Forest invasion by the African Tulip Tree (Spathodea campanulata) in the Hawaiian Islands: are seedlings shade-tolerant?

Abstract

Native to West Africa, Spathodea campanulata (African tulip tree) is frequently viewed as a shade-intolerant invader. It commonly colonizes roadsides, human-disturbed forests, and abandoned agricultural land in tropical islands, where it can then become dominant in secondary forests. Some authors have suggested that the seedlings may be shade-tolerant and able to establish in closed-canopy forest, but the shade tolerance of seedlings has never been evaluated. We identified tolerated light environments of S. campanulata seedlings in wet forests in Hawai‘i by measuring photosynthetically active radiation (PAR) around naturally occurring seedlings (<30 cm height) in the field. We also measured photosynthetic responses of seedlings to light under field and laboratory conditions and determined seedling growth rates in sun and shade. Seedlings were found in shaded conditions in the field, and they consistently had positive net carbon gain at 50 μmol photons m⁻²·sec⁻¹ PAR, with an estimated mean compensation point below 10 μmol photons m⁻²·sec⁻¹, indicating high shade tolerance. The most frequent midday light environments of S. campanulata seedlings in the field were in the range of 50 to 200 μmol photons m⁻²·sec⁻¹ PAR (i.e., 2.5% to 10% of full sunlight). Among seedlings found growing in shade, minimum saturating light (Ek), determined from chlorophyll fluorescence, averaged 260 μmol photons m⁻²·sec⁻¹, suggesting that maximum seedling photosynthesis can occur at less than 13% of full sun. Growth rates of young seedlings in shade and sun were comparable. Widespread wind dispersal of seeds, seedling tolerance of low light, and our observations of some S. campanulata saplings establishing in rain forest without recent disturbance suggest that S. campanulata will be a persistent component of Hawaiian lowland rain forests.