Step inside a hollow tree and you step inside history. The hollow centres of trees have hosted dinner parties, preacher’s pulpits and bullpens. It’s said that the centre of the Bowthorpe oak in Lincolnshire (shown below) once held 39 people within the trunk! Apart from history and bit of fun, what are the benefits of hollow trees and what are the risks?
A hollowing trunk is a natural process and it is not necessarily a sign of an ailing tree. The centre of the tree is deadwood which is slowly decayed by fungi. The fungi is perfectly happy in the deadwood and will rarely touch the living sapwood.
The tree has spent years storing up minerals in the wood in the centre of the trunk. As this wood is decayed the minerals are released and can be used once more by the tree; the hollowing trunk is providing the tree with recycled nutrients, helping it to live longer.
A hollow trunk may react better in high winds, allowing the tree to bend rather than break.
A hollow trunk is an ideal location for wildlife. It provides protection from the weather and a more constant environment than outside. A hollow trunk provides nesting and roosting sites for bats and birds. The base of a hollow trunk provides an ideal location for hibernating hedgehogs, and for snakes to lay their eggs. Fungi, epiphytes and invertebrates will also colonise a hollow trunk.
In brief, hollow tree trunks are an essential habitat for a wide range of our native flora and fauna.
The concept of the t/R ratio was first applied to hollow trees by Mattheck and Breloer in 1994. It has subsequently been extensively reviewed, and latterly revised by the authors.
The t/R ratio is the ratio between the thickness ‘t’ of the remaining sound wood of the hollow tree and the radius ‘R’ of the original stem.
The application on the t/R ratio is nuanced, but in broad terms, based on the evidence available:
- hollow trees are unlikely to fail with a t/R ratio of 0.35 or greater
- a small minority of hollow trees with a t/R ratio of 0.30 will fail
- a third of hollow trees with a t/R ratio of 0.25 will fail (two thirds will not fail!)
- the majority of hollow trees with a t/R ratio of 0.1 will fail.
Hollows are a feature, and a natural part of the life cycle of a tree. They are also an essential component of the life cycle of a wide range of our native flora and fauna. Yes, there is a risk of stem failure, but the likelihood of failure depends on the extent of the hollow as a percentage of the original stem, as well as the wind loading, exposure, crown architecture and species. Many hollow trees can be successfully retained for history, for wildlife or for a bit of fun by undertaking a little judicious pruning.
Felling must be the last resort, surely?
FOUNDATIONS OF TREE RISK ANALYSIS: Use of the t/R ratio to Evaluate Trunk Failure Potential by Jerry Bond