Disentangling water sources in a gypsum plant community. Gypsum crystallization water is a key source of water for shallow-rooted plants

Abstract

Gypsum drylands are widespread worldwide. In these arid ecosystems, different species ability to access different water sources during drought is a key determining factor of the composition of plant communities. Gypsum crystallization water could be a relevant source of water for shallow rooted plants, but the segregation in the use of this source of water among plants remains unexplored. We analyzed the principal water sources used by 20 species living in a gypsum hilltop, the effect of rooting depth and gypsum affinity, and the interaction of the plants with the soil beneath them.We characterized water stable isotope composition, δ 2H and δ 18O, of plant xylem water and related it with the free and gypsum crystallization water extracted from different depths along the soil profile and the groundwater, both in spring and summer. Bayesian isotope mixing models were used to estimate the contribution of water sources to plants xylem sap.In spring, all species used free water from the top soil as the main source. In summer, there was segregation in water sources used by different species depending on their rooting depth, but not on their gypsum affinity. Gypsum crystallization water was the main source for most shallow-rooted species, whereas free water from 50-100 cm depth was the main source for deep-rooted species. We detected plant-soil interactions in spring, and indirect evidence of possible hydraulic lift by deep-rooted species in summer.Plants coexisting in gypsum communities segregate their hydrological niches according to their rooting depth. Crystallization water of gypsum represents an unaccounted, vital source for most of the shallow-rooted species growing on gypsum drylands. Thus, crystallization water helps shallow rooted species to endure arid conditions, which eventually accounts for the maintenance of high biodiversity in these specialized ecosystems.