Why do trunks become hollow?
A hollowing trunk is a natural process and 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.
Trees spend 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. Along with the yearly supply of leaves and any decaying branches, the hollowing trunk is providing the tree with recycled nutrients, helping it to live longer.
A hollow trunk might react better in high winds too, allowing the tree to bend and reduce damage.
From here
