Is it normal for trees to have decay inside the leaders or large branches? How much decay is acceptable?

Question

My tree is 15m tall.

What decay assessment methods are there?

Answer 1

It is normal for aged trees of certain species to have a small degree of decay in the lower parts of the lower branches. Your tree, at 30 metres, does not appear to be an aged tree, unless it is a dwarf variety or has been pollarded.  Unless your tree is an oak, lime or a yew I would be be a little concerned.

How much decay is acceptable depends on the species of tree, the possible progression of the decay and the position of the tree with regard to dangers to person or property  from a sudden shed branch.  

Decay assessment can only properly be undertaken by an experienced person like a tree surgeon. He/she would apply their accumulated knowledge with regard to the species involved and/or do a core drilling into the branches to assess the extent of the damage.