What English would translate to:
「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?