Ecological niches and biogeography of nitrogen‐fixing plants in Europe

Abstract Symbiotic nitrogen fixation by vascular plants represents a major pathway for nitrogen input in terrestrial ecosystems, fundamentally altering nutrient cycles and plant community dynamics. Nitrogen‐fixing plants comprise phylogenetically and physiologically distinct lineages whose ecological niches and responses to environmental gradients remain poorly resolved at continental scales. We investigated the geographic distribution and ecological responses of major nitrogen‐fixing lineages across Europe, focusing on legumes (inverted repeat lacking clade IRLC, characterised by high symbiont regulation ability, and non‐IRLC) and actinorhizal genera. We analysed 707,673 vegetation plots (1970–2021) from the European Vegetation Archive to map lineage density at 30‐km resolution, assess habitat associations, model climatic drivers and evaluate distributions along environmental gradients using ecological indicator values. Non‐IRLC legumes predominated in Mediterranean scrublands and dry grasslands, whereas IRLC legumes extended into northern regions and mesic grasslands. Legumes were associated with high diurnal temperature range, high summer temperatures, low summer rainfall and low soil nitrogen and water availability—patterns pronounced in non‐IRLC legumes, but less distinct or even absent in IRLC legumes. Actinorhizal lineages showed disparate habitat associations and contrasting climatic responses, with temperature seasonality as the strongest predictor—positive for Alnus and Elaeagnaceae and negative for the other lineages. Our findings demonstrate fundamentally divergent ecological niches among European nitrogen‐fixing lineages, reflecting distinct evolutionary histories and physiological strategies. Enhanced symbiont regulation in IRLC legumes likely facilitates persistence where the benefits of nitrogen fixation are limited. Despite sharing a common adaptive trait, nitrogen‐fixing lineages have evolved different strategies to colonise various environments under diverse climatic conditions.