Competition, facilitation, and the distribution of Schizachyrium scoparium along a topographic-productivity gradient

In this study, neighbor-removal and propagule-addition experiments were conducted along a natural topographic-productivity gradient in Kansas grassland to investigate the impact of plant neighbors on recruitment, survival, and growth of a widespread native grass (Schizachyrium scopartum L.). Plant neighbors had contrasting effects on Schizachyrium that depended on the stage of the life history considered, the measure of plant performance examined, and the position of the local habitat along the complex topographic-productivity gradient. Germination of Schizachyrium was strongly suppressed by plant neighbors at high productivity, but facilitated by neighbors at low productivity. Final recruitment (final seedling density in seed addition plots) was facilitated by neighbors in low productivity sites, resulting in differential recruitment success in the presence of neighbors along the productivity gradient. Although transplant survival was also facilitated by plant neighbors in unproductive sites, transplant survival varied independently of productivity in the presence of neighbors. Despite positive effects on transplant survival, neighboring plants strongly suppressed the growth of surviving transplants, with the magnitude of suppression varying independently of productivity. The results suggest that despite strong neighbor effects on established plants, facilitation of seedling recruitment may play a particularly important role in regulating the re-colonization potential and the spatial distribution of Schizachyrium in successional grasslands. The findings support the view that events occurring during the earliest establishment phases of the life history can be of primary importance in regulating species distribution along environmental gradients.