Then leaf subsides to leaf. (5)
This line refers to the early leaf (actually a flower) turning into a normal, green leaf. The word "subsides" lets us know that this transformation isn't exactly a good thing. "Subsides" means to sink down, so the change from early to later leaf is a step down. It would be nice if things always changed for the better, but this poem suggests that might not be so—like it or not.
So Eden sank to grief, (6)
If there's an archetypal example of impermanence, the Garden of Eden is it. The famous biblical story of original sin shows how, at least in the Bible, humans have never been able to exist in a state of paradisiacal perfection. There's something in the nature of the world, and in human nature, that leads to transience.
So dawn goes down to day. (7)
Again, the poem hints that something is changing for the worse. When we think about dawn, we normally think about the sun rising. That's a positive thing, filling us with anticipation of the warmth of the day. But the negative idea of dawn going down to day makes us think about how we'll miss the beautiful colors of dawn once it's gone.