Well, I think the story of that tree is explained in the Silmarillion, but I'm not sure anymore if it is explained in LOTR.
Uhm well the tree in Minas Tirith (I think that's the one you're talking about) is a descendend of the tree Nimloth that grew in Numénor, which in turn was a descendend of the Celeborn, that stood on Tol Eressëa, and that was in turn a descendend of Galathilion, a tree that stood in Tirion.
That tree was an image of Telperion, the white tree, that shed light on Arda in the years of trees (when they were destroyed the first age began).
What a story eh! Got that all from the encyclopedia! But uhm yeah that tree in Minas Tirith is, remotely I give you that
, a gift of the Valar.
And that tree will play some more part in the story.!
Have I answered your question.. I think not but okay
!