The first six and a half stanzas of A. R. Ammon's poem "Easter Morning" are written using an elegiac form. (An elegy is a text which is a memory of times gone and, typically, show the memories of lives lost.)
Within these lines, the following is found:
I must stand and fail,
calling attention with tears
to the branches not lofting
boughs into space, to the barren
air that holds the world that was my world
through the incompletions
(& completions) burn out
standing in the flash high-burn
momentary structure of ash.
Here, the bold-ed phrase refers to the past, the crumbling of life and its turn to ashes. The speaker, therefore, is remembering those who have passed on. The speaker is struggling with the idea of moving on and the ashes represent the rebirth of the speaker and his mentality. (Similar to the phoenix which rises from the ashes to begin a new life and Christ's resurrection on Easter morning.) The momentary standing on the ashes remind the speaker of the importance his need to move forward.
Later in the poem (the second half of stanza six to the end), the speaker's mood changes. Easter morning has brought promise for the speaker's life. No longer is he burdened by the past. He, and his changed perspective, can now look forward to the future.