Natural tree hollows are prime real estate for a huge variety of wildlife species. Woodpeckers, owls and bats use them as roosting and nesting sites. Hollows provide small mammals and birds with a refuge from inclement weather and a safe hideaway from predators. The huge range of insect species that make their home in the decaying wood creates a living larder for birds and animals. In fact, the importance of natural tree hollows for biodiversity cannot be overestimated.
The vast majority of any tree is dead tissue. To survive, a tree needs only the bark and a few layers of living tissue. Throughout its life, it is continually laying down new layers of healthy wood known as sapwood. In very mature trees, the entire centre of the tree can decay away leaving just a living outer shell.
Hollowing, which is now recognised as a distinct phase in the life cycle of a tree, has two main functions.
It enables the tree to recycle nutrients which would otherwise be locked up. The tree allows fungi in through the root system. It then manages the colony by feeding the fungi with the carbohydrates they need to break down the deadwood giving rise to the phenomenon known as Red rot. In return, the fungi release back to the tree a proportion of the nutrients which feeds the tree’s roots.
Hollowing also has an important role to play in keeping the tree standing. The formation of a hollow cylinder creates a stronger structure that enables the tree to withstand torsional stress. This means the tree is able to move in the wind without breaking. When the tree detects that the trunk walls are getting too thin, it kills off the colony of fungi by stopping the supply of carbohydrates.