Isaiah 9:18 (King James Bible) it shall devour the briars and thorns and shall kindle in the thickets of the forest
Here in 2 Nephi 19:18, the Book of Mormon text has the plural forests instead of the singular forest (the King James reading). The plural forests is also found when the same phraseology appears in the next chapter:
The fact that for both these examples of “the thickets of the forest(s)” the Book of Mormon text replaces the singular forest with the plural forests suggests that the difference in number is intentional rather than accidental.
Summary: Maintain the plural forests in both instances of “the thickets of the forests” (2 Nephi 19:18 and 2 Nephi 20:34); this difference in number from the King James forest appears to be intentional.