The challenge you face in building a forest for a stage set is to give it dimension. A flat forest is unconvincing. You can assemble a convincing forest by working in three distinct phases on your stage props. As you build the forest from the back of the stage to the front, you will provide depth of viewing field for your audience, and this will help them suspend disbelief and enter the fantasy of a forest on stage.
Paint a canvas backdrop. Use acrylic paint to paint your backdrop with a distant forest scene. Make sure this scene has some perspective, so you can give the eye the impression of distance. Paint far-off trees and mountains to give the impression that the viewer is looking at a far-away vista.
Place boulders in front of the backdrop. Make the boulders out of paper mache. Do this by forming chicken wire into rough boulder shapes, then applying wet newspaper that has been dipped in a flour-and-water paste. After your boulders dry, paint them brown with acrylic paint.
Create polystyrene bushes. Polystyrene is a foam product you can cut with a knife into desired shapes. Many online stage-supply companies sell this product. Cut bush shapes and paint them green with acrylic paint. Attach a 1-inch by 3-inch piece of lumber to the bask of your stage bushes with duct tape. This will prop them up.
Create hanging leaves for the front of the stage. Cut these out of polystyrene and hang them from the stage battens so that they hang down on to the stage viewing area. Paint them green. A few large leaves will give the impression of foliage the audience is looking through to the scene on stage.
Create paper mache tree trunks for stage left and right. These should be large enough to extend out of sight as they rise. Make these from chicken wire and paper mache. Paint them brown with acrylic paint.
A few sticks and logs from an actual forest can be strewn around the stage floor to add to the realism.