Trees in Sydenham

[Mungo]

Walking around Sydenham, I have noticed a lot of dead and dying trees. Some have been removed but none replaced. What is causing this and what can be done?

[syd-gal]

The horse chestnuts have a disease, a leaf blotch, where the leaves all turn brown, but apparently it doesn't really damage the tree.
I haven't noticed any trees that have been cut down.