Unfortunately, you haven't said where in the UK you are, so what I'm about to say may be completely irrelevant, I don't know without that bit of information. If you live in the South, and had drought this year (as we did) and you didn't water, it MIGHT be that the Birch and the Clematis were competing for water, particularly if the Montana is a large, mature one, and the Montana won.

I don't think Silver Birch is very long-lived anyway. So it's time to say bye bye and use this as a planting opportunity! But don't put the Clematis into a small, new tree. You could take advantage of the lack of tree to cut back the Clematis a bit too.

We had to take a silver birch down many years ago but left a very tall stump and are growing Clematis alpina over it; it lokks great. I wouldn't have thought that a clematis would particularly botyher a 30 foot tree, that is pretty well established. I don't know the answere but our silver birches are still hanging onto their leaves.

Walking in woods in various parts of the UK, we've observed that silver birches don't seem to be very long-lived. We've often found ourselves scrambling over dead specimens that weren't anything like as wide as other species still standing nearby.

They're not, Beattie, 50 to 60 years, with them looking their best between 10 and 20 years, really. But this one wasn't old enough to keel over through age.

Why not give it a little prune here and there to see if the wood is drying out, the feathery end branches will be very brittle if it is dead, whats the base of the trunk like, any signs of desease/fungus here or elsewhere on the tree?, are there any signs of bark splitting?