Question
We have a small piece of woodland behind our cottage in South West Wales. The trees are 20 years old including a number of oaks which are starting to look as if they are dying. One has got what appears to be dead branches and all the leaves have gone brown, There ii damage at the base on one side of the trunk which looks as if it has been taken away but the others show no sign of trunk damage, just the leaves going brown and some of the branches are dying.

Answer
I may have a hard time answering this question. I will start by asking questions and possibly you will be willing to answer back. Has there been any kind of herbicide work done in the area where these trees are growing ? Has the watering changed  or a spring that has opened up ? Is there areas on the trunks of the trees that are weeping or holes that have frass ? ( frass looks like termite droppings or coffee grounds ) If you can answer back I will have a better understanding at least of what it might not be. thanks