Phenology and pollination ecology of Prosopis rubriflora (Leguminosae, Mimosoideae), a species from the semi-arid Brazilian Chaco

Abstract

Prosopis L. is particularly rich and important in the Chaco region where forest vegetation currently persists as fragments. Its species are very important for the conservation and management of Chaco vegetation, and understanding their phenology and reproductive biology is essential. Here, we studied the phenology and pollination biology of P. rubriflora Hassl. in the Brazilian Chaco compared to other species of the genus. Differing from other Prosopis species, P. rubriflora is evergreen, and most phenophases are nonseasonal and bimodal (except fruiting), with a continuous pattern. Prosopis rubriflora is andromonoecious, but only weakly, since plants produce relatively few staminate flowers. Flowers showed three floral stylar phenotypes: short, medium and long style. Short-style flowers are functionally staminate because the stigma is nonreceptive. To compensate for the small size of P. rubriflora red brush flowers, attraction is compressed and transferred to the spike, which functions as a collective pollination unit. This compression may favour self- and intraplant pollination, which may reduce and/or prevent the occurrence of protogyny and partial, i.e. temporal, herkogamy in hermaphrodite flowers. Prosopis rubriflora has generalist pollination, but differs from the other Prosopis species because it is also pollinated by hummingbird, most likely due to its “ornithophilous” characteristics. This species is the first of the genus recorded as pollinated by hummingbirds. Although the exotic bee Apis mellifera L. presents high frequency of visit, this bee is not effective pollinator, due to its foraging style that does not favour cross-pollen flow. Native insects and hummingbird species are main pollinators because they present trapline foraging.