Density-dependent growth interactions between Eleocharis acicularic (L.) R. & S. and Juncus pelocarpus forma Submersus fassett

Interspecific and intraspecific interactions were examined in Eleocharis acicularis ( L.) R. & S. and Juncus pelocarpus forma submersus Fassett, two rhizomatous perennial plai;its which occur in the littoral zones of many temperate oligotrophic lakes. Reciprocal replacement series experi­ments were established in Roach Lake, U.S.A., at 4 ratios and 3 densities during the 1982 and 1983 growing seasons. Changes in biomass and in a variety of morphological characters were measured. Although the species did not differ in biomass change, Eleocharis produced more rosettes, longer rhizomes and more new photosynthetic structures in the first season than did Juncus. Effects of neighbors, regardless of species, were more important than interspecific interactions. Growth interactions between these species were competitively neutral. Differences between Eleo­charis and Juncus growth dynamics lead to contrasting clonal structures that facilitate coexist­ence. The results would not have been interpretable without detailed, plant-by-plant morphological data. Such detailed structural analysis may be essential in studies of interspecific interactions among macrophytes.