Pollination Biology of an Endemic Hawaiian Tree, Erythrina sandwicensis (Fabaceae: Papilionoideae), in a Novel Ecosystem1

Abstract: Tropical dry forest ecosystems are among the most endangered ecosystems globally. In Hawai‘i, dry forest species are under threat due to habitat destruction, invasive species, and a loss of mutualisms including pollinators. Understanding the pollination biology of species at risk can inform conservation strategies. This study aimed to identify the breeding system and types of floral visitors to Erythrina sandwicensis (Fabaceae) or wiliwili, an iconic Hawaiian tree in decline and with little regeneration. We conducted hand-pollination trials and observed visitors in two sites: a botanical garden and a forest site. We compared fruit set, seed set, seed germination, seedling growth rate, and seedling size across four pollination treatments: open control, autogamy, geitonogamy, and xenogamy. We found that wiliwili is visited by a novel suite of non-native visitors. All treatments produced seeds, but the xenogamous (cross) treatment produced significantly more fruit and seeds than the control or other treatments. Seedlings produced from cross-pollinated treatments were also taller and had wider basal diameters after twenty-eight days of growth. These results indicate that wiliwili employs a mixed-mating system, and that trees are pollen limited. Enhancing current populations of wiliwili to promote cross-pollination could help increase population regeneration.