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

The most common effects of increased density in terrestrial plant populatio ns are reduced rates of growth and reproduction. There are contradictory re ports for fucoid seaweeds, whose performance appears to be enhanced by incr eased density. Density is 1 of 2 components of crowding. The second is plan t size. Crowding increases with size for a given density. In modular organi sms there are 2 components of crowding, the within-organism component (whic h increases with ramet density and size) and the extra-organismal component (that is determined by the sizes and densities of whole plants in the stan d). Such complexity presents special difficulties in experimental examinati on 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 th e effects of the 2 components of crowding on the performance of the modular seaweed Ascophyllum nodosum. Plant density was not manipulated. We found t hat low ramet density promoted the formation of new ramets (shoots) and enh anced relative growth rates. As ramet length and biomass are directly corre lated, increases in ramet length translated into increased biomass. Convers ely, high density promoted reproduction and faster ramet elongation. Plant length had fewer significant effects. Longer individuals depressed the grow th of shorter and similarly sized plants. Hence, for A. nodosum, ramet dens ity has a wider range of effects than plant length alone. Also, the effect of density differed among response variables. Nevertheless, it is clear, fr om this and other work, that modular fucoid species respond to crowding in ways that are fundamentally different from terrestrial higher plants.