GERMINATION STRATEGIES DURING GRASSLAND SUCCESSION

1. We analysed the germination of 91 herbaceous species in response to a temperature gradient, and to alternating temperatures and stratification (chilling). A principal-components analysis revealed that the species were distributed along two statistically independent axes, the first of which primary represented the optimal temperature for germination and the second the rate of germination. 2 These results were further related to data on species replacement during 25 years of succession in grassland after the cessation of fertilizer application but with continued hay making. Separate successional sequences were distinguished for the dry parts and the wet parts of the fields. Changes in canopy structure were studied by counting gaps and mapping light at the soil surface. 3 Changes in species composition of species present at the different stages were related to the changes in the germination attributes during the succession towards nutrient-poor grassland. The productive grassland was characterized by rapidly germinating species and in the wetter parts, by those germinating at low temperatures. This might enable these species to escape from light competition by germinating in the autumn or winter, i.e. soon after seed set. The species from the less productive, more open stages germinated more slowly, and responded more clearly to stratification and alternating temperatures. It is therefore likely that germination of most seeds of these species is delayed until the following summer. 4 The importance of the observed interspecific differences in germination characteristics in relation to the observed changes in vegetation structure as an explanation for species replacement during succession is discussed.