why translate to「周囲の林が暗くなっている」?

Question

What English would translate to:
「周囲の林が暗くなっている。」

「暗くなっている」// speaker location unspecified
「林が暗くなっている」 // speaker location implied as being in the forest?
「周囲の林が暗くなっている」 // speaker location implied as being on the perimeter of the forest?

So, when speaking, it's somewhat unnatural for an English native speaker to say:
"The perimeter of the forest is getting dark."
"The forest is getting dark." sounds much more natural.

So, why translate as "周囲の林が暗くなっている。」？.

Well it's to imply the speaker's location (and not a try at a direct translation), right?

Answer 1

「A の B」 = "B of A" ≠ "A of B" ← A fairly common mistake among J-learners.

「周囲{しゅうい}の林{はやし}」 = 「林 of 周囲」 ≠ 「周囲 of 林」

「周囲の林」, therefore, means "the surrounding woods" and not "the perimeter of the woods".

So, a sentence like "The surrounding woods are getting dark." or "It is getting dark in the surrounding woods." would best translate to 「周囲の林が暗くなっている。」.

Answer 2

You've confused 周囲の林.

周囲の林 = The surrounding forest

perimeter of the forest　would be something like 林の周囲 http://ejje.weblio.jp/content/forest+edge

So the english translation would be:
The surrounding forest is growing dark / has grown dark.