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
•
•
•
•
•
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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!