Unravelling the influence of annual weather conditions and Mediterranean habitat types on acorn production, availability and predation

Abstract

Seed production and survival are critical steps in the life cycle of plants, and could become bottlenecks for plant regeneration in case of failure. During regeneration many factors, both abiotic and biotic, intervene and interact with each other. After dispersal, environmental variation affects the success of the plant population, so seedling recruitment can display spatial variability. However, few studies of seed production and availability cover a range of habitat conditions (e.g. mature, transitional and post-fire habitats), which makes it difficult to evaluate the potential of site-production relationships. We study how the heterogeneity of habitat structure and composition affects the phenology, production and availability of acorns, mainly holm oak, in a Mediterranean Natural Park. Our results show that meteorological variables determine the phenology of acorn fall. Habitat types present temporal mismatches in falls since, for example, shrublands required a longer period of time for all seeds to fall. Furthermore, habitats with greater tree density and low understory complexity had higher production but smaller acorns. Lower densities promote higher acorn volume (seedling size) as a consequence of less intraspecific competition and more reserves for germination. The proportion of viable acorns available on the ground also decreased substantially in winter, when the proportion of infested acorns increase because the selective consumption of sound acorns by predators. Acorns from mixed forests and shrublands had less pre-dispersal predation by arthropods and post-dispersal predation by wild boar as a result of oak isolation and understory complexity. Conversely, rodents are more abundant in shrublands, where they find a greater diversity of anti-predator refuges and large nutritious acorns available in autumn and winter. Understanding the reproductive constraints imposed by time and space allows us to infer the consequences of environmental changes on population recruitment and persistence. We thus show that habitats with a high density of oak trees, less plant diversity and understory cover had a greater prevalence of infested acorns, thereby reducing recruitment viability. These habitats also had more available acorns on the ground for seed predators. Therefore, habitat type is an important factor affecting acorn production and should be taken into account in future studies.