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After fire (a frequent phenomenon in the Mediterranean area), numerous seedlings appear from seeds of the soil seed bank that would not appear would there be no fire. Why this sudden advent? To which species do they belong? Vegetation records were made in a 12-year-old mixed pine forest of Pinus halepensis and P. pinaster. At the same time, soil samples were collected (5 cm deep). The direct method (identification with binocular lens of all encountered seeds after soil screening) was used to identify seeds to species. Seeds of species encountered in large quantity were subjected to viability tests. The seeds of two therophytes (Trifolium glomeratum, T. campestre) were subjected to thermal treatments. The soil seed bank was only partially reflecting the floristic composition of the pine forest. Three groups of plants were distinguished: (i) species observed both in aboveground vegetation and in the soil seed bank (21.9%); all were perennials, (ii) species encountered only in the vegetation (58.5%); those preferentially regenerated by vegetative means, and (iii) species observed only in the soil seed bank (19.5%); most reproduced only from seeds. The number of seeds in the soil was high and their distribution very variable. The higher the number of plots in which seeds of a species appeared, the more numerous were the total seeds of that species recorded. Thus, Cistus ladanifer was present in great quantity in all samples. In seeds of species subjected to thermal treatments, low temperatures (50–90 °C) did not increase germination rates, but exposure to high temperatures (130–150 °C) for long duration (10 min) killed the embryo. In conclusion, numerous seeds are present in a dormant stage in the soil. When a disturbance such as fire eliminates temporarily the aboveground vegetation, these seeds germinate. Thus, after a fire, the seedlings of these species proliferate, then these species disappear with representation by seeds in latent form in the soil when perennial aboveground vegetation has reoccupied burned areas. Key words: seeds, soil seed bank, fire, thermal treatments. |
