Co‐existence of three species of amphibious plants in relation to spatial and temporal variation: Investigation of plant responses

Abstract Plant responses in Ludwigia peploides (Kunth) Raven ssp. montevidensis (Spreng.) Raven and Marsilea mutica Mett., and the introduced Myriophyllum aquaticum (Vellozo) Verdc. were observed and related to co‐existence of the species in a freshwater lagoon in the Sydney basin. Water levels vary in the lagoon, and plants of all three species grow readily from stem fragments on freely draining, waterlogged and submerged soils. In Marsilea , sporelings develop rapidly from ruptured sporocarps when wet. In Ludwigia, the seeds germinate in white and in red light, but not in darkness or far‐red light. They germinate readily on soil and in water, floating or when held submerged. At 30°C, germination is rapid, at 40°C slower and less, and at 10°C nil, but seeds imbibed at 10°C and then exposed at 15°C have greater and more rapid germination than seeds maintained at 15°C. In relation to seasonal variation, plants of all three species grew at 10, 20, 30 and 40°C on freely draining, waterlogged and submerged soils, with the optimum temperature for growth apparently lower in Myriophyllum than in the other two species. In relation to water depth, plants of all three species grew when submerged to I m; the stems of Ludwigia and Myriophyllum elongating and reaching the water surface, and the petioles in Marsilea elongating bringing the leaf blades on to the surface, each within a week during summer. The floating leaf blades in Marsilea increase in size with depth of submergence of the subtending shoots. Mean relative growth rates (R̄) under the most favourable experimental conditions were least in plants of Marsilea , the species occurring along more exposed parts of the shoreline in the lagoon. It was concluded that, while water levels in the lagoon were stable, differences in growth form between the species were probably more critical in their co‐existence than quantitative differences in their growth: the floating leaves of Marsilea, being more resistant to wind and wave action than the floating shoots of Ludwigia and Myriophyllum, occupy the more exposed parts of the lagoon 'S edge. The most sheltered parts are occupied by the shoots of Myriophyllum which ride high on the water and overtop shoots of Ludwigia . Shoots of Ludwigia appear to be more resistant than those of Myriophyllum to removal and damage by wind and wave action, and achieve their greatest abundance on the water surface in situations of intermediate exposure to wind and wave action. Using Grubb's (1977) classification of niches, in Bushells Lagoon the three species appear to be primarily differentiated in habitat niche, less in regeneration niche and even less in phenological niche. In the lagoon, their habitat niches apparently overlap in water depth but are differentiated along a gradient of exposure.