Seed Ecology In Montane Forests On O‘ahu: Implications For Conservation
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2019
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University of Hawaii at Manoa
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Hawai‘i is renowned for its unique biota, and for the degree to which that biota is imperiled by extinction. Key threats to the native flora include alien herbivores, the loss of mutualists, and competition with alien species. Ecological restoration on the islands focuses on removing alien ungulates, reducing invasive alien plants, and increasing native diversity and recruitment. Oʻahu is the most populated island and has lost the highest proportion of forest cover compared to other main Hawaiian Islands. Forest communities are dominated by alien plants at lower elevations but retain increasing native diversity at higher elevations. All forest types have lost all native seed dispersers, which may significantly alter the recruitment of native species. Ongoing restoration efforts on Oʻahu, in combination with novel interactions between alien birds and both alien and native plants, provide a unique opportunity to investigate novel interactions and assess the effects of restoration efforts to reduce invasive alien plants and increase the recruitment of native plant species. I conducted three field studies on O‘ahu investigating various aspects of seed ecology. In Chapter 2, I used seed rain traps and vegetation surveys, in three mixed and one native forest community, to investigate the distribution, reproduction, and dispersal of native and alien species adapted for bird dispersal. At all sites, alien seeds were more abundant in the seed rain and more frequently dispersed than native species, even where alien species made up <5% of the vegetation cover. Abundant alien species both in the vegetation and seed rain are among the most invasive alien species in the world. In Chapter 3, I measured and compared the seed rain and vegetation of native and alien bird-adapted species and the wind-dispersed native tree Metrosideros polymorpha in a three-year-old clear-cut surrounded by relatively intact forest to understand whether clear-cutting an invasive tree, Psidium cattleyanum, facilitates native plant regeneration. Two invasive alien understory species, Clidemia hirta and Rubus rosifolius, were the most abundant species in the seed rain in both habitats and the most abundant vegetation in the clear-cut. Seeds of the dominant native tree, Metrosideros polymorpha, were dispersed into the clear-cut, but few seedlings occurred, possibly owing to microsite limitation. In Chapter 4, I investigated the effect of canopy cover on the germination and seedling survival of four common native plant species—Alyxia stellata, Coprosma foliosa, Dianella sandwicensis, and Leptecophylla tameiameiae—in a managed mesic forest. Germination of A. stellata and C. foliosa was positively correlated with canopy cover. Germination of D. sandwicensis, and L. tameiameiae was independent of canopy cover. Invasive alien plants are regenerating more vigorously and more frequently dispersed than natives under current conditions. Two small-seeded alien species, C. hirta and R. rosifolius, are the most prolific invasive aliens in the seed rain and disperse into most forest microsites. Native recruitment is limited in all forest types and native species may benefit from human-mediated dispersal. Seed sowing in appropriate microsites is a potential method for increasing recruitment of common species.
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