After the southern part of an old-growth, mesic southern forest dominated by Fagus grandifolia and Acer saccharum in southwestern Michigan, USA, had been clearcut, it was converted to a pasture. In 1919 grazing ceased on the pasture, and it and the old-growth forest to the north were protected as a natural area, later named Warren Woods State Park. At that time the pasture began to develop into a second-growth forest from root and stump sprouts and seeds. In 2014, 2018, and 2019, the tree species composition of the old-growth and second-growth forest was studied to answer the following questions: (1) How, when, and from where did the current species of the second-growth forest arrive? (2) Is there any evidence that the second-growth forest is trending toward the composition of the old-growth forest? (3) If so, what changes are predicted to occur? Data show that the second-growth forest has become a hardwood forest dominated by A. rubrum, Liriodendron tulipifera, Prunus serotina, and Quercus rubra with a subcanopy of A. saccharum and F. grandifolia. Finally, size class frequency distributions of F. grandifolia and A. saccharum, but not of the current canopy dominants, indicate that the second-growth forest may be trending toward the composition and structure of the old-growth forest that preceded it. As it does, tree species richness and diversity will decrease as basal area increases.