Over the past 200 years, Hawaii’s upland forests have been drastically altered through decline of native species and invasion by non-natives, even where there has been no direct habitat modification by humans. Agencies statewide are attempting to conserve and restore upland forests, but it is unclear to what extent these efforts conserve and restore critical ecological processes, such as plant recruitment. With approximately half of the native forest flora relying on bird dispersal for recruitment, but few (and sometimes no) native fruit-eating birds remaining, many native plants may partially rely on novel interactions with nonnative birds to persist. We monitored two ongoing restoration areas on O‘ahu, Kahanahaiki and Manoa Cliffs, to determine the extent to which native plants are regenerating and if recruitment success varied depending on dispersal mode and seed traits. Restoration areas were located at 630-660 m elevation in mesic and wet forests, with native Metrosideros polymorpha and Acacia koa (neither of which are bird-dispersed) as the dominant remnant canopy at both sites. We surveyed the vegetation, seed rain (148 samples/month), seed bank (148 samples quarterly), and seedlings (72 plots/month) for one year within each site to determine limitation at each recruitment stage (source, dispersal, and establishment).
Results/Conclusions
Nonnative species made up 47% and 45% of total species at Kahanahaiki and Mānoa Cliffs, respectively. Clidemia hirta (22,269 seeds/m2/yr) and Rubus rosifolius (3,803 seeds/m2/year), both fleshy-fruited, small-seeded, nonnative invasives, had the highest recruitment success across recruitment stages at Kahanahaiki. At Mānoa Cliffs, Clidemia hirta (1,115 seeds/m2/yr), and invasive, nonnative, wind-dispersed Crassocephalum crepidioides (212 seeds/m2/year) and Erechtites valerianifolia (187 seeds/m2/year) had the highest recruitment success across recruitment stages. Wind-dispersed Metrosideros polymorpha (1,690 seeds/m2/yr at Kahanahaiki; 1,970 seeds/m2/yr at Mānoa Cliffs), the dominant native canopy tree at both sites, dispersed into the most traps and at the highest density compared to other natives, but was establishment limited. Two common, fleshy-fruited, native species, the large-seeded liana Alyxia stellata (10 seeds/m2/year), and the small-seeded shrub Pipturus albidus (36.24 seeds/m2/yr), were the second most abundant native seeds in the seed rain at Kahanahaiki and Mānoa Cliffs, respectively; but, both species were dispersal limited (trapped seeds still in fruit or only beneath adults). Results suggest that among native, fleshy-fruited species in both sites, the common species are limited by dispersal, while the less common species are source limited.