First - trees should be growing. It's unnatural that there are only fully grown trees, there should be at least three phases of tree growth. For example the smallest gives 1-3 branches (and disappears after collecting?), medium around 5 branches and few blocks after cutting and third as it is.
Second thing about spawning: It should be a little different then suggested earlier. Each fully grown tree sometimes tries to spread on certain area around it, but the more treas are around the spot, the less chance that the tree will sprout. This would work more like in nature - in thick forests new thees have little chances to appear because there's not enough sunlight and water for them to grow. This way new trees will grow mostly on forest borders (like it is in nature).
There's also might be a very small possibility of a tree sprouting from a seed, that someone threw away.
Also a little suggestion about graphics: Birch Tree is too round and thick, it should be more irregular, transparent and willowy.
And apart from trees, why there aren't any flying birds? I like birds, it's lonely without them
