You could randomise the trees a bit colours size shape etc.
is it paint fx or fx converted to polys?
I once did a scene with a lot of trees in it and I converted the paint FX to polys (made about 4/5 trees differing styles, sizes, colours etc) as I wanted to render with mantal ray. then moved the pivots to the bottom of the trunks and then painted a few different particle sets, and attached the trees to the particles as instances and added a random Y rotation to them to vary them up a bit.
Also in the distance, I would just render a tree line/forrest as a single image and attach it to a plane with the alpha for transparancy, cut down on the strain on the system
"No pressure, no diamonds" Thomas Carlyle