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Abstract

Climatic conditions alter the phenology of species, which may threaten the synchrony of biotic interactions. However, how phenological synchrony across entire communities of plants and their pollinators responds to varying environmental conditions remains poorly understood. We recorded the communities and phenology of flowering plants and pollinators in herb-rich meadows at 22 sites between 600 and 1800 m a.s.l. on Calanda massif, Switzerland. During weekly visits, we counted 1 177 396 flowers belonging to 163 taxa and captured a total of 9707 insect pollinators across the study sites. We observed a decline in the flower abundance, numbers of flowering species, and insect abundance with increasing elevation. Additionally, we found strong and slightly differing seasonality in flower and pollinator abundance, resulting in increasing pollinator availability toward the end of the growing season. On average, flowering phenology was delayed by 1.4 days per 100 m of elevation gain, although species-specific responses varied substantially. We also detected a shift in the most contested flowering phenology along the gradient: late-flowering species showed higher overlap in flowering times at low elevations, whereas early-flowering species experienced greater niche overlap at high elevations. Meanwhile, overall synchrony between flowering plants and pollinators increased at higher elevations with shorter growing seasons, as pollinators showed greater temporal overlap with flowering species. At lower elevations, species with the highest flowering-time overlap coincided with periods of high pollinator availability in the late season. In contrast, at higher elevations, contested flowering phenology did not track pollinator availability. This suggests that flowering time is potentially more constrained by biotic factors (pollinator availability) at lower elevations, and by abiotic factors (season length) at higher elevations. These shifting biotic versus abiotic controls of plant–pollinator interactions may change substantially under future climate scenarios.