This tree seems to be one of the most lasting images in my brain after leaving the theater, and I have been dying to ask this question, as I am struggling to dissect its symbolic significance.
Just to sum up what importance this tree holds, firstly, this is Rachel's grave. As such, it bears the "long-buried secret that has the potential to plunge what's left of society into chaos" - the fact that Replicants can give birth. Secondly, said birth (constantly referred to as "the miracle") happened right in front of it.
There is a scene where Officer K and Mariette discuss the tree. Mariette thinks it's beautiful, while K points out it's dead. I'm trying to decipher this particular exchange, and my guess is that it shows what the tree means to each, and how differently. Mariette, a member of the Replicant Freedom Movement, clearly views it as the site of "the miracle", while K's indifference shows how he hasn't seen a miracle yet, in Sapper's haunting last words.
So that's how much I've analysed this movie, and I'm guessing there's A LOT MORE to do. So what symbolic importance does this tree bear?