A multi-scale approach in the study of plant regeneration: Finding bottlenecks is not enough

Using a multi-scale design, we investigated the spatio-temporal variability and concordance across recruitment stages of the Mediterranean fleshy-fruited shrub Daphne gnidium. The patterns of seed rain, post-dispersal seed predation, seedling emergence, survival and growth were studied in two populations in contrasting conditions, in an olive grove and in a dune system, in Mallorca (Balearic Islands) during two consecutive seasons. We considered 20 sites (fine scale) in four microhabitats (medium scale) in each population (large scale). Most seeds were dropped under parent plants, especially in the population with presumably less seed dispersers. Post-dispersal seed predation (ca. 90%) represented the most important bottleneck in plant recruitment in all microhabitats at both populations. Seedling emergence was also similar at both populations although seedling survival was higher in the olive grove. One year after of emergence, seedlings under shrubs were taller – though not necessarily had more leaves – than in the other microhabitats. There was a positive and consistent effect of seed size across microhabitats on seedling emergence, survival and growth (maximum length) although its intensity varied between populations and years. Concordance between seed rain and seedling recruitment was affected by the heterogeneity of population-dependent forces acting upon each recruitment stage. There were stages, which represented a strong bottleneck but were little discordant with seed rain. By contrast, other weak bottlenecks were hardly discordant with seed rain. Our study stresses the importance of considering simultaneously the critical stages (i.e., bottlenecks) and the concordance between seed dispersal and post-dispersal stages in plant recruitment studies.