Tropical forests have undergone widespread degradation and conversion to alternative land uses such as pastureland, despite their value to society for carbon sequestration, watershed hydrology, biodiversity, and numerous other ecosystem services. Globally,~62 million hectares of pasture were abandoned between 1998 and 2013, presenting the opportunity for landscape scale forest restoration. This is particularly true in Hawaii, where a decline in ranching provides the opportunity to restore abandoned pastures to native forest. Many of these efforts use the disturbance adapted, nitrogen (N)-fixing native tree Acacia koa to initiate ecological restoration. However, recent evidence shows that increased soil N associated with A. koa can increase non-native grass biomass, which suppresses native regeneration and results in a stable state characterized by A. koa overstory, non-native grass understory, and an almost complete lack of native recruitment. This study sought to address the question of whether A. koa facilitates or hinders the development of a diverse native forest by examining the impact of A. koa density on leaf area index (LAI; the inverse of light availability), soil moisture, soil N, grass biomass, and native outplant survival at an 11-year-old mesic forest restoration site in Hawaii Volcanoes National Park.
Results showed that A. koa competes with invasive grasses, resulting in a decline in grass biomass with increasing A. koa density. In addition, we found a positive, linear relationship between A. koa density and both LAI and extractable inorganic soil N, and no statistically significant relationship between A. koa density and soil moisture. There was also no significant relationship between A. koa density or grass biomass and outplant survival 6 months after outplanting. A. koa litter increased with A. koa density, but increased litter was not related to grass biomass. Cumulatively, these results indicate that at high densities, A. koa produces low light conditions, increases soil N, and outcompetes non-native grasses. Species chosen for understory enrichment outplanting should, therefore, be ecophysiologically adapted to these conditions to improve restoration success. This also shows that at the highest levels of soil N, grass biomass declined, most likely due to co-occurring low light availability caused by high tree density.Contrary to prior studies, increased soil N at this site did not drive an increase in non-native grass biomass. Restoration using disturbance adapted, woody species at high density in this system is a good first step to "retaking" abandoned tropical pastures from aggressive non-native grasses.