Spatial indicators of plant community assembly from a 453-year sand-dune chronosequence

Abstract

Background : We explored evidence for spatial patterning in vegetation across a sand-dune chronosequence spanning 453 years of primary succession to test for indications of a temporal signature of niche versus neutral processes during community assembly. Methods : The study was conducted on a series of dune-capped beach ridges located in Wilderness State Park in Emmet County of northern Lower Michigan, United States (45° 43′ N, 84° 56′ E). Nearest-neighbour survey data were subjected to species dispersion and association analyses. Results : Both species richness and species evenness showed the classic ‘hump-shaped’ relationship across successional time, suggesting that competition is an important force in structuring this community. However, the only evidence of intra-specific aggregation occurred in the youngest dune. Older dunes displayed random spatial structure and the oldest dunes tended toward a mix of random patterns and intra-specific segregation, i.e. with nearest neighbours more likely than by chance to be inter-specific. In addition, the frequency of species having lower neighbour-species richness than expected by chance was higher in younger, not older dunes. Species with significantly low neighbour richness (based on absolute nearest neighbours) on later dunes tended to have a high proportion of nearest neighbours that were intra-specific. Conclusions : The data provided no evidence that resident species in later succession have greater intra-specific aggregation or more niche differentiation based on horizontal niche space. The data are more consistent with theories of community assembly involving competitive equivalence and/or effects of facilitation.