Temporal and spatial differentiation in seedling emergence may promote species coexistence in Mediterranean fire-prone ecosystems

Mediterranean ecosystems are hotspots of species richness where fire is one of the key processes influencing their structure, composition and function. Post-fire seedling emergence constitutes a crucial event in the life cycle of plants and species-specific temporal and spatial patterns of seedling emergence have been hypothesized to contribute to the high diversity in these ecosystems. Here we study the temporal and spatial patterns of seedling emergence observed for the four dominant species (Cistus albidus, Ulex parviflorus, Helianthemum marifolium, Ononis fruticosa) after an experimental fire in a Mediterranean gorse shrubland. In a first analysis we compared the timing of emergence of each species using the Kaplan-Meier method. The spatial component of seedling emergence and the spatiotemporal relationship between different cohorts of the same species were analyzed using recent techniques of spatial point pattern analyses. We found a bimodal temporal pattern of emergence. Emergence of Cistaceae species (H. marifolium and C. albidus) occurred predominantly early after the fire while Fabaceae (O. fruticosa and U. parviflorus) emerged mainly during the following autumn. Individually, all species showed an aggregated spatial pattern and, when testing for pair interactions, we found that the clusters of individual species were spatially segregated. Additionally, the clusters of individual species showed an internal spatial structure where seedlings of different cohorts were spatially segregated. Theoretical models predict that these patterns will promote species coexistence. We identified a number of mechanisms that all have the potential to contribute to the observed pattern formation. However, the potential interaction among these mechanisms are complex and not easy to predict. Our analyses take a significant step forward in studying seedling emergence in fire prone ecosystems since, to our knowledge, this is the first time that both spatial and temporal patterns of all dominant species have been studied together.