Dynamic properties of recruitment, expansion and mortality of macrophyte patches in streams

Streams are physically perturbed habitats with high demands on the dispersa l and recruitment to maintain plant populations. Yet, little is known about these important demographic processes for stream plants. Therefore, we stu died the monospecific vegetation of Callitriche cophocarpa in a small Danis h lowland stream to determine: 1) the importance of drifting shoots and see ds for recruitment of plants, and 2) the influence of water flow, light ava ilability and patch size on recruitment, growth and mortality processes. We found that the majority (about 90 %) of new colonising patches of plant st ands derived from drifting shoots being caught around protruding stones, wh ile few developed from seeds. Many new patches were lost in the flowing wat er before roots became well established. Flow exposure of the patches resul ted in the main growth taking place in the downstream direction. Combined a real cover of Callitriche patches on the stream bottom reached an upper lim it of about 70%, probably because areal expansion above this threshold was constrained by strong shear forces and coarse substrata developing in the f low channels between the patches. We discuss why efficient shoot dispersal and vegetative growth documented here for Callitriche is an optimal plant s trategy in flow-perturbed streams in contrast to the production of numerous small seeds but limited vegetative spread among ruderal plants in perturbe d habitats on land.