Dispersal, diversity and distribution patterns in pioneer vegetation: The role of river-floodplain connectivity

Question: Are there changes in dispersal patterns in floodplain pioneer vegetation with effects on seedling number, species richness and species composition along a gradient of declining river-floodplain connectivity? Location: Middle Elbe river floodplain, Germany. Methods: An experiment with five treatments was set up along a gradient of declining river-floodplain connectivity, partitioning seedlings into three groups: (1) emerging solely from water dispersed seeds, (2) from wind/animal dispersed seeds and (3) from the soil diaspore bank. Two controls were established: without any manipulation and exclusion of all seeds. The results were compared with those of vegetation and soil sampling to evaluate the representativeness of the experimental sites in terms of species composition, diversity, seedling number and soil parameters. Results: Water dispersal and the soil diaspore bank were the major dispersal strategies shaping floodplain pioneer vegetation at the Middle Elbe river. The number of seedlings, species rich-ness and the variation in species composition in these habitats depend on the degree of connectivity. The seedling number and species richness is highest in sites of permanent or almost permanent exchange with the main channel, where water dispersal additionally contributes to the number of seedlings grown from the soil seed bank. Conclusion: The results underline the importance of river-floodplain ecotones as sink habitats for water-dispersed seeds. Considering the strongly reduced river-floodplain interactions due to dykes and other engineering structures, management strategies are necessary to improve connectivity and the renewal of fluvial land forms. Nomenclature: Wisskirchen & Haeupler (1998).