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Functional traits determine the ecological strategy of plants, their response to site conditions and their influence on ecosystems (Lavorel & Garnier, 2002; Garnier et al., 2007; Suding et al., 2008). The study of functional traits therefore plays an important role in research on the ecology of exotic plant invasions. While many external factors contribute to the spread and establishment of invasive plants (e.g. human dispersal, vegetation disturbance, nutrient enrichment), the trait equipment of plant species is crucial for their impact on vegetation and ecosystem properties (Wardle et al., 1998; Levine et al., 2003; McIntyre et al., 2005). The comparison of traits between coexisting invasive and native plant species can therefore help understanding success of invasion and its impact (Thompson et al., 1995; Funk & Vitousek, 2007).
Associations between plant functional traits and either invasiveness or invasion impact have been reported in many studies (McIntyre et al., 2005; Küster et al., 2008). However, there are also studies that found no relationship (Hastwell & Panetta, 2005). Many traits may contribute to successful invasion (life form, phenology, seed size, polyploidy level etc.) and the significance of individual traits is often context-dependent or species-specific (Thompson et al., 1995; Moles et al., 2008). In particular, traits promoting high reproduction rates and rapid spread are decisive for the colonization of open habitats while those promoting fast growth and resource acquisition are most important for the ability to establish, dominate and displace the resident vegetation (Lavorel & Garnier, 2002; Dietz & Edwards, 2006). Therefore, the most successful invaders are often species with high specific leaf area (SLA), high relative growth rates and high nutrient turnover (Smith & Knapp, 2001; Hamilton et al., 2005; Leishman et al., 2007); these traits are characteristic of a competitive–ruderal strategy in the sense of Grime (2001). Invaders therefore tend to have an advantage over native species under high nutrient supply (Kolb et al., 2002; Rickey & Anderson, 2004; Blumenthal & Hufbauer, 2007), whereas nutrient-poor conditions do not generally favour plant invasions.
Phenotypic plasticity in functional traits can allow invasive plants to benefit from changing environmental conditions, either by greatly increasing their performance under resource-rich conditions or by maintaining their performance under resource-poor conditions (Richards et al., 2006). Some studies found invasive species to be particularly successfully in acquiring and using nutrients under nutrient-poor conditions (Funk & Vitousek, 2007; Muth & Pigliucci, 2007). More often, however, invasive species showed a more plastic response to nutrient enrichment than native species (Milberg, 1999). A possible reason is higher root plasticity, which allows for a more rapid capture of nutrients in soil (Callaway et al., 2003).
The effects of invasive plants on the soil are also variable. Invasive species have often been found to increase soil nutrient availability (Allison & Vitousek, 2004; Vanderhoeven et al., 2005; Dassonville et al., 2007; Rodgers et al., 2008), to exert allelopathic effects (Callaway et al., 2005; Jarchow & Cook, 2009) or to alter the soil microbial community (Belnap et al., 2005), but none of these effects is ubiquitous. Effects on the soil are most likely to occur if invasive plants are functionally different from the native vegetation, especially in terms of litter production, root properties and secondary chemistry (Mack et al., 2001; Levine et al., 2003; Moles et al., 2008).
The role of differences in functional traits for the success of invasive plants and their impacts on ecosystems is partly confounded by the fact that exotic plant invaders often belong to a different functional type than the native vegetation. For example, in Central Europe, the most successful herbaceous plant invaders are annual or perennial forbs (Pyšek et al., 2009), whereas the native herbaceous vegetation is often dominated by perennial graminoids. This implies that functional differences between dominant native and invasive species may just reflect their belonging to different functional types. For example, even though grasses are often highly competitive under nutrient-rich conditions, they generally have tougher leaves than forbs, with a longer life span and lower litter decomposition rates (Cornelissen & Thompson, 1997; Grime et al., 1997; Craine et al., 1999; Dorrepaal et al., 2005). These inherent functional differences must be taken into account in the evaluation of invasion effects. The comparison of exotic forbs with native forbs might underestimate the potential impact of invaders on ecosystems that are naturally dominated by graminoids, whereas the comparison of exotic forbs with dominant native graminoids might overstate the relevance of their functional differences for competitive interactions. It thus seems important to compare plant invaders with native species of both groups to properly assess how different they are.
In this study, we compared major invasive forbs in Switzerland with potentially dominant native forbs and graminoids for functional traits, plasticity and effects on soil under controlled conditions. We cultivated six plants from each group in pots at low and high nutrient supply, either alone or in competition. With these 18 species, we tested the following hypotheses which might contribute to explaining the success of the invaders.
Invasive forbs differ from native species in having functional traits that are more typical of a competitive–ruderal strategy, which allow more efficient resource acquisition or use; they differ more from native graminoids than from native forbs in this respect. Invasive forbs are more plastic than native species in response to variation in nutrient supply (i.e. their biomass production and their competitive ability increase more with increasing nutrient supply); they differ less from dominant native graminoids than from (mostly not dominant) native forbs in this respect. Invasive species increase soil nutrient availability but inhibit the growth of native species through allelopathic effects; they differ more from native graminoids than from native forbs in this respect.