• Longer and less expensive conservation of mature
and ancient trees as important natural habitats.
Taking into account this one aspect of the many allo-
metric (quantitative relations) properties of mature
trees (annually and automatically increasing basic
safety as soon as height growth stops) thus helps
arborists and tree-risk assessors relax at the tree:
the older the mature trees are, the more they can
tolerate defects. Although it may sound counter-
intuitive, we have to learn and accept what nature
tells us: even strongly hollowed trees can be safer
than young intact trees without any defects (Fig. 6).
The One-Third Rule correctly reflects the fact that the
load-carrying capacity of circular cross-sections with
centrally located defects drops significantly as soon
as the ratio of shell-wall thickness (t) over radius (R)
goes below 1/3. That means the One-Third Rule is a
valid and important aspect for understanding general
properties of circular cross-sections with centrally
located defects, and this can be used to evaluate
breaking safety of trees in forest stands and plan-
tations as long as they are still growing in height.
For safety assessments of the typical mature urban
tree, the One-Third Rule usually cannot be applied
because the cross-sections are commonly not circular
and the defects are usually not located in the center.
The t/R ratios of such cross-sections do not provide
relevant information for determining load-carrying
capacity of the corresponding cross-sections. This can
be done only when applying tomographic approaches,
determining the relative loss in load-carrying capac-
ity as compared to the fully intact cross-section.
But this is only one input parameter when evalu-
ating safety, because the load has to be regarded
Figure 6. This tree has stood close to a subtropical coastal shore line for more than
100 years. As shown by the sonic tomogram, the stem has been severely damaged
and hollow for decades but has survived dozens of hurricanes while young intact
trees in the same area broke. This illustrates that the older trees are, the higher
their basic safety and the more decay the can tolerate without being significantly
more susceptible to breakage.
36 CityTREES
