To answer your question, William, I am no expert but I would guess that different kinds of birch trees flourish in Britain. They are known as "pioneer" trees because they are among the first to grow where the land has been cleared by a fire or some other disturbance. They grow so well, in fact, that they can threaten heathlands if the saplings are not grazed down, burned off or thinned out by other methods. Their favored places to grow are lowlands. In the UK, they grow so well that they're called "forester's weeds."

As for your other question, if all birches are white in the center, someone else will have to answer that one!

Tomorrow is a new moon and a new tree - Rowan. So if anyone has anything to add for Birch, this is your last chance.