Question
I am trying to find out why a mature sweet chestnut has developed reduced leaf growth over the last two summers, the tree does not have dead wood or any obvious signs of pests or disease but the leaves are definately too yellow and sparse for this time of year.. I am in South England. There is a mature but smaller one next to it which is doing fine. Some leaves are turning brown already. How do I find out what is happening to the tree? There are no signs of bleeding canker or bracket fungus or the leaves being eaten. The soil in the garden is very clay and gets waterlogged but the sweet chestnut next to it is fine. We had a dry spell early in the summer.
The one on the left in the photo is the healthy one . on the right is the larger declining one.

Answer
Moby:
Unfortunately, your image was not attached to your message.  You may wish to try again. The symptoms that you describe may be related to several things, but the first things that come to my mind may be associated with a root/soil issue.  A previous drought or excessive water, poor fertility, or previous extreme weather events may come into play here.  Difficult to say without seeing your tree in person.  Contact your local nursery/garden center for their comments and ask about how to get a soil test done for nutrients. There may also be a certified arborist in your town that can give you an on-site assessment.  The clay soils that you mentioned can be a problem. They tend to hold water, but they are also difficult to wet after they are allowed to dry down a lot. Best to avoid moisture fluctuations in these types of soils.  Also moby, you may look for any evidence of bark damage or injury to the trunk and/or roots.

Regards
Steve