This is where the forest proper begins. Charlton Flats is a rare level area at the start of the San Gabriel Mountains at around 5,500 feet elevation that's used as a day camp.
John McPhee writes of a "reverse treeline" in southern California: the forest does not begin until 5,000 feet up, and the upper treeline -- 9,000 feet -- can't be seen from the highway.
Like all forest in the San Gabriels, it's quite open, with an arid sandy floor that has green grass in spring. In winter it gets snow, so it's often closed then.
Try the hike to Vettner Mountain, where you can get a 360-degree view of the mountains and chaparral around you from an old firespotter station that's recently been reopened.
Also check out the Nature Trail, a half-mile hike. Both hikes begin beyond the closed-off portion of the paved road.