I wouldn't worry about the polycount. Your level has in this, let's call it, skeleton version 1640 faces (will round it up to 2k). So if you enhance the geometry x10 that's still only 20k Loopix trees have approximately 250 faces each, so if you put 40 trees that's about 10k (20 tress only 5k).

So if you enhance your level geometry by 10 times & place 40 tress that is 30k, & if you enhance level geometry by 15 times & put 20 trees that is 35k

...and with the proper use of lights while baking (subdividing the edges & setting the smooth) you can have level that looks like it has millions of poly's.