Ecological factors related to seed germination and early seedling establishment in Ulmus minor Mill., an endangered riparian tree species

Natural populations of Ulmus minor Mill., an outstanding component of European riparian forests, are endangered due to Dutch elm disease, which has led to the development of several breeding programs throughout Europe. However, the reproductive ecology of this species is scarcely known even though sexual regeneration is an essential factor that must be understood before implementing elm stand restoration. In this paper, the main ecological factors affecting seed germination and early seedling establishment in U. minor are studied through a factorial design with five different factors: Light (levels: Full-sun, Shade), Initial water (levels: Initially-flooded, Initially-irrigated), Subsequent water (levels: Semi-submerged, Later-irrigated), Soil cover (levels: Bare-soil, Litter) and Substrate (levels: Silty-clay, Sandy-loam). Seed germination and seedling survival were monitored for 7 weeks. All factors except Initial water and Substrate turned out to be highly significant. Shade increased seed germination and seedling survival. Response to Initial water fluctuated, with an early higher percentage of germinated seeds in Initially-flooded but a higher seedling survival in Initially-irrigated. Subsequent water did not affect seedling establishment and survival, but Semi-submerged treatments increased seed germination. Bare-soilincreased seed germination too, whereas, in the medium term, Litter affected positively seedling survival. Sandy-loam substrates favoured seed germination, but Silty-claysubstrates favoured seedling survival. Together, these results suggest that the most favourable scenario for U. minor sexual regeneration may be a combination of a not-too-severe late-winter or early-spring flood followed by a wet spring and summer, probably with a higher establishment in heavy-textured substrates located in small gaps and forest edges where the overstorey provides some protection against strong sunlight.