Invaded Landscapes

This first study was designed to explore the global pattern of impact of seven of the most invasive plant species in NW Europe (Impatiens glandulifera (Balsaminaceae, annual), Heracleum mantegazzianum (Apiaceae, hemicryptophyte), Senecio inaequidens (Asteraceae, chamaephyte), Solidago gigantea (Asteraceae, perennial rhizomatous geophyte), Fallopia japonica (Polygonaceae, perennial rhizomatous geophyte), Rosa rugosa (Rosaceae, woody shrub) and Prunus serotina (Rosaceae, tree)) on productivity and soil chemical properties. To take into account the "site factor", for each species, 5 invaded sites were selected in a wide variety of ecosystems across Belgium. In each site, we compared topsoil (0-10 cm) chemical properties (pH, exchangeable cations and P concentrations and organic C and N concentrations) and biomass production between invaded plots and area with the remaining uninvaded resident vegetation (For technical details, see Dassonville et al., 2008 and Vanderhoeven et al., 2005). Therefore, the impact of an invasive species at one site on one parameter was defined as the difference between the value of this parameter in invaded and uninvaded stands.

Contrary to other studies, we did not find a systematic positive impact of the invasive plants on topsoil carbon and nutrient pools. On the other hand, we found a strong correlation of the magnitude and direction of impact with initial nutrient pool in uninvaded plots. This correlation was significant for exchangeable K, Mg, Mn and P and for organic nitrogen concentration (figure 1). In oligotrophic and mesotrophic habitats, our results were in agreement with earlier reports (Musil 1993; Scott et al. 2001; Duda et al. 2003; Vanderhoeven et al. 2005; Chapuis-Lardy et al. 2006; Liao et al. 2008). The nutrient concentration of topsoil was increased consecutively to the invasion (higher nutrient concentration in the invaded soil compared to the soil under the surrounding resident vegetation). However, the magnitude of impact decreased with initial nutrient pool in topsoil and even became negative (lower concentration in the topsoil of invaded plots compared to uninvaded ones) in the most eutrophic sites. Opposite impacts of alien invasive species in nutrient-poor vs. nutrient-rich sites thus result in convergence of invaded plots towards similar values of topsoil chemical properties. Thus, element concentrations in topsoil varied within narrower limits in invaded (I) plots compared to uninvaded (U) ones, most strikingly so for Mn (44-fold variation among U plots vs. 15-fold in I plots), K (45-fold vs. 22-fold), P (40-fold vs. 22-fold), Mg (35-fold vs. 23-fold), N (32-fold vs. 21-fold), Zn (248-fold vs. 174-fold). This is not such a surprising result considering that invasion results in convergence of plant community composition by the replacement of a wide variety of ecosystems by a few dominant species with common functional traits. This study was however the first evidence of homogenization of soil properties across invaded landscapes.

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Figure 1. Impact of invasive species on soil chemical properties (pH, exchangeable cations and P concentrations and organic C and N concentrations) as a function of soil chemical properties in uninvaded plots (U). Impact is calculated as the difference between the value of the parameter in invaded plots (I) minus the value of the parameter in uninvaded plots (U). Positive values of I-U indicate increased element concentration in the invaded soil. Pearson correlation coefficient (r) between U and I-U. Significance levels (*: p<0.05, **: p<0.01, ***: p<0.001). Log-transformed data for Mg, P, Mn, Zn. ♦ S. gigantea, ■ F. japonica, ▲ H. mantegazzianum , A I. glandulifera, o R. rugosa, ◊ S. inaequidens, □ P. serotina. Figure from Dassonville N., Vanderhoeven S., Vanparys V., Hayez M., Gruber W., Meerts P., 2008. Impacts of alien invasive plants on soil nutrients are correlated with initial site conditions in NW Europe. Oecologia, DOI : 10.1007/s00442-008-1054-6. Copyright SpringerVerlag Heidelberg. Reproduced with permission.

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