A succa must be directly under the sky; if it's built underneath anything else, it's invalid. Even if the foliage of the tree is very thin, it could still invalidate an otherwise valid succa according to some authorities. Therefore, caution should be taken when building a succa to avoid placing it under a tree.
- Shulchan Aruch, Orach Chaim 626:1
- Aruch Hashulchan, Orach Chaim 626