Density-Dependence in Salvia Lyrata, A Herbaceous Perennial: The Effects of Experimental Alteration of Seed Densities

Abstract

(1) A demographic study of seedling populations of the perennial herb, Salvia lyrata L., was conducted from June 1979 to May 1983. The number of seedlings establishing varied widely from year to year. The half-life of the seedling population was about 10 months and seedling mortality rate declined with time. Correlation of seedling mortality with population density was low. (2) Field experiments showed that established S. lyrata individuals significantly reduced the proportion of seedlings emerging and surviving. Seed density had no demonstrable effect on the proportion of seedlings emerging over a range of densities (0-1440 seeds dm-2). There was thus no evidence of regulation by limitation of 'safe sites' in this species in nature. (3) Seedling mortality depended strongly on seed and seedling density. Fewer individuals survived in densities above 720 seeds dm-2 compared with densities of 240 and 480 seeds dm2. (4) Seedling growth was negatively density-dependent. Growth reduction, suffered at initially high densities, persisted after a large fraction of individuals had died. (5) Regulation of numbers of S. lyrata seedlings occurs primarily through the negative effects of adults. The densities of seedlings and year-old individuals are sufficient to reduce survival and growth only in infrequent clumps.