Eventually it returns to forest within a lifetime. In certain parts of the midwest you see fields of farmland and occasionally squares of trees in them. Chances are in the early 19th century all of what you saw was farmland and at one point not as much was actively farmed and certain fields no longer plowed. All the trees you find in that plot of what looks like the holdover of some carved up midwestern forest are actually all less than 100 years old and relatively recent growth.