We were recently asked to inspect a very large Holm Oak (Quercus ilex) for a client near Princes Risborough. The tree was huge by any standards with a large crown spread of around 25 meters and the trunk diameter close to ground level of about 2.5 meters. We estimated the age to be around 300 years. The tree stood in a garden situation with other property and gardens around it. The owner had noticed a large fungus growing at the base of the tree and was concerned about the safety of the tree.
We were able to identify the fungus as Inonotus dryadeus which is a lumpy brown bracket type fungus often found on old Holm Oaks as well as other species. This fungus digests the lignin content within the lower stem and buttress roots and so can have a significant effect on the structural integrity of any affected tree. There were also cavities and rot pockets higher on the tree and we needed to get a clearer picture of the extent of the decay, particularly in the buttress and lower trunk area. Sounding with a mallet indicated that much of the wood was affected but we decided to get more reliable evidence by the use of a tool called a Resistograph.
The Resistograph drives a microdrill into the wood to a depth of 40 centimeters. The drill measures the resistance that the bit encounters as it passes through the wood. This in turn gives a print out that can be used to determine if the wood is affected by the decay fungus. With experience and training it is possible to build a three dimensional ‘picture’ of the quality of the wood inside the tree.