Life history strategies of two lemnaceae, Lemna minor and L. trisulca at Delta Marsh, Manitoba

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McIlraith, Alexander L.
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Lemna minor L. and L. trisulca L. flowered abundantly in Delta Marsh, Manitoba. Flowering in L. trisulca began in early May, peaked by about June 1 and was finished by early July. The phenology of L. minor lagged behind that of L. trisulca by about three weeks. Pollen grains and seeds of both species were examined with scanning electron microscopy. Although pollen morphology of the two was similar, L. trisulca seed differed from those of L. minor in having a logitudinally ribbed surface. Seed-bearing fronds of both species were observed. Seed set varied between years and within sites, and in some sites plants produced no seed. Peak seed densities occurred 3-6 weeks after biomass of flowering plants peaked, indicating that seed may take 3-6 weeks to develop. Seed of both species exhibited dark dormancy, and tolerated drying and freezing. A large Lemna seedbank was present at Delta Marsh, but seedlings were not observed in nature even though greater than 95% of seed extracted from sediment was viable. In the laboratory, seeds floated after being dried, and germinated at the water surface. This suggested that drought, followed by rain might liberate seed from sediment and permit it to germinate. It is suggested that the retention of sexuality and tolerance of seed to drought and freezing may function as insurance, allowing some of a clone's genes to survive periods of evnironmental change. Replacement series, addition series and 'introduction' experiments were conducted to assess interactions between L. minor and L. trisulca in the field. In addition, a transplant experiment was conducted to determine whether various sites were capable of supporting a codominant community of L. minor and L. trisulca. Vegetative biomass trends of natural populations were monitored in three sites. Results suggested that the two species may compete for light and nutrients. One-way competition for light may give L. minor a competitive advantage in eutrophic habitats, while nutrient competition may favor L. trisulca in sunny oligotrophic habitats. In a sunny eutrophic habitat, L. trisulca dominated in spring and fall, while in summer L. minor dominated. Dominance patterns, in time and space, of L. minor and L. trisulca were explained with a resource competition model (for light and nutrients), and by considering environmental factors and life history.