Relevant rule is
805.01 Establishing a Position.
If the disc is suspended above the ground, whether in a tree or a low bush, you mark on the ground directly below the disc and play from there. Not where it ends up after you knock it down.
In a low bush, you mark the lie underneath the disc and you do your best to take a stance on that lie that does the least to disrupt or damage the bush. In other words, don't break branches and don't hold them out of your way, but take a stance that puts a supporting point (foot, knee, hand, etc) on the lie.
If you don't want to play from in/under the bush, you can relocate the lie with a penalty. It is not "out of bounds" however. It is either
optional relief (line of play relief with penalty) or
abandoned throw (re-throw from previous lie with penalty).