We investigated the processes of oak-savanna redevelopment in an abandoned sand mine site in Indiana Dunes National Lakeshore. Our Detrended Correspondence Analysis (DCA) revealed a divergence of vegetation between the prairie and the woodland portions of the study area. Eastern cottonwood (Populus deltoides) and black oak (Quercus velutina) dominated the woodland canopy. The shallow water table (0.90–1.35 m below ground surface) appeared to support the formation of tree canopy. The prairie, with deeper water table (2.09–2.62 m), was sparsely vegetated by shrubs; mostly choke cherry (Prunus virginiana), sand cherry (P. pumila) and fragrant sumac (Rhus aromatica) without apparent tree canopy. Development of woody vegetation, black oak overstory and sassafras (Sassafras albidum) understory was initiated with the colonization of cottonwoods. No evidence was found to support the ‘facilitation effect’ of cottonwoods for black oak seedling recruitment, survival or growth. The tree canopy formation, promoted by water table depth, appeared to be the major factor for divergence in herbaceous vegetation and soil characteristics. The herbaceous vegetation was dominated by C₄ and CAM species (e.g., little bluestem Schizachyrium scoparium and prickly pear cactus Opuntia humifusa) in the prairie, whereas horsetails (Equisetum arvense), asters (Aster spp.) and other C₃ forbs were the dominants in the woodland. Our curve-linear regressions revealed that the cover and diversity of herbaceous species reached their peaks at about 40% tree canopy. The regression analysis also showed strong correlations between the tree canopy and soil organic matter, cation exchange capacity, nitrogen, phosphorus and pH. The 70-y old woodland site had accumulated 1.4% organic carbon and 820 ppm of nitrogen, similar to the ‘mature’ soil of oak-savanna/woodlands near our study site. This suggests a rapid accumulation of major nutrients during the early vegetation development. Nitrogen would likely continue to accumulate beyond the ‘mature’ level. The nitrogen enrichment, particularly under the changing climate, adds uncertainty to the future trajectory of vegetation in the oak-savanna on the Lake Michigan sand dunes.