BIRD-DISPERSED SEED RAIN AND SEEDLING ESTABLISHMENT IN PATCHY MEDITERRANEAN VEGETATION

The composition and spatial patterns of the seed rain produced by bird dispersers and of the seedlings of fleshy-fruited plants have been studied in patchy Mediterranean vegetation in southern France. The seeds of fleshy-fruited plants were collected in seed traps during a 17-month period (1981-1983) and the fecal samples of bird dispersers mist-netted on the same 2.5-ha site over a 25-month period (1981-1984) were analysed. The locations of the 5 sets of seed traps and the 6 mist-nets were chosen in order to take into account the various plant communities and the successional gradient from old fields to Quercus ilex coppice. Seedling establishment of fleshy-fruited plants was censused in 1983 adjacent to the seed traps. In 1992 seedlings were again censused in the open coppice and in the old fields. The seeds of 38 fleshy-fruited plants were collected, including 13 alien and cultivated species. Among them, 25 species were dispersed by Sylvia atricapilla, which dispersed the most diverse and mixed seed rain of the various bird dispersers. Species richness of the seed rain increased log-linearly with seed density, ranging from 3 to 21 species per 0.25 m2. Seed density was very heterogeneous in space. The maximum density of seeds was observed under the canopy of isolated trees and saplings in the old fields (up to 829 per 0.25 m2) which are the favoured perching places for the dispersers. Minimum density was observed in the same old fields outside the canopy of these same trees and saplings (down to 12 per 0.25 m2). In the Quercus ilex coppice, the higher the canopy, the higher the seed density. Dispersal was a short distance process. A large majority of the seeds of Pyracantha coccinea and Sambucus nigra were dispersed by birds less than 100 m from the maternal plant. However, a dispersal distance of > 300 m was observed for a seed of Ligustrum vulgare, In 1983 the < 1 yr-old seedlings of 17 fleshy-fruited plants, including only one alien species, and the 1-2 yr-old seedlings of 7 native species were censused. Though important components of the seed rain, no 1-2 yr-old seedlings of several alien (e.g. Pyracantha coccinea) or native (e.g. Rhus coriaria) species were observed in the studied vegetation. Seedlings were more numerous under tree cover than in the coppice clearings. In the old fields, < 1 yr-old seedlings were more numerous under the canopy than in open areas. No such difference was observed under saplings in 1983 and very few seedlings were observed in the old fields. The 1992 census suggested that the positive role of the cover of pioneer plants on seedling establishment progressively increased with successional development. This is probably due to an increase in the nurse effect and a decrease in the herbaceous cover under the canopy. It is suggested that. in old fields. the establishment of fleshy-fruited plants is favoured when seeds are deposited under pioneer woody plants rather than in open areas. The bird dispersers thus trigger dynamic processes initiated by the pioneer woody plants in Mediterranean old field succession. More generally, dispersal of fleshy-fruited plants by birds is more significant in the central stages of succession gradient than it is in open herbaceous communities and in dense forest when both have an homogeneous structure.