Do you remember how Jack said that Aku let a single tree stay in the middle of his devastation to remind people of what they lost and go full despair? And them later we discover that the tree was standing there since Jacks time?
Jack and the tree have paraleles, both are from a time before Aku, and both are the only ones in the middle of the devastation to remember it.
In the same way that Jack in the lonely tree, all the other trees are the lives of the people that he saved, by destriying Aku. The sacrifice that Ashi did, and Jack completing his mission ensured that the tree and all the forest will still exist on the future and flourish.
I thought it was pretty obvious with the last shot. >>92711495
But we loved it?