Development of a native plant agroforestry system to restore the understory of a lowland wet forest in Hawaiʻi

Abstract

Native plant agroforestry enrichment systems established in already degraded or altered environments have the potential to protect existing native habitat and provide plant material for cultural purposes while potentially favoring the reestablishment of other non-target native species. Limited information is available on the survival and growth of native Hawaiian plants planted in the understory of secondary forests dominated by nonnative species. This study investigated an understory agroforestry system that utilized native plants with both economic and cultural value and was designed to provide desired plant material while also complementing restoration activities with the removal of invasive species. Research objectives were: 1) Measure establishment and growth response of three native species planted in removed or intact nonnative understory and determine to what extent light availability, soil moisture and nutrient availability influenced these responses; 2) evaluate the effects of understory nonnative species removal, particularly the dominant invasive species Ardisia elliptica, and the planting of native species on nonnative and native seedling recruitment; and 3) examine the photosynthetic light-response and functional differences of the native fern, Microlepia strigosa, planted in the two understory treatments. Results of this research suggest that within lowland wet mesic forests dominated by nonnative species, light availability is the most critical resource limiting establishment and growth of understory native species. Successful and sustainable cultivation of these native species will require some level of canopy opening or manipulation to ensure adequate light levels. The relatively high survival of M. strigosa under varying understory light conditions suggests it is a good species for establishing an understory groundcover. Additionally, M. strigosa shows increases in photosynthetic capacity when planted within removed understory, but is photosynthetically limited to relatively low light and shaded environments. Results from this research also indicate that established native plantings may provide some resource competition for nonnative species but manipulation of environmental resources, such as decreasing available light, plus continued weeding is more effective at reducing nonnative species recruitment. Consequently, management for a more homogenous nonnative canopy cover can provide more uniform reductions in light transmittance to the understory and limit nonnative seedling recruitment.