Nature's first green is gold,
Her hardest hue to hold.
Her early leaf's a flower;
But only so an hour.
Then leaf subsides to leaf.
So Eden sank to grief,
So dawn goes down to day.
Nothing gold can stay.
Cyclic Monotony 
 As the leaves change color, and fall, and grow again, one leaf becomes another and another, around and around in an endless cycle. 
 
 While at first beautiful, the varied hues and nuanced shades become a monotonous reminder of time and of age. 
 People grow older, and no longer do they marvel at the trees and leaves with the wide-eyed wonder of children.  Instead, the seasons and colors become a mundane, monotonous background. 
 The first part of the poem focused on verbal transition—from green to gold, from leaf to flower. With this line the poem makes its careful shift: leaf to leaf, to grief.
Nothing Gold Can Stay