Introduction

The strong negative impact of alien invasive species (AIS) on indigenous biodiversity has long been acknowledged by biologists (Braithwaite et al. 1989; Pysek & Pysek, 1995 ; Dunbar & Facelli, 1999; Belnap & Philips, 2001 ; Alvarez and Cushman 2002; Maertz et al., 2005 and many others). Beyond their impact on plant and animal communities, invasive plants are also able to deeply modify the functioning of invaded ecosystems (Ehrenfeld 2003; Liao et al. 2008 and references herein). Invasive plants modify the functioning of ecosystems because they differ from indigenous vegetation by several important functional traits (Wilsey and Polley 2006; Liao et al. 2008). Most of the time, they have a higher productivity than the vegetation they invade and their leaves have a higher nutrient concentration. These two characteristics often result in an accelerated turnover of nutrients and a higher availability of nutrients in invaded soils (Musil 1993; Scott et al. 2001; Duda et al. 2003; Vanderhoeven et al. 2005; Chapuis-Lardy et al. 2006; Liao et al. 2008). While most studies on alien invasive plants report increased carbon and nutrient concentration in invaded habitats, others report the opposite pattern (Christian and Wilson 1999; Leary et al. 2006). Several authors have pointed out the high variability of response of the ecosystem to invasion (Ehrenfeld, 2003; Liao et al. 2008). However, the role of the environment ("site factor") in this variability has been somewhat neglected. Most studies are "species-oriented" and focus on only one site. However, the few authors who worked in several sites pointed out that this variability can even be observed within a single species. For instance, Scott et al. (2001) observed that Hieracium pilosella tends to increase soil organic matter content in some sites and do the opposite in other sites without being able to explain the differences. While it was commonly admitted that the impact of an invasive species could vary depending on the environment (Ehrenfeld, 2003), no general pattern was identified until our study on the impact of invasive plant species in Belgium (Dassonville et al., 2008).

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