Let us just remember the overall context of this poem. It begins by describing the way in which a woman was punished by death for committing adultery in her time period. The corpse of this woman was found in 1951 in a peat bog, having been preserved for centuries and therefore open to analysis and investigation. The stanza you have identified relates the formation of the peat with the preservation of the corpse. Peat is of course formed by dead trees that, over a long period of time, becomes transformed in to peat. Thus the stanza points out the way that her existence became intermingled with those of the trees that formed the peat that preserved her. This is why the stanza says she was at first "a barked sapling." The last line points towards the curious fusion of human matter with trees, reinforcing the way that peat has preserved the corpse over all fo these years.