The mechanisms implied in these soil properties modifications are probably diverse and are not completely understood. However, our data allow us to formulate some hypotheses. The biomass production of all studied plants was always higher than that of the resident vegetation (Dassonville et al., 2008). Higher biomass was achieved either by larger shoots and faster growth rate (F. japonica, I. glandulifera, S. gigantea) or by increased ground cover compared to the resident vegetation (S. inaequidens). Despite lower or equal nutrient concentration in aboveground biomass of the invasive plants compared to uninvaded vegetation, the nutrient standing stocks were always higher due to the higher biomass (Dassonville et al., 2008). These higher aboveground nutrient stocks can be achieved by the invasive plants probably through access to nutrient sources inaccessible for the resident vegetation. Nutrient uplift (sensu Jobbagy & Jackson, 2004) by a deeper root system is certainly the explanation for Fallopia japonica which can root at more than 2 meter deep (Dassonville et al., 2007). This is also the case for H. mantegazzianum and probably also for R. rugosa and P. serotina surrounded by herbaceous vegetation. Active mobilisation of nutrient forms that were inaccessible for resident vegetation is also possible as shown by Chapuis-Lardy et al (2006) for P. In absence of a strong translocation, this leads to a higher nutrient return to the soil through litter fall. Thus the invasive species steadily enhance specific nutrient fluxes in invaded ecosystems, including net uptake rates from soil and annual returns in dead organic matter. This higher nutrient return could explain the increased nutrient concentrations in the topsoil of the originally most oligotrophic sites. However, it must be strong enough to compensate the increased nutrient leaching in invaded plots. Invaded plots are more sensible to nutrient leaching because of the absence of soil cover during nearly 6 months in winter and early spring contrary to the resident vegetation, mostly composed of perennial grass species (Hooper and Vitousek 1998; Ridley et al. 2001). This phenomenon may explain why the magnitude of impact decreases with increased initial nutrient concentration in the soil. In addition, in the most fertile sites, nutrients may be less strongly immobilised in the topsoil compared to nutrient-limited sites (Tamm et al. 1995) and result in "nutrient leakage". Organic matter turnover rates and nutrient leaching losses are often increased in eutrophic sites (Wedin and Tilman 1996; Debusk and Reddy 1998).
We essentially discussed the "site factor" which in our opinion explains a high proportion of the variability of impact (Dassonville et al., 2008). However, all studied species differ in their strategies and even for two plants (S. gigantea and F. japonica) with the same growth form (perennial rhizomatous geophytes), we found some divergence in their impacts. For these reasons, the impacts of these two species were studied in more details.
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