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Juveniles vs. Adult Competitive Abilities in Plants: Size‐dependence in Cattails (Typha)

Grace, James B.
Ecology 1985 v.66 no.5 pp. 1630-1638
Typha, adults, flowering, juveniles, plant growth, planting, seeds, water table
This paper tests the hypothesis that plant size can affect a species' relative competitive ability; i.e., species that are good competitors as adults may not also be good competitors as juveniles. Seeds of two Typha species were sown along a miniature gradient in water depth, both in monocultures and in mixtures, to ascertain the relative competitive abilities of juveniles. In monoculture, T. latifolia was found to germinate and grow better at slightly higher elevations than T. domingensis. In mixture, T. latifolia was the better competitor above the water table while T. domingensis dominated below the water table. Summed over the entire gradient, the two species did not differ significantly in their overall competitive abilities despite the fact that T. latifolia has larger seeds. Adult competitive abilities were studied experimentally in a large artificial pond by using transplants planted at low initial densities. When each species was grown in monoculture, T. latifolia grew best in shallow water with little growth in water ≥1.0 m deep, and T. domingensis had peak shoot growth in deep water (0.8—0.9 m) with comparatively less growth in shallow water. When grown in mixture, T. latifolia was the dominant competitor over virtually the entire gradient. Therefore, I conclude that relative competitive ability in Typha is size dependent. The factors most likely to cause the observed shift in relative competitive abilities are (1) the onset of flowering, (2) changes in morphology and physiology associated with growth and development, and (3) changes in the factors limiting growth as plants increase in size. Regardless of the cause, the observed relationship between size and competitive ability suggests that the type of initial propagules used in competition experiments can affect the outcome, and that in some cases, the outcome of competition may be dependent upon the initial density of propagules.