Decay is sometimes thought of as bad for the tree. However, decay of the wood at the heart of the trunk, which eventually causes hollowing, is called ‘heart-rot’, and this can be beneficial for the tree, releasing nutrients for growth. It provides important habitats for fungi, invertebrates and vertebrates, and may enhance the longevity of the tree.
Heart-rot is determined by age, not size. Bonsai trees can be hollow. With beech, hollows are forming by 100 years, and become widespread between 100 and 300 years. In oak, it happens between 200-400 years.
Heart-rot is a problem for trees grown for timber, and often timber trees need to be felled when young to avoid the problem. This led to the view that it is a pathogenic process; we now know it is part of the natural process. We don’t know HOW it happens, but we do know WHY.
Cavities usually form through the activities of heart-rot fungi, although woodpeckers and termites can hasten the process. The conditions in heart wood are usually harsh with low pH, toxic chemicals and high levels of water and CO2. Only a few specialised fungi can tolerate the conditions. They include Fistulina hepatica in oak and sweet chestnut. Amillaria gallica, the Honey fungus, can also decay trees.
Heart-rot is usually in the roots, the lower trunk (butt) or the main trunk. Sometimes, a fungus only affects one of the areas, some affect two or all three.
Heart-rot is generally a slow process, taking many decades, even centuries. The
fungi that start the process are usually replaced by secondary colonisers. The limited information we have is from a study of 68 beech trees from a forest plantation. Four different species of fungi were found in samples of wood taken from the trees, from the initial coloniser to those involved at the latter stages.
The habitat of decaying wood is home to thousands of species of saprophytic species. Some are host-specific, others are more general. In the UK, 1700 invertebrates (7% of the UK total) depend on decaying wood. Mammals, such as bats, voles and shrews, and birds, also depend on hollow trees.
As an old tree hollows, the conditions change and its value, in terms of habitat, increases. Hollow trees matter.