Human impacts, such as invasive species introductions and land use changes, have the potential to alter the functional composition and diversity of plant communities. We can make and test clear predictions of how these processes should affect community-level trait values, yet very few studies have done so. We examined how three invasive species and succession from savanna to woodland because of fire suppression may change functional composition and diversity in the State Line serpentine barrens on the Pennsylvania-Maryland border. We characterized native and invasive plants in eight key functional leaf traits and quantified trait overlap between natives and invasives. We compared plots in two successional stages—savannas and woodlands—in functional composition and diversity. The invasive species Elaeagnus umbellata Thunb. and Microstegium vimineum (Trin.) A. Camus had extreme trait values and low overlaps with native species. Invasive species and plant assemblages including invasive species had traits more characteristic of fast growth, competitiveness, and inefficient resource use. Assemblages with invasive plants had higher functional diversity. The conversion of savannas to woodlands also shifted communities toward more fast-growing, competitive, and inefficient traits as well as decreasing functional diversity. Both invasion and succession appear to reduce the distinctive stress tolerance of plant communities on serpentine soils, with the potential for strong ecosystem-level impacts and positive feedbacks.