Foreground trees, as the name implies, are on the front of the layout and easily subject to close inspection. I use several meathods to make them.
Woodland Scenics Fine-Leaf Foliage:
Woodland Scenics Fine-Leaf foilage is a natural plant material. It comes attached to branches. It can either be stripped off the branches and glued to WS tree armatures or You can break the branches into smaller pieces and plant them on your layout, as I have done.
Fine-Leaf foilage makes a very realistic tree. However, it's not cheap and one box only covers a relativly small area.
The tree in the above photo was completely home made. I purched some material used for making dried floral arangments at Michael's, cut several pieces and lashed them together with wire. I them covered the wire with Woodland Scenics Flex Past. Once dried I painted the lower trunk brown and planted the tree on the layout.
While inexpensive, this trees are not as realistic as some of the others. The are also quite labor-tensive and tedius to make.
An easy way to mass-produce trees cheaply is to make them out of goldenrod. Wait for the goldenrod to turn a white-ish color in the fall and harvest it. Take the individual flower heads and spray-paint them the desired color. Then it's simply a matter of cutting off individual pieced and trimming them into a tree shape.
These trees work best when packed together tightly into a desne forrest. They can be produced very quickly for very little money. However, before you go out harvesting goldenrod, make sure you have permission to do so. You don't want to be cited fro tresspassing or worse, end up on the wrong end of a gun.
Woodland Scenics tree consist of plastic armitures, which you glue foam foliage to. These are available both premade and in kit form. The kit form is cheaper but tedius and messy to assembe. They work well for accent trees but can get expensive when covering a large area.
Background trees are not as subject to close inspection as foreground trees. They are used for covering hillsides and mountains. Only the canopy is modeled. giving the illlusion of a distant, dense forrest.
One popular meathod for making background trees is to make "puff balls" out of polyfiber and green turf. However, I prefer to use lichen. I find it much more life like than puffballs. However, it's not cheap. Michael's sells it for about $7 a bag. However, you can usally print out coupons from their web site.