Different junk causes different amounts of lag, lots of overlapping translucent textures causes HUGE lag (which is why the full tree's and hawaiian trees cause much lag), whereas those skinny mosstrees cause close to no lag even with 1000 of them, because theres very little translucency for the engine to deal with.
Generally, if your getting any FPS drop while your testing your map, that area needs some adjustment, because it means as soon as your on a 30 player server, the FPS drop will increase by around 2x.