Native ecosystems globally face constant pressure from nonnative, invasive plants that are often competitively superior. Evidence exists that nonnative plants typically outcompete natives under conditions of high resource availability. Therefore, one potential restoration technique for favoring native species over nonnatives is to decrease soil nutrient availability via carbon amendments (e.g., sucrose and sawdust). 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 in tropical ecosystems and with woody species. We evaluated survival, growth, and ecophysiology of native and nonnative, invasive species from Hawaiian wet and dry ecosystems in both greenhouse and field competition experiments. In the greenhouse, the density of plants was held constant at 2 plants/pot, and native species were grown with a conspecific and 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). Informed by the greenhouse experiment, carbon amendments were added in the field to both wet and dry ecosystems to alter nutrient availability to determine if this restoration technique is successful in less well-controlled environments.
Results from the greenhouse showed that biomass, whole plant carbon gain, and foliar nutrient content of all species across both ecosystem types increased with increasing soil nutrient availability. When individual growth and carbon gain responses were examined, all native species had a non-significant response to soil nutrient level, except one, indicating a conservative resource acquisition strategy. In turn, one native species and both nonnatives responded similarly with total biomass and whole plant carbon gain increasing with increasing soil nutrient availability, indicating an exploitative resource acquisition strategy. Moreover, one of the native species experienced very high mortality in both nutrient addition treatments (mean of 86% mortality). The nonnative grass tested displayed a sharp decrease in reproductive output with decreasing soil nutrient availability (65% flowering rate in nutrient addition treatments and 3% flowering rate in nutrient reduction treatments). Photosynthetic capacity (Vcmax and Jmax) was constant for all species across varying soil nutrient levels. Finally, resource use efficiency (RUE) of native species remained constant, while that of invasive species decreased with decreasing soil nutrients. Collectively, these results suggest that reducing soil nutrient availability via carbon amendments is a promising technique for restoring tropical ecosystems where nutrient availability is high and invasive species occur.