Before World War II there was a lot more cleared farmland than there is now. Before chemical fertilizer and lime was readily available, per acre yields for farm crops were much lower and so more land was needed to farm. This was not a problem, as most farms had large families with a built-in labor force. But over the years the land eroded, kids left the farm, and the farmer got older. So gradually the steeper, rougher fields or field edges were let go, and the forest reclaimed them. There are indicators you can look for to tell if a forest was once a field.
One obvious indicator are rock piles in the old field woodland where farmers were constantly picking them up out of the field. Another indicator is that some tree species are geared to jump onto open areas quickly and grab the sunlight before everybody else. Foresters refer to them as pioneer species: the first to immigrate to a new area. The most common pioneer trees are cedar and pine, which often form pure stands on old field sites. Both species must have full sunlight to grow, and so their survival tactic is to seed into fields from birds or the wind and put on height growth quickly to stay ahead of competitor trees. Eventually their crowns grow together and “close canopy” as foresters call it, shading the forest floor and capturing all of the sunlight, leaving other species in the dark to eventually die out. Yellow poplar is a broadleaf that are also a successful pioneer tree, often forming in pure stands or in a mixture. Other trees you’ll often see in old field forests are sassafras, sourwood, red maple, dogwood and hickory.