Elevational patterns in plant mating systems and pollen limitation in Afrotropical montane grasslands

Abstract

O_LIPlant mating systems and pollen limitation often vary along elevational gradients, yet empirical evidence remains mixed and rarely comes from multi-species experimental studies in tropical mountain ecosystems. C_LIO_LIWe tested how elevation affects natural seed set, pollen limitation, and selfing capacity, and whether these patterns are associated with variation in flower visitation, in Afromontane grasslands on Mount Cameroon. We conducted a hand-pollination experiment on seven zoogamous plant species at four elevations above the timberline (2,300-3,800 m), applying autonomous selfing, geitonogamous selfing, outcrossing, and open-pollination treatments to 1,776 flowers and quantifying seed set. In parallel, we quantified pollinator visitation on unmanipulated plants to estimate visitation frequency, morphospecies richness, and functional-group richness. C_LIO_LINatural reproductive success of the studied plants exhibited a pronounced mid-elevation peak and declined sharply towards the summit, where pollen limitation increased strongly. At the highest elevation, some species produced few or no seeds even under outcross-pollination, indicating physiological reproductive constraints. C_LIO_LIIndices of autonomous selfing and geitonogamy varied among species, with no consistent elevational patterns. Despite detected partial self-compatibility, summit populations did not exhibit expected shifts towards selfing, suggesting limited reproductive assurance under high-elevation conditions. C_LIO_LIPollinator visitation frequency and diversity declined at the highest elevation. Natural seed set was positively associated with visitation frequency and mildly negatively associated with morphospecies richness, whereas pollen limitation and selfing indices showed no clear relationships with visitation metrics, consistent with the influence of additional physiological and developmental constraints on reproduction. C_LIO_LISynthesis. Our study shows that in isolated Afrotropical montane grasslands, plant reproductive success at high elevations is jointly constrained by declining pollination service and abiotic limitations, which is not compensated by increased selfing. This suggests that successful plant reproduction near upper vegetation limits depends on both sufficient pollination service and favourable physiological conditions. Under ongoing climate change, upslope range shifts of plants may therefore not guarantee reproductive success if plant and pollinator responses to warming are asynchronous or if a part of extreme high-elevation conditions remain limiting. These findings advance understanding of how biotic interactions and abiotic constraints together shape plant reproduction along tropical elevational gradients. C_LI