Question
This 20ft Birch tree is beautiful.  However at the very top it has developed two main branches.  Friends have advised that the top of the tree should never be trimmed. The "V" at the top is comprised of two main branches; one about thumb diameter, the other about the size of your index finger. My concern is that the winter winds will split the tree if left in this condition.  Should one or both main branches be trimmed?  Thank You

Answer
No do not cut either. If the tree is 20 feet tall then even if the tree continues to be forked at this point the likelihood of it splitting is very low. If the fork was at ground level then it might split at some future time. Cutting the top out of trees is damaging to the growth. The height growth of a tree is in the meristem (top). Leave them alone one of the two branches might even become dominate and be the top and the other a side branch. Birch trees are for the most part  resistant to wind and ice damage due to stout branching habit.