Posted: 14/03/2014 at 14:40
there has to be a blance between the amount of leaves a tree has and the amount growth those leaves can sustain. If you think that growth is all over the tree from the tips to the girth at the base then there's an increasing amount of growth each year and not a balanced increase in the canopy of the tree so part of the tree will die once that balance tips in the direction of growth over leaf. Not easy to explain.
I'm assuming the cause isn't disease of course. If it is disease then you will need to cut off all dead or disease bit into clean wood with a clean saw.
We had a really old Laburnum that had lost it's main trunk, it was still a beautiful small shrub for years. So I agree with nut. Yours is a very beautiful tree so only take off what you have to.