Gypsophile vegetation patterns under a range of soil properties induced by topographical position

Iberian gypsophile plant communities are considered a priority for conservation by the European Community because of their highly specialized flora in gypsum outcrops in arid and semiarid regions. Despite the ecological importance of these ecosystems, the edaphic factors that constrain plant communities on gypsiferous soils remain unclear. It has been proposed that both the chemical and physical restrictive conditions of gypsum soils determine gypsophily in plants. Here we hypothesize that the rigors of the gypsum soil environment depends on topography, decreasing from flat areas on hilltops to south-oriented slopes and finally to slopes oriented to the north. We also hypothesized that the relaxation of the rigors of the gypsum soil environment with topography affects both to individual plant and community characteristics of gypsophile vegetation: we expect a reduction of gypsophyte abundance, an increase of diversity and the amelioration of facilitative interactions of plant species. We analysed the physical and chemical properties of gypsum soils that have been proposed that determine the rigors of the gypsum soil environment (i.e.: unbalanced ion concentrations and superficial soil crust). The predicted rigor gradient along topographical locations was confirmed and was mainly caused by superficial soil crust. The decreasing rigor gradient was accompanied by a fall in the abundance of gypsophytes. However, when gypsophytes were considered separately, several patterns were observed, indicating distinct tolerance to relaxation of rigor of the gypsum soil conditions and different competition abilities between gypsophytes. Plant species were more clumped, and gypsophile communities presented higher diversity, evenness and richness values where rigor of gypsum soil conditions were maximum (flat hilltop positions). Relaxation of rigor (north-oriented slopes) was characterized by loss of facilitative interaction between species and the dominance of the gypsovag Rosmarinus officinalis L., although richness was still very high, which can be attributed to the coexistence of gypsophytes and gypsovags. We conclude that the rigor of gypsum soil environment gradient with topography is mainly determined by superficial soil crust, and it is a crucial determinant of gypsophile plant communities.