In times of drought, the extreme dry conditions can put an old oak tree under great stress. It responds by killing off some of its own branches, leaving behind antler-like skeletons. After a few years these branches will decay and the tree's canopy shrinks. Meanwhile its trunk becomes fat and covered with new wart-like growths. Some of these are a natural feature of ageing, but others are produced by fungi. Bit by bit, the fungi eat out its core and the tree is hollowed out. Yet these particular fungi are as old as the tree itself and are not a threat. The active part of the fungus will have been growing for centuries, eating away the tree's heart wood which is actually dead. It leaves the living part of the tree untouched. The hollow centre can now act as home for more creatures, including a colony of hornets.