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Marine Ecology Progress Series

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MEPS 174:257-267 (1998)  -  doi:10.3354/meps174257

Components of crowding in a modular seaweed: sorting through the contradictions

M. Laura Lazo, Anthony R. O. Chapman*

Department of Biology, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada

ABSTRACT: The most common effects of increased density in terrestrial plant populations are reduced rates of growth and reproduction. There are contradictory reports for fucoid seaweeds, whose performance appears to be enhanced by increased density. Density is 1 of 2 components of crowding. The second is plant size. Crowding increases with size for a given density. In modular organisms there are 2 components of crowding, the within-organism component (which increases with ramet density and size) and the extra-organismal component (that is determined by the sizes and densities of whole plants in the stand). Such complexity presents special difficulties in experimental examination of the effects of crowding. Using stands made up of transplants, and by manipulating the length and density of ramets in the plants, we examined the effects of the 2 components of crowding on the performance of the modular seaweed Ascophyllum nodosum. Plant density was not manipulated. We found that low ramet density promoted the formation of new ramets (shoots) and enhanced relative growth rates. As ramet length and biomass are directly correlated, increases in ramet length translated into increased biomass. Conversely, high density promoted reproduction and faster ramet elongation. Plant length had fewer significant effects. Longer individuals depressed the growth of shorter and similarly sized plants. Hence, for A. nodosum, ramet density has a wider range of effects than plant length alone. Also, the effect of density differed among response variables. Nevertheless, it is clear, from this and other work, that modular fucoid species respond to crowding in ways that are fundamentally different from terrestrial higher plants.

KEY WORDS: Ascophyllum nodosum · Brown seaweed · Crowding · Density · Growth · Intraspecific competition · Ramets · Reproduction

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