In the poem "Birches," what do the bent birches prompt the speaker to imagine?

The speaker is prompted to imagine boys swinging the birches left and right until they touch the ground. But in fact, the speaker is aware that they wouldn't stay down because of the boys swinging them. It is actually ice-storms that do it. For the boys, it's just a game. For nature, it is a serious matter of being restrained and pulled down to earth.