Early plant establishment following experimental removal of invasive Morella faya (AIT.) Wilbur stands in a Hawaiian forest

Patterns of plant community re-establishment following removal of a dominant woody invader have been rarely studied. In Hawaii, Morella faya, a small nitrogen-fixing tree, has invaded native forest in Hawai'i Volcanoes National Park, forming near monocultures in many areas. The purpose of this research was to determine how different Morella faya removal treatments could influence plant community re-establishment. Three removal treatments (logging all trees, simultaneous girdling of all trees, or incremental girdling of all trees over 20 mo.) were applied to 30 x 30m plots in order to remove Morella faya stands at different rates, resulting in different environmental and resource conditions. Logging resulted in instantaneous opening of the forest canopy followed by rapid invasion of alien species. Girdling resulted in slow canopy defoliation followed by slower rates of alien plant establishment. Native plant establishment was low in all treatments. However, native understory fern and native woody species establishment were higher in the two girdling treatments. Light availability best explained differences in plant composition among treatments. The dominant alien colonizers were mainly herbaceous species (e.g., Kyllinga brevifolia, Anemone hupehensis ) or shrubs (Rubus spp.) that had formed a persistent seed bank able to quickly take advantage of Morella faya removal. In contrast, there were few fastgrowing native colonizers found in the soil seed bank or seed rain. Morella faya failed to re-establish in the log treatment, but some seedlings survived in the girdle treatments, suggesting Morella faya might regain dominance. Seed addition and transplant experiments demonstrated that the ability of native and alien species to establish among the treatments was context dependent if barriers to seed dispersal were overcome. Native species could outperform alien species under some conditions. Based on these results, managers can facilitate native forest recovery in areas overwhelmed by Morella faya by (1) removing individuals slowly (e.g., girdling, incremental girdling) in order to slow down rates of alien species establishment and allow some native plant establishment, (2) preventing the establishment of highly disruptive alien weeds (e.g. Psidium cattleianum) through early detection and eradication, and (3) seeding and outplanting with native species to supplement the scarce native seed supply.