Gaugemaster produce this bulk pack of 25 model trees with mixed shades of green and heights ranging from 50mm to 90mm.
Each tree has plastic roots and can be glued into place.
With the addition of suitable ground cover you have a small OO gauge woodland in a pack
Don't forget that trees are rarely uniform in size. So, although these packs may be most suited to OO an HO scales, they may well be suitable for applications in other scales too.