Strategic light manipulation as a restoration strategy to reduce alien grasses and encourage native regeneration in Hawaiian mesic forests

Question: Is there a light level at which alien grass biomass is reduced while still supporting growth and survival of native woody species, allowing for native species regeneration in abandoned pastures? Location: Island of Hawaii, USA. Methods: In a two‐part study we examined the effect of light availability on common native woody and alien grass species found in secondary forests in Hawaii. A field survey was conducted to examine the relationship between light availability and canopy type (open pasture, planted canopy and secondary forest) on understory grass biomass and litter accumulation. We then experimentally manipulated light levels to determine the effect of light availability on growth and survival of six native woody species and three alien grasses. Low‐light (5%), medium‐light (10%) and high‐light (20‐30%) treatments were created using shade structures erected beneath the existing secondary koa canopy. Results: In the field survey, alien grass biomass was greatest under the open pasture and lowest in the secondary forest. There was a positive correlation between understory light availability and alien grass biomass. In the experimental study, large reductions in relative growth rates were documented for all of the grass species and four of the six woody species under the lowest light level. Although growth at 5% light is substantially reduced, survival is still high (84‐100%), indicating that these species may persist under closed canopy. Conclusion: Low‐light conditions result in the greatest reduction in alien grass biomass while creating an environment in which native woody species can grow and survive.