Here's the core of what the case said about AG.
"Examination has revealed that where it joined the main stem of the tree there was a depression or cup on the upper surface and at either side of that point of the branch there was the phenomenon of adaptive growth (“AG”) in the form of what Mr Forbes-Laird calls adaptive growth flares, and which some others call “bulges” or “ears”. These, it is agreed, would have been visible from the ground, and are very much at the heart of this action. In simplified terms AG is a biomechanical response by the tree to load stress on a branch, or to gravitational forces causing deflection of the branch, or as a “repair” response, for example, to some internal damage to the cambial cells on the outer surface of the wood of the branch. This autonomic response causes the normal annual growth ring process, programmed into every tree, to become exaggerated in thickness in order to reinforce or add strength to the branch at this point. In other words it is autonomic response by a tree to a stressed branch.
"All agree that inspection of trees for safety purposes is carried out from ground level – a system called visual tree assessment or VTA, first defined by Professor Mattheck. It is based on inspection from ground level and does not require any aerial examination. The depressed area I have described above would not have been visible from the ground. The claimant’s expert estimates that the AG flares had been in place for 5-10 years. At some later stage a crack or fissure had developed in the central upper surface of the join as a primary failure due to stress and this had led to oxidation and water ingress into the wood of the branch at this point. This too would have been impossible to see from the ground. Once this crack process started it is said that a secondary failure leading to the shearing from the branch of the stem was inevitable.
"At the heart of the claimants’ case is the proposition that the mere presence of the AG at this point was a warning sign of a possible failure and meant that further investigation of the join was indicated. The defendant’s case is that AG is a frequently found feature on many if not most mature beeches of the age of this tree."
The case is worth a read as it has all the drama and suspense of a Detective Dendro story...