Long-term change in ground-layer vegetation of deciduous forests of the North Carolina Piedmont, USA

1 In Southeastern North America residual mature deciduous forests have long been assumed to represent the stable endpoint of succession, with local compositional variation reflecting primarily variation in soils and topographic position. However, the long-term stability of understorey vegetation in this region has rarely been examined due to the lack of reliable baseline data. 2 We resampled a series of permanently marked vegetation plots spanning a range of soil and site conditions in the Piedmont of North Carolina to evaluate compositional change in the understorey of putative 'climax' hardwood forests over a 23-year period. We examined all ground-layer species for change in total and native species richness at 25 m(2) and 1000 m(2), change in species composition and variation in richness and composition with environment. 3 Although native and total species richness remained nearly constant, there was a significant decline in herb richness balanced by a strong increase in tree seedling richness. Species composition changed significantly across all plots with strong evidence of consistency in the direction of plot-level compositional change. 4 Change in herb species richness over time was strongly correlated with environment at 25 m(2). Sites with higher pH, percentage sand and soil cations exhibited less decline in herb species than other sites. Understorey richness tended to increase more over time on low-elevation, mesic sites as opposed to higher, dry sites. 5 The systematic changes in herbaceous species richness and woody seedling abundance do not support the expectation of stability and may be due to the elimination of ground fires and widespread grazing in the early twentieth century, exotic species invasion or increases in white-tailed deer populations. 6 The lack of stability we observed for mature vegetation of the Carolina Piedmont is likely to be representative of temperate forests in general. Our results suggest that the common practice of projecting vegetation change based on contemporary mature vegetation as the expected endpoint is unreliable and results obtained must be viewed with caution.