Over the last 200 y the tallgrass prairie of the Midwestern United States has experienced widespread conversion to agricultural production. Today, the few remaining tracts of unplowed grassland persist as small isolated patches within a landscape of row-crop agriculture. The small size and isolation of these prairie remnants raises concerns over their long-term sustainability. In this study I examined changes in the upland plant community that have occurred since state acquisition of one of Iowa's oldest prairie preserves. I found that 50 y of management has succeeded in reducing the frequency of exotic species and, thus, improved the overall integrity of the native plant community. However, during this same period dramatic changes in the frequency of many native species have also occurred. A general increase by mesic and late flowering species and a decrease by xeric natives was observed. Changes coincided with a documented shift in management from mid-summer haying prior to state ownership to spring burning, suggesting that burning may have exerted selective forces altering the composition of the native community over the past 50 y. My results emphasize the need to merge our current understanding of the processes that help sustain diversity into implemented management practices that will prolong the diversity of our remaining small isolated prairie preserves.