Could you post a better photo that is not so dark?
Did you see any flowers or fruits?

I'll hazard a guess at porcelineberry (Ampelopsis brevipedunculata) by the way you describe it.

Thank you, kchd. Here are close-ups of a leaf, from front and back.
(I don't know how to get more than one photo in a post.)

There are no fruits or berries.

Back of leaf.

Oops, neither fruits nor flowers, I meant to say.
But are these little buds where those would have formed?

I have now looked up porcelain berry vines (thanks for that lead). At first I thought the leaves were the wrong shape, but a closer look sometimes showed one or two heart-shapes leaves in with the photographed clusters. Maybe my vines are very young? We had an unbelievably wet summer, which could have spurred sudden growth. The stem was very thick and woody, so I guess this was crawling across the upper tree branches for a long while before we noticed it.

I second the porcelainberry id. My neighbor and I have been fighting it for years, and it took us forever to ID it as the flowers and berries were very few and far between. I have a feeling that the flowers were higher up in the tree. It's a beast, and I swear that every invisible berry sprouts. Unfortunately, until the people behind us actually weed, all we can do is damage control. I hope your situation is better.

Thank you both so much! Luckily, it seems limited to that one tree. Since we cut away the bottom part of the vine, the higher branches seem to be wilting. We will keep an eye out for any resurgence.

Iook Cucumber

I like it

Wild