The Visual Tree Assessment One-Third Rule:
Frequently Applied, but Mostly Irrelevant
Shortly after it was introduced, the
"Visual Tree Assessment (VTA) One-Third Rule"
(see for example: Mattheck et al. 1993) became a
popular criterion for evaluating the breaking safety
of urban trees around the world. The One-Third
Rule (t/R>1/3) posited that as soon as the thick-
ness of the outer intact shell-wall (t) of a hollow
or decayed tree stem is less than 1/3 of the local
radius (R), this stem section was supposed to be
significantly more likely to break under wind loads.
Many, if not most, arborists interpreted the One-
Third Rule as defining a clear line between "safe"
and "unsafe," making risk assessment seem cor-
respondingly clear and easy. As a central part of
the VTA concept (Mattheck & Breloer 1994), the
One-Third Rule is still the most commonly applied
tree-breakage safety threshold around the world.
However, since its introduction, the One-Third
Rule has been increasingly criticized, especially by
experts preferring the Static Integrated Assessment
(SIA) method for evaluating tree safety (Wessolly
& Erb 1998). Gruber (2007; 2008) for example,
criticized the VTA One-Third Rule as "scientifically
unproven," "mono-causal," and "untenable failure
criteria for trees." On www.dasgruen.de and other
websites, publications and statements from vari-
ous authors of note can be downloaded in support
of Gruber's criticism of the use of the One-Third
Rule and other VTA thresholds, such as the ratio of
tree height over diameter at breast height (H/D).
In addition, many arborists have observed trees
with small or no defects breaking (Fig. 1), while
mature trees stand on thin shell walls for decades
(Fig. 2). These observations are commonly inter-
preted as confirming Gruber's criticism that the VTA
One-Third Rule is not correct, and resulted in many
arborists switching from VTA to SIA for tree-safety
purposes. This shift from one method to the other
was accelerated since the SIA concept is supported
and applied by many tree safety experts and univer-
sity professors. As a consequence, the freely avail-
able SIA tree-safety calculation form is used by an
increasing number of arborists around the world. >>
Figure 1. Two of many examples of broken conifer trees (in
these cases, of Picea abies). The tree on the left had a small
defect in the center at the stem base with t/R≈1/2 but broke
on a height where t/R≈4/5 (>1/3). The tree on the right was
fully intact (t/R=1) and broke in a combination of torsional
and dynamic loading in a thunderstorm.
by Frank Rinn
Owner of RINNTECH,
Executive Director of
ISA Germany, Heidelberg
www.urban-forestry.com 31
