Abstract

Loading from the wind and gravitational pull gives rise to adaptive growth in trees through the process of thigmomorphogenesis. In the case of bifurcations in trees set at varying angles from a vertical orientation, they will have developed under quite different loading regimes. However, previous studies do not agree as to the influence of angle upon the strength of such bifurcations. In this study, 97 bifurcations of hazel (Corylus avellana L.) were cut from a hazel coppice in Lancashire, England, based on collecting bifurcations whose arising branches were growing at a range of angles, from vertical (0°) to horizontal (90°). These bifurcations were then subjected to rupture tests until they failed, using a Lloyds EZ50^® testing machine. It was found that as the angle of inclination increased from 0° to 90°, the arising branches formed above the bifurcation became 5% more elliptical in the vertical plane, theoretically increasing the bifurcation's bending strength in this plane by 2.5%. Similarly, the parent stems formed below the bifurcations became 14.6% more elliptical in cross section between the angles of 0° to 60° in inclination from the vertical, theoretically increasing the bending strength in the vertical plane by approximately 5%. The diameter ratio was also found to be a significant factor in determining bifurcation yield strength in the samples tested, predicting 23% of the variation in breaking stresses, but had a significantly negative relationship to angle of inclination. These findings provide a resolution to disagreements in former studies that analysed the effect of angle upon bifurcation strength in woody plants. It was concluded that a subtle adaptation to junction morphology occurs in response to additional gravitational loading in young bifurcations of hazel that are growing at angles diverging from the vertical.