The first two quatrains of Shakespeare's Sonnet 73 feature extended metaphors comparing the speaker's situation in life to some aspect from the natural world, like autumn trees and darkening skies. The general idea is that the speaker's youth is behind him, that age is approaching, but that he still has some spark of life left. All that natural imagery really hammers home one of the central ideas of the poem—that oncoming death is relentless, inevitable, and, well… natural.
Because fire is something humans can control, the imagery in quatrain 3 isn't nature imagery in the same way that the imagery from quatrains 1 and 2 is.
There is no nature imagery in the couplet because Shakespeare wants to focus our attention there directly on human concerns.