Functional Group Identity Does not Predict Invader Impacts: Differential Effects of Nitrogen-fixing Exotic Plants on Ecosystem Function |
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Authors: | Stephanie G Yelenik William D Stock David M Richardson |
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Institution: | (1) Institute for Plant Conservation, University of Cape Town, Rondebosch, 7701, South Africa;(2) Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, CA 93106, USA;(3) Botany Department, University of Cape Town, Rondebosch, 7701, South Africa;(4) Centre for Ecosystem Management, Edith Cowen University, Joondalup, 6027, WA, Australia;(5) Centre for Invasion Biology, University of Stellenbosch, Private Bag X1, Matieland, 7602, South Africa |
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Abstract: | The introduction of exotic plants can have large impacts on ecosystem functions such as soil nutrient cycling. Since these
impacts result from differences in traits between the exotic and resident species, novel physiological traits such as N cycling
may cause large alterations in ecosystem function. It is unclear, however, whether all members of a given functional group
will have the same ecosystem effects. Here we look at a within functional group comparison to test whether an annual (Lupinus luteus) and a perennial (Acacia saligna) N-fixing exotic species cause the same effects on soil N cycling in the fynbos vegetation of South Africa. We measured litterfall
quantity and quality, and soil total nitrogen and organic matter for each vegetation type as well. Available nitrogen was
quantified using ion exchange resin bags monthly for 1 year. We used microcosms to evaluate litter decomposition. Although
both exotic species increased the available nitrogen in the soil, only Acacia increased the total soil N and organic matter. This could be explained by the slow decomposition of Acacia litter in the microcosm study, despite the fact that Acacia and Lupinus litter contained equivalent N concentrations. Presumably, low carbon quality of Acacia litter slows its decomposition in soil, resulting in retention of organic nitrogen in Acacia stands after clearing for restoration purposes. The differences in long term impacts of these annual and perennial species
highlight the fact that not all N-fixing exotic species exert equivalent impacts. Ecologists should consider multiple traits
rather than broadly defined functional groups alone when predicting invader impacts. |
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Keywords: | Acacia saligna available soil nitrogen exotic plants fynbos litter decomposition Lupinus luteus organic soil nitrogen South Africa |
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