Individual study: Fertilization with nitrogen increases abundance of non-native species in montane forest adjacent to Thurston Lava Tube, Hawaii, USA
Ostertag R. & Verville J.H. (2002) Fertilization with nitrogen and phosphorus increases abundance of non-native species in Hawaiian montane forests. Plant Ecology, 162, 77-90
In disturbed natural or semi-natural habitats, application of fertilizers may change the composition of species present in the community, and may also influence the success of invasive species. This present study focused on changes in the understory community after fertilizer application in two Hawaiian montane forests in order to assess, amongst other things, the potential (or not) of applying fertilizers to encourage native species. One forest was on a young soil where tree growth is limited by nitrogen (N), while the other is on an older substrate where tree growth is limited by phosphorus (P). Both sites contained an on-going, long-term fertilizer addition experiment in which plots were fertilized semi-annually with N and/or P. The study undertaken on the N-limited site is summarised here.
Study site: The study was undertaken in moist forest at a N-limited site (1,176 m altitude) adjacent to Thurston Lava Tube just outside Hawai'i Volcanoes National Park on the island of Hawai'i (19º25'W, 155º15' N).
Experimental design: Long-term fertilizer addition experiments have been ongoing since October 1985. A 15 × 15 m area of each plot was fertilized semi-annually at a rate of 100 kg/ha/yr of N (50% urea, 50% ammonium nitrate) and/or 100 kg/ha/yr P (as triple superphosphate). Control plots (C) had no fertilizer addition. There were four replicate plots for each treatment (N, P, NP, and C).
Vegetation monitoring: To assess changes in understory community structure in response to the treatments, vegetation data was collected in 1996. No data on understory species composition were taken before fertilization began.
In each plot, four transects of 15 m length were established. For plants >0.5 m tall (longer lived stems e.g. shrubs and trees), density was estimated by counting all individuals in a 1 m belt along each transect. Ground cover and vegetation < 0.5 m tall (i.e. herbaceous vegetation and seedlings), five 0.25 mÂ² quadrats, were placed every 3 m along each transect (20 quadrats/plot). In each quadrat, number of seedlings and percent cover of higher plants, moss, leaf litter, fine woody debris, logs, stems/rhizomes, and bare ground, were recorded.
N addition reduced species richness at this N-limited site. Species diversity and evenness were not affected by fertilizer additon. Plots fertilized with NP had higher densities of the non-native ginger Hedychium gardnerianum (well known for its vegetative spread through rhizomes). Other effects included declines in moss cover with fertilizer addition, and reduced abundance of native seedlings in response to N and NP addition.
The authors consider that continued long-term fertilizer addition could lead to greater dominance of non-native species (especially H.gardnerianum) by encouraging their growth at the expense of native species, which may also suffer decreased recruitment as increased abundance of the non-native species may reduce suitable substrates for seedling establishment.
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