I have to answer that question, I'm going to use Waiting for Godot and Cat on a hot tin roof because i saw a representation where trees were used.
For now, I've found that trees are used to show the passing time and that they help represent the outside. I need to find two other uses, so if anyone could help me it would be great.
In Waiting for Godotthe tree could represent the possibility of suicide. Vladimir and Estragon spend a good deal of time commenting on the tree, specifically the strength of the tree's limbs, to be able to kill themselves by hanging themselves from the tree. The two men are spending their time contemplating their circumstances and come to the realization that they don't have very much to live for so they contemplate a dual suicide. The irony of the situation is that the tree is even "conspiring" against them -- it looks like it will be too weak to support their bodies and that if they did attempt to hangthemselves, the limb could merely break and they would survive the attempt! They are ultimately left to just continue to wait for Godot to come and give them the direction they so desperately desire.
The tree does also suggest the passage of time and/or the absurdity of the passage of time in the play. During Act 1 the tree has no leaves, but at the start of Act 2, which is seemingly the next day, the tree has sprouted leaves. We know that that is an impossibility, thus we are left questioning the men and their understanding of their circumstances and the passage of time.