Seed dynamics and seedling establishment in the invading perennial Solidago altissima under different experimental treatments

1 Population increases in Solidago altissima, an aggressive invader in central Europe, can be due to clonal growth by rhizomes or sexual reproduction by seeds. We studied transitions between different reproductive stages, to assess the potential of S. altissima to colonize new habitats by seed, and the influence of annual mowing on the dynamics of seeds and juvenile plants. 2 The dispersal of the numerous achenes, their subsequent fate on and in the soil, and the growth and survival of seedlings and juvenile plants were followed from 1988 to 1990 in an old-held population. 3 The seed rain was 49 000 seeds m(-2) per annum in unmown and 1800 seeds m(-2) per annum in adjacent mown plots. Survivorship and germinability of seeds in the soil decreased rapidly in the first 3 months after dispersal. Thereafter the declines were much slower. Germination within the established vegetation was very rare: only 12 out of 150 000 experimentally sown seeds (0.008%) became seedlings. 4 The survival and growth of seedlings transplanted into closed vegetation increased with age and developmental stage. Nevertheless, no transplants flowered during the 34 months over which their fate was followed. 5 Mowing affected neither seed survival nor germinability, nor did it have any effect on the growth, survival and reproduction of seedlings or juvenile plants. 6 The creation of l-m(2) openings in the vegetation strongly increased the number of emerging seedlings. Production of achenes was high (c. 20 000 per sexual shoot) and seed rain data suggested that 80-90% of all seeds may have been dispersed outside the study area. The colonization of nearby open sites is therefore quite probable. 7 Thus, mowing in late summer to control the growth of S. altissima does not provide opportunities for further seedling establishment in existing populations and may prevent the colonization of new sites by reducing seed supply from the source population.