After many years thinking about significant of Christmas Tree; hear is my intuition account derived from Bible verses:
Genesis 2: 9, And the Lord God made all kinds of trees grow out of the ground-trees that were pleasing to the eye and good for food. In the middle of the garden were the tree of life and the tree of the knowledge of good and evil.
Genesis 2: 16-17, And the Lord God commanded the man, “You are free to eat from any tree in the garden; but you must not eat from the tree of the knowledge of good and evil,for when you eat of it you will surely die.”
Well, after the serpent deceived Eve to eat from tree of the knowledge and good, God banished them from the Garden of Eden.
Genesis 3: 22, And the Lord God said, “The man has now become like one of us, knowing good and evil. He must not be allowed to reach out his hand and take also from the tree of life and eat, and live forever.”
From above biblical chronology in Old Testament; I opine Jesus came as banished tree of life, hence the Christmas Tree to depict this.
I base this from Jesus revelation in the New Testament; I quote:
John 6: 51; I am the living bread that came down from heaven.If anyone eats of this bread, he will live forever. This bread is my flesh, which I will give for the life of the world. says Jesus.
John 6: 58 This is the bread that came down from heaven. Your forefathers ate manna and died, but he who feeds on this bread will live forever.” asserts Jesus.
So, Christmas Tree signifies tree of life, thus handshake between God and man.