If the trees are far enough away from the viewpoint, they don't need to be accurately represented-- this might do the trick: 

union { 
  sphere { y*2,1 
    pigment { rgb <.25,.5,0> } 
    normal {granite scale .25} 
  } 
  cylinder { 0,y,.25 
    pigment { 
      crackle 
      color_map { 
        [0 rgb <.35,.2,.05>] 
        [1 rgb <.5,.35,.2> ] 
      } 
      scale <1,3,1> 
    } 
  } 
} 

You can change the sphere to a cone { y/2,1,y*2,0} to simulate a pine tree.  You'll want to randomly scale each tree from .8 to 1.2, to make the forest canopy a bit more random-looking. 

Remember, these are down-and-dirty models for trees, but if there are a whole bunch of them a long ways from the camera, nobody will be able to tell.