Figure 2. This mature oak tree (Quercus rubra) has a diameter at breast height of more
than 2.5 meters (8.2 feet). As can be seen by the sonic tomograph (Rinn 1999, 2014b) and
the resistance drilling profiles (Rinn 1988, 1990, 2016), the intact outer shell wall ranges
from a few to approximately 30 cm (12 inches). The average shell-wall thickness is less
than 25 cm (10 inches), which means t/R<1/5 (<1/3). This tree is standing despite heavy
defects, being hollow for decades, and achieving a height of more than 25 m (75 feet).
Arborists strictly applying the One-Third Rule as commonly understood usually condemn
such trees as being unsafe or tend to recommend strong pruning and even cabling.
However, often there is no need for that—and heavy reductions can significantly
contribute to an accelerated spread of internal fungal decay (Rayner & Boddy 1983).
Applying the tree-safety concept presented in this article leads to different conclusions
for such mature trees and mostly results in nothing to be done or just pruning the crown
symmetrically in order to prevent wind-induced torsional loads.
32 CityTREES
