Metaphor one:Frost uses birches as climbing high and then returning back to earth.In this metaphor he wants readers to think about the risks we take in life
Metaphor Two: You´d think the inner dome of heaven have fallen, meaning one sometimes think everything is over till it starts again.
Metaphor three:Onee could do worse than be a swinger of birches.A person has to try different things and face challenges, not doig so is much worse than trying.