Colour similarity to flowering neighbours promotes pollinator visits, pollen receipt and maternal fitness

While co-flowering plants often compete for pollination through pollinator visits and pollen transfer, recent studies reveal potential for the facilitation of pollination. Pollination may be promoted when similarities in the floral traits of co-flowering species enhance pollinator visitation and seed set. Floral similarity, however, could also lead to heterospecific pollen transfer, creating a fitness trade-off that is seldom explored empirically. Here we construct experimental communities of Oxalis to investigate the trade-off between enhanced visitation and heterospecific pollen receipt, which can impact seed set at varying relative plant abundance. Our experimental communities comprise O. purpurea that is matched or mismatched to congeners in flower colour at low and high relative plant abundance. We investigate the consequences of colour overlap for pollinator visitation and the receipt of conspecific, heterospecific, and mixed pollen loads, and assess seed set in natural plots. Flower colour similarity increases visitation by shared pollinators at both low and high relative abundance when O. purpurea occurs in matched flower colour communities. The receipt of pollen analogues from conspecifics is also greater at both abundances when neighbours are matched in floral colour. Although conspecific pollen in matched communities is mostly delivered in mixed pollen loads, hand pollinations confirm the receipt of mixed pollen carries no fitness costs for seed production. Consequently, plants in natural populations plots of O. purpurea experiences increase maternal fitness when growing in matching flower colour communities. We demonstrate a positive net effect of flower colour similarity, relative to flower colour dissimilarity, that can facilitate plant reproduction. Through field experiments we systematically test previously observed patterns of clustered flower colour assembly in Oxalis communities to provide a comprehensive assessment of facilitative co-flowering interactions.