It's true that there aren't a whole lot of different English words that mean "lots of trees." However, you don't necessarily need to label every mountain range "The <Name> Mountains." If your mountains look like mountains, it may be just as good to simply call them "The <Name>s." For instance, if you were looking at a map of Europe, and there was a large label over the mountains that said "Alps," you'd probably get that those were the mountains called the Alps.
Tad Williams' fantasy trilogy Memory, Sorrow and Thorn featured a large forest called Aldheorte ("Old Heart"). Sometimes it was referred to as the Aldheorte forest, but on the maps, it didn't say "Forest," as the depiction of trees made that obvious.
Many forests are simply named for the country they are in. The word Amazon refers to the rain forest, the river, and the basin in which forest and river reside.
Of course, on the other end of that, are features that pick up additional descriptors as time goes by: Sherwood Forest is a bit redundant.