I think it would actually be Man vs. man, or person vs. person. The myth is about Atalanta's struggles with Hippomenes; she's a young woman, he's a clever man. He creates a situation that tricks her, or puts her into the conflict, so it'd be strictly him that she's got the conflict with. Not nature, society, or supernatural.
He uses that line to describe the birches themselves. Because when the girls are in that position air drying their hair, it is the same way that these trees bend in the strong wind.
Assonance.<span>in poetry, the repetition of the sound of a vowel or diphthong in nonrhyming stressed syllables near enough to each other for the echo to be discernible</span>
