Aerial roots

Beech trees with aerial roots in woodland at Alderley Edge, Cheshire

The heartwood of trees is made up of dysfunctional wood whose only purpose is to provide rigidity for the tree. It is no longer able to transport nutrients and water from the roots to the leaves for photosynthesis and then sugars and starches back to the trunk and roots for growth.

As the tree becomes very mature the need for this volume of rigidity diminishes and heartwood decay begins to takes place. This is a natural process that all old trees go through.
The decay fungi breaks down the structure of the wood, which then enables the tree to reabsorb the nutrients locked away in the heartwood for centuries.
This is done by producing internal aerial roots which grow inside the decaying wood. These can become visible if the decay process extends all the way to the outside in fragmented sections of the trunk.
The decaying wood acts like a slow release fertiliser for the tree.