Nectar traits of Echium vulgare in urban habitats are associated with local temperature, floral traits, and surrounding water bodies

Abstract Background Urbanization exposes plants to multiple environmental stressors, including landscape-level changes in land-cover composition (domination of impervious surfaces, reduced proportion of green areas and water bodies), elevated temperatures associated with the Urban Heat Island effect, and altered water availability. These factors can alter nectar production and quality, with potential consequences for plant–pollinator interactions and the stability of ecosystem services in urban environments. However, the relative importance of different components of urban environmental variation — including landscape composition, temperature, and conditions experienced at the plant level — remains poorly understood. Results We quantified nectar volume and sugar concentration in 800 flowers and analyzed their relationships with floral morphology, flower position within inflorescences, air temperature, and the proportion of impervious surfaces, green areas, and water bodies (within radii: 150, 600, and 1000 m). Coarse urbanization metrics did not explain variation in nectar traits, highlighting limited predictive power of land-cover structure for floral reward dynamics in urban ecosystems. Neither impervious surfaces nor green area cover significantly influenced nectar volume or sugar concentration at any spatial scale. Instead, nectar production was better explained by local thermal conditions, proportion of water bodies cover, and floral morphology. Nectar volume increased with air temperature and with the proportion of water bodies within 600 m, while sugar concentration showed a marginal positive association with water bodies at the 1000 m scale and tended to decrease under warmer conditions. Larger flowers produced greater nectar rewards, and flower position within the inflorescence significantly affected both nectar volume and concentration. Conclusions Our results suggest that nectar production in urban E. vulgare populations is influenced more strongly by local thermal conditions, proportion of water bodies cover, and floral traits than by broad land-cover composition, indicating that fine-scale environmental heterogeneity and specific landscape elements may play a key role in maintaining floral rewards for pollinators in urban ecosystems.