MICROHABITAT REQUIREMENTS AND SEED MICROSITE LIMITATION OF THE RARE GRANITE OUTCROP ENDEMIC AMPHIANTHUS-PUSILLUS (SCROPHULARIACEAE)/

Effective management of rare species often requires extensive biologic al knowledge. This study was undertaken to explore microhabitat requir ements, describe density-fecundity relationships, and examine the role s of seed and microsite limitation in determining the microdistributio n of the federally-threatened plant, Amphianthus pusillus. Microhabita t variables such as soil depth, maximum water depth, soil texture, soi l nutrient status, and number and fecundity of Diamorpha smallii plant s correlated poorly with Amphianthus density and fecundity. Density-fe cundity relationships were explored by thinning dense populations of y oung Amphianthus. Mean capsule production/plant declined at densities greater than 1.4 plants/10 cm(2), indicating strong intraspecific comp etition. Capsule production per plot was constant at densities greater than 7.1 plants/l0 cm(2). Transplanted A. pusillus were significantly less likely to survive to produce seeds when placed in non-Amphianthu s pools compared with plants transplanted into Amphianthus-containing pools, implying that microsite limitation occurred. However, 33% of th e plots in non-Amphianthus pools produced capsule-bearing plants, indi cating that unoccupied suitable habitat may exist and that the distrib ution of A. pusillus may also be dispersal-limited.