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

Weexamined the effects of fertilization on the diversity, abundance, and cover ofthe understory plant community of two montane wet forests in Hawaii. One siteoccupies a young substrate, where aboveground tree growth is limited bynitrogen(N), while the other site is on an older substrate, where aboveground treegrowth is limited by phosphorus (P). Both sites contained an on-going,long-termfactorial fertilization experiment in which plots were fertilized semi-annuallywith N, P, or N and P in combination. In each fertilization treatment, wemeasured density of species ≥ 0.5 m tall and percent cover ofspecies < 0.5 m tall. Fertilization with N reducedspeciesrichness at the young, N-limited site, but none of the nutrient additionsaltered species richness at the older, P-limited site. Species diversity andevenness were not affected by fertilization at either site. At the site withlowN availability, plots fertilized with NP had higher densities of the non-nativeginger Hedychium gardnerianum, and at the site with lowP-availability, densities of the exotic shrub Rubusargutuswere higher in P- and NP-fertilized plots. Other effects included declines inmoss cover with fertilization at both sites, and reduced abundance of nativeseedlings in response to N and NP addition at the N-limited site. Continuedlong-term fertilization could lead to greater dominance of non-native speciesbyencouraging their growth at the expense of native species, which may sufferdecreased recruitment as fertilization and increased abundance of thenon-nativespecies may reduce suitable substrates for seedling establishment.