Microhabitat requirements and seed/microsite limitation of the rare granite outcrop endemic Amphianthus pusillus (Scrophulariaceae)1

amine the roles of seed and microsite limitation in determining the microdistribution of the federally-threatened plant, Amphianthus pusillus. Microhabitat variables such as soil depth, maximum water depth, soil texture, soil nutrient status, and number and fecundity of Diamorpha smallii plants correlated poorly with Amphianthus density and fecundity. Density-fecundity relationships were explored by thinning dense populations of young Amphianthus. Mean capsule production/plant declined at densities greater than 1.4 plants/10 cm2, indicating strong intraspecific competition. Capsule production per plot was constant at densities greater than 7.1 plants/10 cm2. Transplanted A. pusillus were significantly less likely to survive to produce seeds when placed in nonAmphianthus pools compared with plants transplanted into Amphianthus-containing pools, implying that microsite limitation occurred. However, 33% of the plots in non-Amphianthus pools produced capsule-bearing plants, indicating that unoccupied suitable habitat may exist and that the distribution of A. pusillus may also be dispersal-limited.