What is the significance of the tree in "Waiting for Godot"?

'A country road. A tree' is all the setting for WFG. And, in Act 1, the tree has no leaves:

ESTRAGON:
Where are the leaves?

VLADIMIR:
It must be dead.

ESTRAGON:
No more weeping.

VLADIMIR:
Or perhaps it's not the season.

ESTRAGON:
Looks to me more like a bush.

VLADIMIR:
A shrub.

ESTRAGON:
A bush.

You notice how, like everything else in the play, it changes and is hard to define and pin down. They're not even sure the tree is a tree. And in the second half, it sprouts "four or five leaves":

VLADIMIR:
(Silence. He looks at the tree.) Everything's dead but the tree.

ESTRAGON:
(looking at the tree). What is it?

VLADIMIR:
It's the tree.

ESTRAGON:
Yes, but what kind?

VLADIMIR:
I don't know. A willow.

Estragon draws Vladimir towards the tree. They stand motionless before it. Silence.

ESTRAGON:
Why don't we hang ourselves?

It's not a symbol that works in a lateral way. It might represent the gallows tree on which Christ was crucified (i.e. the "one of the thieves was saved" interchange), and it certainly is associated with hanging. Death might be an escape from waiting, or a redemption. A defeat, or a hopeful prospect. It just isn't clear.

And what are we to make of its sprouting leaves? Is it coming back to life? Is to simply to signify a lot of time has passed? Or is it a sign that hope has bled into the play - that it isn't entirely pointless? Make of it what you will. But it's all of these things, I think - a symbol of nature and of life.