Consequences of differing competitive abilities between juvenile and adult plants

The competitive ability of perennial plants can change with life-stage, but whether these changes have fitness consequences is unknown. We present a simple model of two components of fitness, mortality and flowering rates, for two grassland species with very different patterns of competitive ability and life-stage. Achillea millefolium seedlings are poor competitors while the adults are good competitors. In contrast, Solidago missouriensis seedlings and adults have similar competitive ability. Models of the two species show that the overall effects of competition on growth are more important than interspecific differences in competitive ability in determining mortality and flowering rates, though the higher seedling competitive ability of S. missouriensis relative to A. millefolium seedlings does result in slightly lower mortality and higher flowering rates for the former species. Simulations where both average competitive ability and relative seedling and adult competitive ability are varied predict that dominant species with high overall competitive ability should experience no advantage or disadvantage from varying competitive ability through development. When overall competitive ability is moderate, the relative costs and benefits of differential competitive abilities among adults and seedlings are variable. High seedling competitive ability relative to adult competitive ability should be favored among species with low overall competitive ability. We predict that communities with high intensity of competition should have a high frequency of species with high seedling competitive ability, while communities with lower intensity of competition should have species with a wide range of relative seedling and adult competitive ability.