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?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

• Stitching

• Fork

geometry

• Inclusions

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

In general, wood is more dense at the join between two branches or limbs

Wood density (kg/m3)

400 500 600 700 800 900

Forkwood Stemwood

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

A lower diameter ratio results in higher stresses at the apex of the fork when

pulled apart

Stronger Weaker

Slimming down

A rapid slimming down of the parent stem after the attachment results in higher

stresses too

Stronger Weaker

The ‘Fillet’

The geometry of the top of the fork is very important in determining the stresses at

the apex

Stronger Weaker

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!