Which line in the excerpt from “out, out—” by robert frost uses metonymy?

Frost's poem contains the perfect image of Vermont's spring landscape. The hardwoods lose their leaves in autumn and stay bare through the winter. In spring, the first green to appear is really gold as the buds break open. The willows and maples have this temporary gold hue. In only a few days, the leaves mature to green.

Identify the theme of the story in your sentence