Native ecosystems across the world face constant pressure from nonnative, invasive plants that are often competitively superior. In a variety of ecosystems, evidence exists that nonnative plants typically outcompete natives under conditions of high resource availability (e.g., increased soil nutrient availability following disturbance). Therefore, one potential restoration technique for favoring native species over nonnatives could be to decrease soil nutrient availability via carbon amendments such as sawdust or sucrose. This is a particularly appealing concept in Hawaii where native species often have conservative growth strategies, but this idea has received very little attention to date, particularly in tropical ecosystems. We evaluated survival, growth, and ecophysiology of native and nonnative, invasive species from Hawaiian wet (Acacia koa, Metrosideros polymorpha, and Psidium cattleianum (invasive)) and dry (Dodonaea viscosa, Metrosideros polymorpha, Sophora chrysophylla, and Cenchrus setaceus(invasive)) ecosystems in a greenhouse competition experiment. The density of plants was held constant (2 plants/pot) and native species were grown with a conspecific and with the ecosystem-specific invasive species in each of five soil nutrient treatments (control; high and low nutrient addition via fertilizer; and high and low nutrient reduction via carbon amendments).
Results show individualized species responses to the soil nutrient manipulation treatments, with a general trend of increased growth with increasing nutrient availability. However, the native tree M. polymorpha experienced high mortality in the nutrient addition treatments (mean of 86% mortality across the nutrient addition treatments). The native D. viscosa and the nonnatives C. setaceus, and P. cattleianum all had significantly higher total biomass in the nutrient addition treatments. In contrast, the natives A. koa and S. chrysophylla had no significant differences in total biomass across the soil nutrient manipulation treatments. The nonnative grass C. setaceus displayed a sharp decrease in reproductive output with decreasing soil nutrient availability, with a 65% flowering rate in the nutrient addition treatments and a 3% flowering rate in the nutrient reduction treatments. Collectively, these results suggest that reducing soil nutrient availability as a restoration tool to restore Hawaiian ecosystems where nutrient availability is high and invasive species are problematic is promising, but that this approach may be species and/or community dependent.