Assistance to Arboricultural Professional in Preserving Ancient Trees

William M. Fountain

   It is common for people to be concerned that trees will fail and cause harm to property or result in personal injury.  While the prudent action is to obtain a tree risk assessment by a qualified arborist with an International Society of Arboriculture™ Tree Risk Assessment Qualification® prior to construction, this is rarely the reality.  

   Situation:  As part of the infill process of development of Lexington, a developer purchased approximately 8 acres of previously undeveloped land.  This land had not been developed because of the steep grade and the presence of two ancient bur oaks (Quercus macrocarpa) that predated European settlement.  Construction had already begun on two houses that encroached on the dripline when the contractor contacted a local consulting arborist. These trees had a DBH (trunk diameter measured at 4.5 feet above the ground) of 66 and 74 inches.  Both trees had open cavities and internal decay in the crown due to ancient lightning strikes.  The concern of the arborist consulting with the developer was over the soundness of the buttress roots and lower portion of the trunk.  Sounding with a mallet with a nylon head indicated that the buttress roots were sound.  This simple and cost effective technique was not appropriate for a trunk of this diameter.

   The outer third of a trunk contains a little over 50% of volume but carries approximately 90% of the load.  Sounding only allows an experienced arborist to detect cavities in the outer 4 to 5 inches of the trunk, far short of the minimum of 10 to 11 inches of wood necessary in the shell wall of a tree of this diameter.  Use of a Picus® tomography unit indicated that there were no significant pockets of decay in the lower portion of the trunk and that the tree cold be safely retained.