Individual study: Phosphorus addition reduces invasion of a longleaf pine Pinus palustris savanna by a non-indigenous grass Imperata cylindrica, University of Mississippi Forest Lands, Stone County, Mississippi, USA
Brewer J.S. & Cralle S.P. (2003) Phosphorus addition reduces invasion of a longleaf pine savanna (southeastern USA) by a non-indigenous grass (Imperata cylindrica). Plant Ecology, 167, 237-245
The grass Imperata cylindrica (native to Asia) is becoming increasingly invasive in southeast USA. Its presence may lead to reduced biodiversity through exclusion of native plants and altered fire regimes. In this study, rates of clonal spread by I.cylindrica were measured from a roadside into the interior of two longleaf pine Pinus palustris savannas; at one of the sites, application of nitrogen (N) and phosphorus (P) fertilisers were undertaken to assess their effects upon invasion.
Study area: The study was undertaken in two longleaf pine savannas with similar vegetation (Henly Park and Wolf Branch) at the University of Mississippi Forest Lands, Stone County, Mississippi, southeast USA.
Clonal invasion: Rates of invasion were measured in 20, 0.5 × 0.5 m quadrats (established in May 1998 at Wolf Branch and May 1999 at Henley Park) at the advancing border of the I.cylindrica swards; quadrats contained at least three Imperata shoots when established. Shoots were counted in May 1998 and May 1999 at Wolf Branch, and in May, June and September 1999, and May 2000 at Henley Park. Impact on native species richness was quantified.
Fertilizer addition: In mid-May 1999, 60, 0.5 × 0.5-m quadrats were established along the Imperata border at Henley Park. The two fertilizer treatments (applied in May 1999, September 1999 and March 2000), and an untreated control (20 of each), were:
i) N (nitrogen) addition - 3 broadcast applications of 60 g of 32% pelletized ammonium nitrate (equivalent to 20 g N/application);
ii) P (phosphorous) addition - 3 applications of 42 g of 46% pelletized superphosphate fertilizer
(20 g P/application).
Shoots were counted and height of the tallest shoot measured in May, June and September 1999, and in May 2000. Dry aboveground mass was measured in July 2000 at the end of the study period: green Imperata shoots from all plots were harvested dried and weighed.
Native plants: In July 2000, all plants were identified to species, where possible, and species richness, percent cover and height of the tallest stem quantified in each plot.
Clonal invasion of Imperata occurred at both sites at similar rates; the number of shoots/plot increased by an average over 12 months of 5.8 shoots at Henley Park and 4.8 shoots at Wolf Branch. The sward interior where Imperatahad been present longer contained on average fewer native pine-savanna species (1.2) than plots along the border (6.2 species).
Growth rates and aboveground mass of Imperata were reduced by P addition relative to controls by the second growing season. However, it had no noticeable effect on species richness, percent cover, or height of native vegetation overall. Legume cover relative to native non-legumes decreased with increasing P limitation (i.e. from P-fertilized to controls to N-fertilized plots) and
Species richness of native plants was lower in N-fertilized plots (2) than in controls (6) or P-fertilized plots (5), likewise percent cover: N-plots (3%); controls (14%); P-plots (11%). Species richness of native plants was negatively correlated with final aboveground mass of Imperatain control and P-fertilized plots, but not in N-fertilized plots.
Conclusions: The results suggest that I.cylindrica is a better competitor for P than native pine-savanna plants, especially legumes, and that short pulses of phosphorus addition may reduce this competitive advantage without negatively affecting native species. However, the authors of the study are uncertain that the addition of phosphorus increased the intensity of competition between I.cylindrica and native plants, as results were not always clear-cut. Long-term effects of nutrient addition would need to be investigated.
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