There are many ways to build model trees. But when it comes to building a forest, modeling one tree at a time could take a lifetime.
When modeling large groves of trees, the secret is to focus on the canopy and the edges. Using these simple techniques, you can create a rather large woodland in an evening. Best of all, many of the materials are inexpensive or free.
Begin by preparing the forest floor. You can build your scenic base out of just about anything. You can use the tried-and-true hardshell method or use a base of inuslation foam. Paint the base a dark color and sprinkle in a little ground foam near the edges. Details will only be seen for about the first 1/2 inch into the forest, but any bright spots will stick out even deeper into the forest.
Time to start planting some trees. The easiest, cheapest and most realistic tree trunks come from trees. Start by heading outside and gathering some appropriate twigs and branches. The sizes you choose will depend on your scale and taste.
Trees come in all shapes and sizes. But one common mistake made on model railroads is making trees too small. A 45 foot tall tree would measure about 6 inches in HO scale. Even 12 inch trees would not be out of scale for some mature species. Of course the right size for you depends on your scale, the trees you're trying to replicate, and even the sense of distance you want to portray.
Insert the trunks into the scenic base. You may need to pre-drill holes in hardshell scenery to allow the fragile twigs to poke through. A small nail will work well. Pack the trunks close together to support the canopy.