Read the excerpt from "Birches" by Robert Frost. When I see birches bend to left and right
Across the line of straighter darker trees,
I like to think some boy's been swinging them.
But swinging doesn't bend them down to stay.
Ice-storms do that. 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.
What is the best description of the theme of this excerpt?