Geographic Patterns of Fog‐Driven Isotopic Niches and Root Adaptations in Cacti Across the Arid‐to‐Hyperarid Gradients of the Atacama Desert

ABSTRACT Aims Hydrological niche specialisation supports plant survival in hyperarid deserts by enabling access to distinct water sources in space and time. We investigated the isotopic hydrological niches of Eriosyce cacti across arid to hyperarid zones along a 1200 km gradient in the Atacama Desert. Using stable isotopes (δ 2 H, δ 18 O) in root water, we identified their primary water sources and evaluated how geographic and morphological factors shape water‐use strategies under extreme environmental constraints. Location Atacama Desert, Chile. Time Period From June 2021 to November 2023. Data collection was conducted as a single sampling event per population. Major Taxa Studied Eriosyce (Cactaceae). Methods We quantified the isotopic niches of 323 individuals across 62 populations representing 49 Eriosyce taxa. Deuterium excess was calculated to measure evaporative enrichment, and Bayesian mixing models were applied to characterize water source contributions (fog, rainfall, and snow). We then examined how these patterns relate to root morphology (fibrous, taproot, tuberous) and geographical gradients, using generalized phylogenetic least squares models based on Eriosyce Bayesian phylogeny. Results Overall, Eriosyce cacti spanned a broad isotopic spectrum. Fibrous‐rooted forms had notably negative Deuterium excess (D‐excess), while species with tuberous or taproots displayed more positive values (ANOVA, F = 32.05, p < 0.05). Coastal and central valley populations showed higher D‐excess, indicating reliance on isotopically enriched fog, whereas Andean taxa presented lower values (ANOVA, F = 179.6, p < 0.05), reflecting predominant use of rainfall and snow. Geographic variables, especially longitude ( p < 0.05), altitude ( p < 0.05), distance from the coast ( p < 0.001) and the minimum and maximum topographic position indices ( p < 0.05) were significant predictors of D‐excess in PGLS models. Main Conclusions Our findings reveal diverse, locally adapted water‐use strategies among Eriosyce cacti, highlighting how hydrological niche specialisation enables these species to endure in one of the world's most hyperarid deserts. This underscores the critical role of heterogeneous water sources, especially fog, in maintaining cactus diversity under harsh conditions.