Senecio congestus (R.Br.) DC. in the Lake Yssel Polders

1. In order to find an explanation for the rapid establishment and the ephemeric occurrence of S. congestus vegetations in the Lake Yssel polders several ecological characteristics of the winter annual life-form of the species were studied. In this area only the winter annual plays an important part in the vegetation. 2. On five sample plots in Eastern Flevoland the vegetation-cycle of S. congestus was studied in the years 1957-1959. In view of these observations we may suppose that the decline in vitality should mainly be attributed to changes in the edaphon under the influence of the first S. congestus generation. Apart from S. congestus in Eastern Flevoland Ranunculus sceleratus showed the highest degree of presence in the vigorous vegetations of the first species. 3. Taking seed output, dry matter production, stem height and number of flower stalks at base of stems as criteria, the vitality of the first generation of S. congestus in Eastern Flevoland proved to be high. Usually, the second generation showed a considerable decline in vitality. 4. It was observed that the diaspores may be disseminated over long distances by wind and convection currents, viz. at least 90 km and in all probability as far as 200 km. This must be the explanation of the rapid appearance in new and well suited habitats, far remote from fruiting sources of the species, e.g. in 1959. As to dissemination over short distances, it was found that hydatochorous transport of diaspores may be effective as well. 5. The seed of the winter annual life-form loses viability within three months after dissemination owing to high temperatures and moisture contents. On account of rather low temperatures, seed of the autumn flowering summer annuals may survive the winter. Wet soil surfaces appeared to be favourable to germination. Moreover, these soils ought to be muddy and bare for a vigorous growth of seedlings. It seems possible that the NaCl-tolerance of the germination- and seedling-stage depends on the mineral nitrogen content of the soil. 6. Beside the wetness of the soil, the high vitality of the first generation should be attributed to the high mineral nitrogen content of the soil at the time of its emergence from the water. The rapid decline in vitality of the next generation is due to the decrease of the mineral nitrogen content of the soil under the influence of the first generation.