There’s so much going on in this photo that’s it a bit difficult to see what is going on. You might have to take my word for it that there are 3 different clematis growing up this half dead dogwood tree.
You can see all the clematis vines snaking up the trunk right at the base of the photo. And just before the large branch forks off to the right, there’s another cluster of vines shooting straight up into the air, heading for the crown of the tree.
What on earth made me do this? This tree was supposed to die. It’s been dying since 1996. I even planted the Japanese maple next to it to replace it, so to speak.
Well, the branches on the Japanese maple are now up to the second story and the dogwood is still hanging on. You can see the seedlings from the maples coming up under the dogwood. They give the clematis a hand (a leaf?) up on their way to the light and tree canopy so they can stay.
And meanwhile, the dogwood hangs on. I pruned the maple back a bit this year, thinking that maybe I would give the dogwood a fighting chance. I probably will kill it with kindness now.
As for the idea of growing vines in trees, I am not sure where I got that from. I suspect I read it in in British gardening book. It seemed like a good idea at the time.
Gardendaze 
On larger trees, it might not be so bad, especially for docile vines that die back for winter, such as perennial pea or morning glory.
This has seemed to work out quite well despite the fact that the tree is already compromised. I surely wouldn’t try it with something crazy like wisteria or trumpet vine, obviously. Clematis seemed the obvious choice because it liked shaded roots.
Interesting that you mention morning glory. Lots of people consider these invasive, although they really are not, by definition. They’re just pesty. I grew that lovely purple one ‘Grandpa Otts’ probably 20 or more years ago and it’s seedlings are still coming up in my garden. Luckily, I don’t mind–but you can see where people might find that annoying, to say the least.
I am rarely annoyed by bonus flowers.
Karla