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 st udied in patchy Mediterranean vegetation in southern France. The seeds of fleshy-fruited plants were collected in seed traps during a 17-mon th period (1981-1983) and the fecal samples of bird dispersers mist-ne tted on the same 2.5-ha site over a 25-month period (1981-1984) were a nalysed. 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 communiti es and the successional gradient from old fields to Quercus ilex coppi ce. Seedling establishment of fleshy-fruited plants was censused in 19 83 adjacent to the seed traps. In 1992 seedlings were again censused i n the open coppice and in the old fields. The seeds of 38 fleshy-fruit ed plants were collected, including 13 alien and cultivated species. A mong them, 25 species were dispersed by Sylvia atricapilla, which disp ersed the most diverse and mixed seed rain of the various bird dispers ers. Species richness of the seed rain increased log-linearly with see d 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 di spersers. 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 see d 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 d istance of > 300 m was observed for a seed of Ligustrum vulgare, In 19 83 the < 1 yr-old seedlings of 17 fleshy-fruited plants, including onl y 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. S eedlings were more numerous under tree cover than in the coppice clear ings. 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 field s. The 1992 census suggested that the positive role of the cover of pi oneer plants on seedling establishment progressively increased with su ccessional development. This is probably due to an increase in the nur se 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 p rocesses initiated by the pioneer woody plants in Mediterranean old fi eld 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 bo th have an homogeneous structure.