The Mechanical Properties of Junctions in Trees Duncan Slater BSc BA(Hons) MSc PGCE MIfL MICFor AA Conference - Sept ‘11 What’s the Forking Point? • Debunking • Stitching • Fork geometry • Inclusions Forks with included bark are prone to failure and can ruin the structure of a tree, and can cause damage to property or person Debunking Shigo’s diagram of branch anatomy Previous junction pulling tests “Medicalization of the fork” Uniform stress distributions FEA visualisation Trees still have lots to teach us! Strength of the fork = Geometry + Material + Probability of flexure Stitched up! Wood Density Forkwood Stemwood 400 500 600 700 800 900 Wood density (kg/m3) In general, wood is more dense at the join between two branches or limbs Wood Anatomy at Junctions Output of CT Scanning Images captured at the Henry Moseley X-Ray Imaging facility, University of Manchester Hazel Stem Fork Radial sections – images from HMXIF Oak Stem Fork Tangential sections – images from HMXIF Stitching… Dense, twisting grain stitches junctions together Tortuosity At the central apex of each junction, the grain is more twisted and tortuous, making it harder to split Quantifying the Effect The central region of a set of hazel forks was drilled or sawn out, to find its contribution to fork strength The shape of things to come Diameter Ratio Stronger Weaker A lower diameter ratio results in higher stresses at the apex of the fork when pulled apart Slimming down Stronger Weaker A rapid slimming down of the parent stem after the attachment results in higher stresses too The ‘Fillet’ Stronger Weaker The geometry of the top of the fork is very important in determining the stresses at the apex Fork Geometry The fork is weakened if it has: • An open-topped bark inclusion • A poor fillet shape • A low (near 1:1) diameter ratio • A slimming profile after the attachment CAUTION! THE TREE CAN REMODEL ITS JUNCTIONS Thoughts included… Bark Inclusions – Why? Better at conductance of sap (Wolf Trees) Probably caused by phototropism (Being drawn to the light) Never felt much tension (Won’t reinforce junction) Not wasteful of resources (Clever old trees! ) Bark Inclusions – How? We can cause bark inclusions to become a problem! Growing trees very close together in sheltered locations will give rise to many bark inclusions; even more so if we breed them to be fastigiate, upright cultivars. Then we complain when the junction snaps a few years later, after we thin or move it (Poor old trees! ) Remodelling My Best Guidance on Forks • The fork is adapted for the location in which it has developed. If it has been sheltered for many years, avoid suddenly exposing it to far windier conditions • Forks with bark inclusions, a poor fillet shape, a low diameter ratio and that slim down rapidly after their point of attachment are not safe, where found in exposed situations and/or connected to an elongate limb Acknowledgments • • • • • • Dr. Roland Ennos Dr. Robert Bradley at HMXIF Phil Benn & Mike Carswell David Elwell & Mike Heys Myerscough College BSc. (Hons) students Joe Barnes, Claire Harbinson & Ian Williams Thank You!