Decay opens with three short lines that move from a forest to a "dammed" forest to a concrete one. The setting seems to shift from nature to something built and blocked, and the poem never says exactly where we are. Judging by the imagery, the forest might be a place, or it might stand for the world we live in.
After that, the poem stays on one idea: whatever enters will rot. Flesh decays, bones wither, dust is carried off by the wind, and nothing is left. The staggered, stair-step layout of the lines makes this feel like a slow fall or a fading out.
The speaker sounds calm and resigned rather than angry or afraid. There is no pleading and no comfort, just a flat statement of fact. The repeated opening lines return near the end like a cycle, but the last words, "no longer remain," leave a cold, empty mood. The poem is bleak, quiet, and final.