tree

Tree leaves turn yellow because of a lack of chlorophyll (leaves use chlorophyll to carry out photosynthesis). In the winter, when the amount of sunlight decreases, photosynthesis can't occur so the leaves lose their chlorophyll and fall off. The tree then goes into a dormant (or sleeping) state in which it lives off of stored energy for the remainder of the winter until Spring, when there is enough sunlight again for the chlorophyl to return to the leaves.

What is interesting about the speaker comparing himself to a tree in the Fall is that the tree isn't really dying, it's just going into dormancy. Is the speaker trying to suggest a rebirth or reincarnation here?

Sleeping is brought up again in line 8 as "death's second self."