There are separate aprite limits:
1500 things in the air (arrows, birds)
2000 people + animals
2000 trees + scrubs
2000 rocks.
Unfortunately, there is no way to raise the limit, though in reality a map with all 2000 trees + scrubs would be considered far too cluttered even for a forest map. You CAN use pitch to mass-burn trees, though it takes a few game years for them to actually rot and disappear, and some will never go away until cut down (I found this in a few of my maps!).
..."religion is the opium of the masses"... - Karl Marx.