Native Species Regeneration Following Ungulate Exclusion and Nonnative Grass Removal in a Remnant Hawaiian Dry Forest

Hawaiian lowland dry forests have been reduced by >90% since first human contact. Restoration has focused on protection from fire and ungulates, and removal of invasive grasses as ways to stimulate native forest regeneration. Despite these efforts, natural regeneration of native plants has been infrequent. To assess effects of previous restoration treatments on natural regeneration, we monitored seed rain and dynamics of seedlings and juveniles for a period of 3 yr (2004–2007) within three restoration units within a remnant forest on the island of Hawai‘i. All units had been protected from fire for many decades but differed in time since ungulate exclusion and grass removal. The units were as follows: (1) long-term restoration (fenced 1956, grass removal initiated 1995), (2) shortterm restoration (fenced and grass removal initiated 1997), and (3) unmanaged (fenced 1997, no grass removal). Overall juvenile plant abundance was highest in the short-term unit, but native abundance was highest in the long-term unit. Native woody seedlings established in all units, but recruitment into larger size classes was restricted to units with grass removal, primarily the long-term unit. Native seed rain explained much of the variation in native seedling abundance between units with grass removal. Nonnative grass seed rain was extensive but was reduced by an order of magnitude with grass removal. This study suggests that natural regeneration may enhance restoration actions in sites with native canopy, but this is likely only when restoration activities have been maintained for several years to coincide with favorable rainfall conditions that are highly unpredictable over time. Tropical dry forests are one of the most endangered ecosystems in the world. Almost all of the remaining tropical dry forests (P97%) are at risk from multiple threats such as fire, fragmentation, and human land use (Miles et al. 2006). In many regions, <0.1% of the original dry forests have been protected ( Janzen 1988, Bullock et al. 1995). In Hawai‘i, tropical dry forests have been reduced to less than 10% of their original area (Bruegmann 1996). This decline represents a substantial loss of biodiversity because these forests were once described as the most diverse in the Hawaiian archipelago (Rock 1913). Currently, remnant dry forests have the highest proportion of endangered taxa Pacific Science (2010), vol. 64, no. 4:533–544 doi: 10.2984/64.4.533 : 2010 by University of Hawai‘i Press All rights reserved 1 Funding for this study was provided by the U.S. Forest Service and U.S. Department of Agriculture National Research Initiative Grant 2002-00631 to R.J.C., S.C., and D.R.S. Manuscript accepted 24 December 2009. 2 U.S. Department of Agriculture Forest Service, Institute of Pacific Islands Forestry, Hilo, Hawai‘i 96720. 3 Department of Biology, University of Puerto Rico, Mayagüez, Puerto Rico 00681. 4 Three Mountain Alliance, Hawai‘i Volcanoes National Park, Hawai‘i 96718. 5 Divison of Science and Math, Brevard College, Brevard, North Carolina 28712. 6 Department of Biological Science, California State University, Fullerton, California 92831. 7 Department of Natural Resources and Environmental Management, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i 96822. 8 Corresponding author (e-mail: jthaxton20@gmail