Investigating phenotypic plasticity in biological invasions and implications for the invasive success of tetraploid Solidago gigantea (Giant Goldenrod, Asteraceae)

Angela M. Walczyk and Erika I. Hersch-Green, Department of Biological Sciences, Michigan Technological University, Houghton, MI

Background/Question/Methods:
Biological invasions are a global ecological and socio-economical threat. It has been hypothesized that relative to native species, invasive species exhibit increased phenotypic plasticity (PP) and tolerances in response to a wider range of environmental conditions; increased PP is thought to contribute to invader success. However, empirical support that invasive species/populations exhibit increased PP relative to related non-invasive species/populations or that increased PP confers fitness advantages that could be selectively favored are rare. In this study we examined whether invasive forms have higher PP relative to non-invasive forms, whether PP is associated with increased fitness, and whether pre-adaption or post-introduction selection influences PP of traits associated with biological invasions. To address these questions, we grew native and invasive genotypes of tetraploid Solidago gigantea in a controlled field experiment with low, medium, and high soil nitrogen and phosphorus availability and measured five growth traits (above- and belowground biomass, root/shoot ratio, height, clonal shoot production), three physiological traits (photosynthetic capacity, transpiration rate, water use efficiency), and two defensive traits (foliar terpene concentrations, herbivore damage). We chose to vary these nutrients because invasive species tend to colonize in nitrogen and phosphorus altered areas.
Results/Conclusions:
Both native and invasive genotypes experienced gains in biomass, height, clonal shoots, and photosynthetic capacity as nutrients increased. However contrary to our expectations, native genotypes displayed more PP to the nutrient environment in terms of above- and belowground biomass accumulation and height and also had greater mean values for these traits than invasive genotypes. Both invasive and native genotypes displayed similar PP responses and mean values for clonal shoot production and root/shoot ratios. PP for physiological traits was similar between genotypes, but invasive genotypes displayed greater photosynthetic capacity and transpiration rates but less water use efficiency. Analyses of defensive traits are pending. These preliminary results do not support our original hypotheses and suggest that PP in invasive tetraploid populations of S. gigantea might have been reduced by genetic drift and/or by selection for specific trait values that are better adapted to invasive habitats. This study is one of the few controlling for the potential effects that polyploidy might have on PP by comparing only native and invasive populations of the same ploidy level. Additional studies in this system are needed to explicitly test the influence of polyploidy on PP and to better understand why only tetraploid S. gigantea became invasive.