It'll figure it out and faster than you'd expect :-) Other than training them towards the tree trunk, I've never tied or attached a clematis vine to the trunk or branches. They scramble up very easily on their own.

If you value the dogwood, I may reconsider planting the SAC to grow through it. This will become a very large and very vigorous vine, producing a LOT of foliage growth, and has the potential for overwhelming a smallish tree like a dogwood. And I'd hate to consider removing the mass of entangled vines when pruning time comes. This can be an issue for growing any sort of clematis through something as large and as complex in branch structure as a tree - all of my clematis that are growing through trees are vines that I do not need to prune back or very infrequently.

I agree with gardengal. A SAC could very easily overwhelm a dogwood depending on its current size. It will easily wind its lengthened side branches around things much larger than an inch or so in size.

I personally don't have an issue growing clematis into trees and removing the branches at pruning time however since they become very brittle as the plant goes into dormancy. Not only that the wind will easily pull any of the unreachable vines out of the tree branches over the course of the winter--in my area anyway.

Thanks for the input. This dogwood badly needs to be overwhelmed--though it's still a sturdy structure, most of the flower and leaf has gone to anthracnose. My husband and I were mulling over taking the whole thing down, except there are two branches that won't quit; we didn't have the heart. The clematis seemed a good way to cover up most of its bareness and give it some green.