Environmental heterogeneity and species diversity of forest sedges

1 A ®eld experiment was designed to investigate the relationship between environmental heterogeneity and species diversity in a group of sedges (Cyperaceae: Carex) growing in old-growth forest. 2 A measure of environmental quality, as perceived by the sedges, was obtained from the survival of clonal ramets of 11 species of Carex planted at 10-m intervals along each of three 1-km transect lines. 3 The resident assemblage of sedges was censused along the same three transect lines and along a further 24 km of survey lines in the same forest. 4 The general state of a site was represented by the overall survival of the experimental implants at that site. The general environmental variance between sites provided a measure of environmental heterogeneity. This could be partitioned into a speci®c variance (mean environmental variance of species) and an environmental covariance. The rate of increase of the general and speci®c variances with distance between sites re ̄ected environmental structure. 5 The three transects di€ered in scale. The species diversity of the resident Carex assemblage was correlated with general environmental quality both among and within transects. 6 The three transects di€ered in structure. The number of resident species, relative to the number expected from the number of individuals sampled, was greatest on the most coarse-grained transect (steepest increase in general environmental variance with distance). 7 Within each transect, species diversity increased with general environmental variance because the speci®c correlation of performance (correlation among species of survival in pair-wise combinations of sites) decreased as the general environmental variance increased. 8 The e€ect of speci®c environmental variance was weaker. Overall survival of a species on the transects was not correlated with its abundance in the forest. Neither the transects nor a targeted implant experiment provided evidence for a close relationship between the distribution of species and the state of the environment. 9 As a general explanation of our results, we propose a `marginal-specialist' model in which the species that dominate the most productive sites also have the broadest ranges, whereas other species are superior in a more restricted range of less productive sites.