Limitations to recruitment of native species in hydroseeding mixtures

To ensure the success and sustainability of restoration actions, native plant species are increasingly recommended due to their presumed ability to adapt to local abiotic and biotic conditions. However, topography, substrate characteristics and water shortage are known to cause failure of native species in Mediterranean areas, and even when such factors are not limiting their establishment and growth can still be very low or null. This study intended to identify causes for such failures.A greenhouse experiment was set up to simulate the hydroseeding procedures used in a Mediterranean quarry, and to study the behaviour of five target species. Seeds of native species (including generalists) were mixed in the usual slurry and applied to pots filled with quarry substrate, using three treatments: (i) each target species sown alone; and each target species and four generalists, (ii) at a low seeding density (15 g/m2), and (iii) at a high density (30 g/m2, the usual seeding density at the quarry). Plant germination, density, development and cover were monitored for two growth seasons to evaluate the effects of the presence of other species and seeding density on the success of the target species.The results explained the previously observed low success of the target native species in the quarry. Germination and establishment were not limited by the substrate and slurry components. With enough water available after seeding, the major constraint on target seedling recruitment was the morphology and fast growth of the co-seeded generalist species, despite the fact that the latter were also native and had not been previously expected to strongly outcompete the former. The dominance of the generalists might be partly explained by the significantly higher proportion of their seeds in the mixture. The experiment also indicated that seeding densities currently used for restoration of Mediterranean degraded areas may be reduced without compromising the development of the plant cover. However, this reduction alone does not ensure the success of all native species. The effective introduction of a given native species requires that adequate mixture compositions and densities be determined taking into account the species' germination speed and seedling morphology (leaf and stem sizes). This is most relevant when the co-seeded species (e.g. generalist grasses) display more competitive growth traits and should therefore be included in lower proportion.