experienced at the junction. Second, where two branch-
es or stems run in parallel and close together after aris-
ing from a branch junction, the probability of the upper
sections consisting of twigs, branches, and stems com-
ing into physical contact with each other to form "natural
braces" across that junction is much higher.
By being braced by the touching or rubbing of branches,
the normal gravitational and dynamic loading that the
junction experiences will be lowered: some junctions
come to be held completely static: and they do not get the
mechanical stimulus to grow normally. My most recent
research work has found that the formation of these natu-
ral braces explains more than 93% of the bark inclusions
present in a cohort of nearly 2,000 trees (Slater, 2017).
Natural braces come in a number of forms, which
my associated publication describes in some detail
(Slater, 2016). Three types of natural bracing in the
crowns of trees are shown in Figure 3. Note the associ-
ation between these different bracing structures being
bark formed at the top of the junctions gave rise to the
weakest branch junctions.
The question then arises: "What causes these malformed
bark-included junctions to form?" Some authors have sug-
gested that simply the angle of attachment could be the
main cause of bark inclusions forming in trees. However, one
can easily find tightly angled branch junctions with a well-
formed branch bark ridge and, contrasting with that, wider
angled junctions with included bark within them (Fig. 2).
Component parts of trees (and other plants) respond
to mechanical stimulus—a process known as 'thigmo-
morphogenesis' (Jaffe, 1973; Telewski, 2006)—resulting
in changes to branch, stem, and junction morphology,
depending on the loading that they experience as they
develop and grow. It is thus more common to find bark
inclusions in tighter-angled junctions and this is for two
main reasons: First, where the two branches or stems
ascend vertically, almost in parallel from the junction,
this greatly reduces the gravitational loading that is
Figure 3
Three types of nat-
ural brace found
in urban trees: A:
Entwining stems; B:
Crossing branch on
stem; C: Complex
brace, involving
other objects (a
wall in this case)
and other trees.
Type A
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