Tales from the underground: Soil heterogeneity and not only above-ground plant interactions explain fine-scale species patterns in a Mediterranean dwarf-shrubland

Abstract

Questions: The current paradigm of plant community assembly relies on a set of processes operating at particular spatial scales. It is assumed that as the spatial scale becomes finer, environmental filtering loses its importance in favor of biotic interactions and neutral processes. Thus, at the very fine spatial scale represented by a rectangular plot of 72.25 m2 in a Mediterranean semiarid dwarf-shrubland, we ask: (a) are the spatial distributions of individuals of the different species explained by neutral models; (b) are these distributions dependent on above-ground plant interactions with the dominant species in the community; and/or (c) are they responding to the spatial variation of different soil variables, in a kind of fine-scale environmental filtering?. Location: Central Spain. Methods: To assess the correlates of fine-scale (i.e., from 0.05–2.00 m) spatial patterns of the species in the community, we fully mapped all perennial individuals inside the rectangular plot. For each species, we fitted one complete spatial randomness (CSR) model that does not assume spatial heterogeneity and three weighted-average inhomogeneous Poisson process (IPP) models using six soil covariates, distribution patterns of the four above-ground dominant plants in the community or both types of covariates. All models were evaluated and compared to select the best-fitting weighted-average model. Results: We recorded 7,988 individuals of 22 species. Patterns of all species were appropriately explained by IPP models. For most species the best-fitting weighted-average model included both soil and dominant plants (i.e., 15) or only soil covariates (i.e., 6). The improvement provided by the best-fitting weighted-averaging model in comparison with the CSR model was consistently high (81%). Conclusions: Our approach suggests that species in this dwarf-shrubland are mainly structured by soil heterogeneity and modulated in some cases by the interactions established with the dominant species.