I would suggest using your directional light for some overhead moon light, unless the forest is so dense that no light is coming through the canopy. Emitting a little light from the directional light will cause shadows as well and will enhance the scary factor. You could cycle the directional light with cloud cover to make periods of complete darkness if you wanted. Think of it as a real forest, it is usually darker than an open field but there is still light coming from the moon unless it has clouds blocking it or the canopy is too dense.