Factors driving species assemblage in Mediterranean soil seed banks: from the large to the fine scale

Warming reverses directionality in the richness-abundance relationship in ephemeral Mediterranean plant communities

Recent findings in forests worldwide have demonstrated how directionality in the richness-abundance causality shifts along global climate gradients: The so-called more-species hypothesis (richness determines abundance) prevails in Earth's most productive climates, whereas the opposite, the so-called more-individuals hypothesis (abundance determines richness), is more likely to prevail in climatically harsh conditions. Since temporal variability is the norm, a critical question is whether this directionality shift is also a function of temporal climatic fluctuations locally. Here, we analyze whether directionality in the richness-abundance relationship is contingent on temporal variability over 10 annual consecutive realizations in ephemeral plant assemblages. Our results support the idea that the more-species hypothesis prevailed in the most benign years, whereas the more-individuals hypothesis did so during less productive years, which were significantly linked to the warmest years. These results support the idea that rising temperatures can reverse directionality in the richness-abundance relationship in these annual plant communities, and therefore, climate warming can have a significant effect on the relationship between diversity and ecosystem functions, such as productivity, by altering the prevalence of primary mechanisms involved in species assembly.

Effect of aridity on species assembly in gypsum drylands: a response mediated by the soil affinity of species

Previous studies found that plant communities on infertile soils are relatively resistant to climatic variation due to stress tolerance adaptations. However, the species assemblies in gypsum soil habitats require further investigation. Thus, we considered the following questions. (1) Do harsher arid conditions determine the characteristics of the species that form plant assemblages? (2) Is the selection of the species that assemble in arid conditions mediated by their ability to grow on gypsum soils? (3) Is the selection of species that assemble in harsher conditions related to phylogenetically conserved functional traits? Perennial plant communities were analysed in 89 gypsum-soil sites along a 400 km climate gradient from the central to southeastern Iberian Peninsula. Each local assemblage was analysed in 30 × 30 m plots and described based on taxonomic, functional (soil plant affinity) and phylogenetic parameters. The mean maximum temperatures in the hottest month, mean annual precipitation and their interaction terms were used as surrogates for the aridity conditions in generalized linear models. In the hottest locations, the gypsophily range narrowed and the mean gypsophily increased at the community level, thereby suggesting the filtering of species and the dominance of soil specialists in the actual plant assemblies. Drier sites had higher taxonomic diversity. The species that formed the perennial communities were close in evolutionary terms at the two ends of the aridity gradient. The mean maximum temperatures in the hottest month had the main abiotic filtering effect on perennial plant communities, which was mediated by the ability of species to grow on gypsum soils, and thus gypsum specialists dominated the species assemblies in the hottest locations. In contrast, the perennial communities on gypsum soils were relatively resistant to changes in precipitation. Our findings suggest that the warmer environmental conditions predicted by global change models will favour gypsum specialists over generalists.