Experimental restoration trials in Nakula Natural Area Reserve in preparation for reintroduction of Kiwikiu (Pseudonestor xanthophrys)

Warren, Christopher
Mounce, Hanna
Berthold, Laura
Farmer, Chris
Leonard, David
Duvall, Fern
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Pacific Cooperative Studies Unit
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The native montane mesic forest in the Kahikinui region of Maui, Hawai‘i USA has been degraded by non-native ungulates for over a century. This has resulted in large areas of non-native grassland and savanna with small intact native forest patches, mainly in steep gulches. The Nakula Natural Area Reserve (NAR), on the southwestern slope of Haleakalā volcano, was selected as the site of the reintroduction of Kiwikiu (Pseudonestor xanthophrys), a critically endangered songbird currently found only in a small range on the northern slope of the volcano. This area was selected for the reintroduction because it is located within a mesic koa (Acacia koa) forest representing some of the best potential habitat outside of the current Kiwikiu range. Historic accounts noted the Kiwikiu’s affinity for koa as a foraging substrate, although little koa forest remains on Maui. Intensive forest restoration has created new habitat and enhanced the existing habitat in Nakula to the point where the reserve may now be capable of supporting a small population of Kiwikiu and other native birds. As a precursor to reintroduction efforts, we designed experimental trials to inform managers of the most efficient and effective techniques to restore the forest in Nakula NAR and surrounding region. Trial plots were established in open grass-dominated areas within a fenced, ungulate-free portion of the reserve to investigate natural regeneration, outplanting, and seed broadcast as restoration techniques under a number of conditions. Treatments to suppress and/or remove non-native grass were implemented as these grasses likely reduce germination of native seedlings and potentially influence outplanting success. Some plots were treated with herbicide and the dead grass biomass was removed to expose bare topsoil in a subset of these plots. Additional plots were established under mature koa trees to investigate natural recruitment and the success of these same restoration techniques in this microhabitat. In two years, natural regeneration was largely limited to ‘a‘ali‘i (Dodonea viscosa) and koa, and was enhanced by the application of herbicide followed by the removal of the grass biomass. Outplanting survivorship was high in most species, exceeding 80% after two years in five of seven species. Treatment application had little effect on survivorship, but the growth rates in four of the seven species planted was greatest in plots where herbicide was applied prior to planting. Seed broadcast was not found to be an effective treatment of producing seedlings. Based on our results, we recommend non-native grass biomass removal combined with outplanting as the primary method of forest restoration in Nakula NAR and the surrounding region.
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Outplanting, natural regeneration, mesic forest, seed scatter, invasive grass removal
Warren, C.C., H.L. Mounce, L.K. Berthold, C. Farmer, D.L. Leonard, and F. Duvall. 2019. Experimental restoration trials in Nakula Natural Area Reserve in preparation for reintroduction of Kiwikiu (Pseudonestor xanthophrys). Pacific Cooperative Studies Unit Technical Report #199. University of Hawai‘i at Mānoa, Department of Botany. Honolulu, HI. 102 pages.
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