Isaiah 10:34 (King James Bible) and he shall cut down the thickets of the forest with iron
The 1953 RLDS edition has the singular forest. This change may have been the result of consulting a King James Bible, although it should be pointed out that previously in 2 Nephi 19:18 the RLDS text retained the plural forests. The fact that the earliest textual sources for both 2 Nephi 19:18 and 2 Nephi 20:34 have the plural forests rather than the King James forest implies that the original change in number was intentional.
It should be noted that the original 1611 King James Bible actually read in the plural for Isaiah 10:34 (spelled as forrests) but in the singular for Isaiah 9:18 ( forrest).
Summary: Maintain the plural forests here in 2 Nephi 20:34 as well as in 2 Nephi 19:18; the change in the Book of Mormon text from the King James singular forest to the plural forests appears to be intentional.