Planting diversity and density shape native plant survival and community assembly in tropical forest restoration

Abstract Introduction Forest restoration in Hawaii is underway due to past large‐scale conversion of forests to pastures dominated by invasive grasses. Secondary succession is limited by grass dominance and a lack of native seed rain, making outplanting a viable restoration strategy. Objective We conducted a multi‐year field experiment at Hakalau Forest National Wildlife Refuge to test how species richness and planting density influence outplant survival, cover, and growth. Methods We planted eight native species representing trees, a shrub, a fern, and a vine in combinations of two, four, and eight species at single and double planting densities. We measured survival and growth of outplants as well as percent cover of all plants in the experimental plots for 3 years. Results Overall survival across all species was high (76 ± 2.5%) but varied by species; shrubs and vines had the lowest survivorship, likely due to grass competition, while a four‐species combination of trees and a fern had the highest survivorship. Outplant cover increased over time, with the greatest cover observed in double‐density plantings of four and eight species. Invasive grass cover also increased across all treatments, with little evidence that outplantings reduced grass cover relative to controls. Relative growth rates were unaffected by species combination or density. Conclusions While overall cover differences were modest by the final monitoring period, planting density and species selection shaped early establishment. These results support multi‐species, high‐density planting as a strategy to enhance survivorship, accelerate canopy development, and support habitat creation.