The young poplars fringing the Loire are also tapered trees, like the candles and the coffin. The Loire here is a river which has seen many bodies being used as fish-bait, which gives "the breath of baited bodies" an added layer of meaning.
[The repeated "l" and "b" sounds in this stanza could also be a reference to the length x breadth required to find area of a shape- probably slim and far-fetched]