Shifting Demographic Control of a Perennial Bunchgrass along a Natural Habitat Gradient

The relative importance of competition vs. other environmental factors in determining the local distribution of a perennial bunchgrass species, Danthonia sericea, was investigated in two experiments, one in 1984 and another in 1985. Seeds were planted in a two—way split—plot experimental design under field conditions on the campus of Duke University, Durham, North Carolina, USA. One treatment, four planting regions in distinct demographic zones along a soil/vegetation gradient, was used to investigate the effect of gradient position on germination rates, early establishment success, growth rates, and early reproduction of Danthonia sericea. The second treatment, vegetation left intact or vegetation removed, was used to assess the relative importance of competition from neighboring herbaceous plants in regulating the same demographic processes. The demographic success of Danthonia sericea was enhanced at all positions along the gradient by vegetation removal. However, the magnitude of the response varied with year of experiment and with gradient position, indicating that the intensity of competition is dependent upon a variety of factors that shift over time and space. It is suggested that a complete understanding of the importance of any one factor, such as competition, in regulating species distributions can only be obtained by conducting experiments over a range of habitats and years.