EDIT: Actually, in retrospect, this is not quite right. It maps every tree to a natural number, but not every natural number to a tree. Regardless, it can be handy in certain circumstances.

Another simpler way of doing this is to encode a tree with n levels as 2^n - 1 bits. The first bit is 1 if and only if this tree has a root. The next 2^(n-1) - 1 bits represent the left tree. The following 2^(n-1) - 1 bits represent the right tree. Recurse as needed until you get to a tree with only one level (which is either a single node when the bit is 1, or is nothing when the bit is 0).

I'm fairly sure this is not optimal, but it is much simpler.