Fertilization with nitrogen and phosphorus increases abundance of non-native species in Hawaiian montane forests

We examined the effects of fertilization on the diversity, abundance, and cover of the understory plant community of two montane wet forests in Hawaii. One site occupies a young substrate, where aboveground tree growth is limited by nitrogen (N), while the other site is on an older substrate, where aboveground tree growth is limited by phosphorus (P). Both sites contained an on-going, long-term factorial fertilization experiment in which plots were fertilized semi-annually with N, P, or N and P in combination. In each fertilization treatment, we measured density of species greater than or equal to0.5 m tall and percent cover of species <0.5 m tall. Fertilization with N reduced species richness at the young, N-limited site, but none of the nutrient additions altered species richness at the older, P-limited site. Species diversity and evenness were not affected by fertilization at either site. At the site with low N availability, plots fertilized with NP had higher densities of the non-native ginger Hedychium gardnerianum, and at the site with low P-availability, densities of the exotic shrub Rubus argutus were higher in P- and NP-fertilized plots. Other effects included declines in moss cover with fertilization at both sites, and reduced abundance of native seedlings in response to N and NP addition at the N-limited site. Continued long-term fertilization could lead to greater dominance of non-native species by encouraging their growth at the expense of native species, which may suffer decreased recruitment as fertilization and increased abundance of the non-native species may reduce suitable substrates for seedling establishment.