Individual study: Fertilization with phosphorus increases abundance of non-native species in montane forest within Na Pali Kona Forest Reserve, Kaua'I island, 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 P-limited site is summarised here.
Study site: The study was undertaken in moist forest at a P-limited site within Na Pali Kona Forest Reserve (1,134 m altitude) on the island of Kaua'i (22º08' W, 159º37' N), Hawaii, USA.
Experimental design: Long-term fertilizer addition experiments had been ongoing since March 1991. 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).
Plant 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.
None of the nutrient additions altered species richness at this P-limited site; likewise species diversity and evenness. Densities of the exotic shrub Rubus argutus (which resprouts from basal and subterranean shoots and may form dense thickets out-competing native vegetation) were higher in P- and NP-fertilized plots. Moss cover declined with fertilizer addition.
The authors consider that continued long-term fertilizer addition could lead to greater dominance of non-native species e.g. R.argutus 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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