Comparative studies have revealed that invasive alien plants tend to have a greater specific leaf area (SLA) and higher nutrient concentrations in biomass and litter than the native species they displace, or than co-occurring, non-invasive alien plants (Daehler 2003; Ehrenfeld 2003; Lake & Leishman 2004). High SLA and nutrient-rich biomass are characteristic traits of plant species with a ruderal–competitive growth strategy: species that grow quickly, propagate effectively, and tend to dominate in interspecific competition (Grime 2001). It is therefore plausible that such traits contribute to making plant species invasive (Thompson, Hodgson & Rich 1995), especially in nutrient-enriched habitats (Daehler 2003; Lake & Leishman 2004). The SLA and the nutrient concentrations in biomass and litter also play a role for a species’ influence on nutrient cycling, as these traits correlate with the rate of litter decomposition and nutrient mineralization from the litter (Cornelissen & Thompson 1997; Wardle et al. 1998). Accordingly, invasive alien plants often accelerate nutrient cycling, increase nutrient availability and modify soil microbial activity at the invaded sites (Kourtev, Ehrenfeld & Häggblom 2002; Ehrenfeld 2003; Allison & Vitousek 2004).
An important question is why plant invaders have these traits: are they inherent properties of the species, or new properties that have developed in the introduced range? In many cases, contrasting functional traits of plant invaders obviously reflect inherent properties of the species. This is particularly the case when native and alien species represent different growth forms, for example when herbaceous plants invade woody vegetation (Mack & D’Antonio 2003), or when annual grasses invade perennial grasslands (Ogle, Ojima & Reiners 2004). However, there might also be differences between native and introduced populations of the same species, especially if environmental conditions place different selection pressures on functional traits in the two regions. It is commonly observed that introduced populations of a given species are more vigorous and competitive than native populations of the same species (Bossdorf et al. 2005). Functional plant traits such as SLA and nutrient concentrations may also differ between native and introduced populations. It is well known that genotypes of the same species can differ considerably in these traits as adaptive responses to variation in climatic and edaphic factors (Ryser & Aeschlimann 1999; Treseder & Vitousek 2001; Oleksyn et al. 2003). Likewise, more favourable climatic conditions, more fertile soils, or a reduced need for defence against herbivores and pathogens in the introduced range of an invasive plant species might select for genotypes with greater SLA and higher nutrient concentrations (Blossey & Nötzold 1995; Rogers & Siemann 2004). To date, this hypothesis has hardly been tested, as research has focused more on antiherbivore defence than on nutrient-related traits (Willis, Thomas & Lawton 1999; Rogers & Siemann 2004).
Solidago gigantea Aiton (Asteraceae) is a perennial forb native to North America, which has recently invaded a wide range of habitats (roadsides, forest edges, old fields, grasslands, wetlands and riversides) in central Europe (Weber & Jakobs 2005). The species occupies a wide geographical range (Weber 2001), and at least part of the phenotypic variation observed within this range is known to have a genetic basis (Weber & Schmid 1998). A field survey in the native and introduced ranges has shown that the performance of S. gigantea populations (in terms of shoot density, shoot size, reproduction and clonal growth) is on average greater in the introduced range (Jakobs, Weber & Edwards 2004). Common-garden experiments in Europe suggested that differences in average performance between European and American populations have a genetic basis, as they persisted when plants were grown in the same environment (Jakobs 2005). Furthermore, a common-garden experiment in the USA indicated that European plants are more vulnerable to pathogens and herbivores, probably due to reduced defence mechanisms (Meyer, Clare & Weber 2005).
The purpose of this study was to test whether native and introduced populations of S. gigantea also differ in functional plant traits that typically characterize invasive plant species: biomass production, SLA, nutrient concentrations, litter decomposability and nutrient mineralization from the litter. We hypothesized that measures of all these properties would be higher in European (introduced) than in American (native) populations. To test our hypothesis, we compared plant traits between 20 European and 22 American populations in a common-garden experiment.