Across the lines of straighter darker trees,
I like to think some boy's been swinging them.
But swinging doesn't bend them down to stay
As ice-storms do. Often you must have seen them
Loaded with ice a sunny winter morning
After a rain. They click upon themselves
As the breeze rises, and turn many-colored
As the stir cracks and crazes their enamel.
Soon the sun's warmth makes them shed crystal shells
Shattering and avalanching on the snow-crust--
Such heaps of broken glass to sweep away
You'd think the inner dome of heaven had fallen.
They are dragged to the withered bracken by the load
And they seem not to break; though once they are bowed
So low for long, they never right themselves:
You may see their trunks arching in the woods

 bracken  (n.) 
 
 A widespread, often weedy fern (Pteridium aquilinum) having large, triangular, pinnately compound fronds and often forming dense thickets. 
 An area overgrown with this fern. 
 
 (American Heritage Dictionary) 
 
 
 The speaker means the birches are bent (“dragged” down) by the weight of the ice, so low as to touch the surrounding underbrush, which has been “withered” by winter.
Birches