex by hand, by bending away from each other the two arising branches. These were left to grow within the tree’s crown for three years prior to testing (Figure 1C). Effect of remodeling after the cracking of the top part of the bifurcation. 25 2011 3 - Table 3.
Wood density of samples taken from different bifurcation types tested, by location. Letters (A, AB, B, and C) below the mean in each entry identify differences between these means across bifurcation type and location of xylem extraction, as identified by a GLM ANOVA with the sample volume as a covariate and post hoc Tukey test at a 5% confidence limit (F13,357 = 93.94; R2 = 78.09%; P < 0.001). Sample volume was not a significant factor in the differences found in wood density between groups (P = 0.509).
Bifurcation type Wood density of extracted sample (Kg m−3), by location Apex Side Stem Normally formed 644.9 ± 4.3 SE 632.8 ± 5.8 SE 493.0 ± 6.9 SE A A C n = 36 n = 36 n = 36 Bark-included N/A 628.6 ± 5.3 SE 490.9 ± 5.2 SE AB C n = 57 n = 57 Newly drilled N/A 628.9 ± 5.8 SE 488.7 ± 7.6 SE AB C n = 30 n = 30 Pre-drilled N/A 595.0 ± 4.7 SE 494.8 ± 12.2 SE B C n = 19 n = 19 Pre-split N/A 614.6 ± 5.7 SE 503.2 ± 22.5 SE AB C n = 5 n = 5 Braced 619.9 ± 8.3 SE 619.9 ± 11.0 SE 480.0 ± 6.7 SE AB AB C n = 14 n = 14 n = 14
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