First, remember that the boundaries of the Cleveland National Forest are property lines: the name is a designation for parcels of conservation land not unlike a national park or a designated Wilderness Area. Many of these property boundaries aren't detectable by hikers unless there's a sign (which there would be on a trail or at a trailhead). It sounds, however, like you're looking for information that will characterize the forest as you approach from the road.
When you're talking about the forest as an ecosystem (not as the boundary of a conservation area), knowing the elevation of the particular forest can help identify how it changes from field to forest. And knowing the impact humans and natural forces have had on the trees in the last 10 to 200 years can help determine whether you'd be entering an area of dense brush or walking into a stand of trees without much of an understory.
In New Hampshire (the area I'm most familiar with), for example, you could identify the transition between a field and a low-elevation forest by stepping into the shade of white pines and mixed hardwoods--maybe maples, birch, and even a few apples depending on the area. The understory brush would likely be thick since the sunlight would reach the ground easily on the fringe, and there'd be a mix of anything from hobblebush to verbena to poison ivy depending on the exact location. Any place where there's been logging or where trees are growing back from weather events in the past few decades, you'd find thicker underbrush. These areas would also be more densely populated with thinner trees. Further on, in areas where the trees haven't been cut for many years, the forest would open out--the trees would be less dense, thicker around, and easier to see between. The oldest trees--trees in areas that have never been logged--can be astoundingly large, with branches high and the canopy and very little growing underneath at all. As you progress in elevation, the tree types would change, their heights and widths would lessen, and other factors could become more important in determining how the forest is structured.
The USFS website is usually a good place to start for forestry research. You can learn about the history of the forest to which you're referring. You can read a bit on the history of logging in the area, of the types of trees, and even the times of brush/shrub you could find. That way, you know whether you'd be sending your characters through rhododendron brush (the Mid-Atlantic), a stand of hobblebush (New England) or something else entirely. Each ecosystem will be described a bit differently, so the *best* word won't be a one-size-fits-all option.
Hope that helps!
