Nonnative plant invasions have been at least partially attributed to increased nutrient availability which can occur following disturbances to native ecosystems. While nonnative plant control is a commonly used technique, it is labor intensive and does not address long-term underlying changes in soil nutrients. A potential restoration technique to reduce soil nutrient availability and promote native over nonnative plants is the application of a carbon substrate (such as sucrose and sawdust) that promotes uptake of available inorganic nitrogen by soil microbes. This method has potential as a restoration tool where native plant species are resource conservative and invasive species are resource exploitative. We manipulated soil nutrient availability and invasive species presence in two montane tropical ecosystems, Hawaiian wet forest and Hawaiian dry shrubland, to assess the impact on competitive tradeoffs between native and nonnative plants. Nutrient manipulation and weeding treatments were applied in a fully factorial design (control- no manipulation; carbon- carbon substrate added; and fertilizer- NPK fertilizer added) and two weeding treatments (weeds- nonnative plants present and no weeds- nonnative plants removed). Treatment effects on soil nutrient availability and native vs. nonnative plant survival, cover, growth and reproduction were quantified for 18 months following treatment application.
Results/Conclusions
Wet forest: Carbon substrate addition significantly decreased plant available N compared to control while fertilizer significantly increased N compared to the carbon and control treatments. Percent biomass of nonnative ginger increased by >50% in fertilizer compared to control and carbon treatments. Nonnative invasive woody seedlings had reduced growth in carbon treatments and significantly greater growth in fertilizer compared to control treatments. Native woody seedlings had a neutral growth response across nutrient treatments, but significantly greater survival and growth where nonnative plants were removed. Mortality of native seedlings was extremely high (95%) in fertilizer + weeds treatments.
Dry shrubland: Carbon substrate addition decrease N only slowly and values were similar to control treatment; fertilizer increased N compared to carbon and control. Invasive species cover did not vary across nutrient treatments. Native woody seedlings grew similarly in carbon and control treatments and had significantly greater growth in response to fertilizer and weed removal treatments. Reproduction of dominant native and nonnative species increased markedly in fertilizer compared to carbon and control treatments.
Overall, increased nutrient availability favored nonnative plants while weeding had a positive effect on native plants. Effects of nutrient manipulation were stronger in wet forest than in dry shrubland ecosystems.