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 stratific ation (chilling). A principal-components analysis revealed that the sp ecies were distributed along two statistically independent axes, the f irst of which primary represented the optimal temperature for germinat ion and the second the rate of germination. 2 These results were furth er related to data on species replacement during 25 years of successio n in grassland after the cessation of fertilizer application but with continued hay making. Separate successional sequences were distinguish ed for the dry parts and the wet parts of the fields. Changes in canop y structure were studied by counting gaps and mapping light at the soi l surface. 3 Changes in species composition of species present at the different stages were related to the changes in the germination attrib utes during the succession towards nutrient-poor grassland. The produc tive grassland was characterized by rapidly germinating species and in the wetter parts, by those germinating at low temperatures. This migh t enable these species to escape from light competition by germinating in the autumn or winter, i.e. soon after seed set. The species from t he less productive, more open stages germinated more slowly, and respo nded more clearly to stratification and alternating temperatures. It i s therefore likely that germination of most seeds of these species is delayed until the following summer. 4 The importance of the observed i nterspecific differences in germination characteristics in relation to the observed changes in vegetation structure as an explanation for sp ecies replacement during succession is discussed.