...With *HER* explanation. :-) I am woman, hear me roar.
In any case, I really think you're nitpicking. This was the kind of cedar tree I envisioned in the poem, which is quite large enough for darkness, snow-protection and whatever.
http://www.gutenberg.org/files/13011/13011-h/images/12.jpg
I'd like to remind you that it's also twilight on a foggy day, which is really quite dark; the poem deals with the transition from black shadows against gray to entire darkness.
I think, however, that you misunderstand the general idea of the poem. It is meant to be told from the point of view of someone who has been dead for some time, and is now lying - perhaps in a grave? - beneath a cedar tree. Perhaps re-reading will clarify.