The role of alien tree plantations and Avian seed-dispersers in native dry forest restoration in Hawaii

Abstract

Foresters often plant tree plantations to speed rehabilitation of degraded forestlands. I compared mature plantations of four alien species (Casuarina spp, Eucalyptus robusta, Fraxinus uhdei, and Grevillea robusta) to remnant patches of native dry forest in the Honouliuli Forest Preserve on Oʻahu, Hawaiʻi to determine if plantations aid the recovery of native plants in disturbed dry habitats. I measured soil pH and nutrients, forest floor structure, vegetation structure, plant species richness and abundance, seed rain species richness and abundance, native seed germination success and seedling survival, fruit availability, alien and native bird species richness and abundance, and avian seed-dispersal. Alien tree plantations do not appear to promote native plant restoration in dry areas in Honouliuli. There were several limitations to native plant establishment in tree plantations including low native seed dispersal (by both birds and wind), low seed germination rates, and low seedling survival in plantations. The understory communities of the five forest types indicate that their futures differ greatly (Ch 3). Casuarina and Fraxinus understories had dense self-regeneration and low proportions of native or invasive species. These forest types will likely remain stable over time. In contrast, Eucalyptus and Grevillea plantation trees are not replacing themselves. Native tree saplings were rare in Eucalyptus and Grevillea plantations, and these forest types seem destined to be replaced by invasive trees as the planted trees senesee. Grevillea plantations comprise a large proportion of the planted trees in Honouliuli, particularly in the southern end of the Preserve. These stands will beconverted to open invasive woodlands. Remnant patches of native lowland dry forest in Honouliuli face an uncertain future. While around 30% of the understory tree seedlings and saplings were native, the remaining belonged to invasive species (Ch 2). Psidium cattleianum, in particular, is overtaking the native forests. As the Psidium stands expand they will further reduce the size of native forest patches. Future management efforts for lowland forests in Honouliuli should focus on control of invasive species in the remnant patches of native forest, and on dry forest restoration in the Grevillea plantations in the south end of the Preserve.