Natural areas are intended to preserve biodiversity and understanding how well they fulfill this role is crucial as pressure from anthropogenic disturbances like habitat loss increases. We re-inventoried the flora of Baker Woodlot in East Lansing, Michigan, 40 years after an initial study to document changes in community composition. Both inventories were based on vascular plant collections vouchered by specimens and deposited in the Michigan State University Herbarium. We compared results with 10 similar floristic change studies in northeastern North America to understand patterns in species turnover, accounting for gains and losses of natives and nonnatives. In Baker Woodlot and across the comparable studies, total richness stability masked a consistent pattern of native losses and nonnative gains. In Baker Woodlot, total richness increased marginally (3%; 11 species), reflecting a net loss of 19 natives and a gain of 30 nonnatives. Across the comparable studies, richness increased by 3.6%, but seven native species were lost per decade on average. Nonnative turnover was higher compared to native turnover and positively correlated with time between inventories. Despite net loss of natives, Baker Woodlot experienced only a slight decline in floristic quality. Baker Woodlot remains a high-quality beech-maple forest, which is a prominent component of Michigan's biodiversity. Species richness alone does not adequately address biodiversity concerns. In Baker Woodlot and comparable forests, natives are being replaced by nonnatives.