The prospective rise in atmospheric CO2 and temperature may change the distribution and invasive potential of a species; and intraspecific invasive lineages may respond differently to climate change. In this study, we simulated a future climate scenario with simultaneously elevated atmospheric CO2 and temperature, and investigated its interaction with soil salinity, to assess the effects of global change on the ecophysiology of two competing haplotypes of the wetland grass Phragmites australis, that are invasive in the coastal marshes of North America. The two haplotypes with the phenotypes ‘EU-type’ (Eurasian haplotype) and ‘Delta-type’ (Mediterranean haplotype), were grown at 0‰ and 20‰ soil salinity, and at ambient or elevated climatic conditions (700 ppm CO2, +5 °C) in a phytotron system. The aboveground growth of both phenotypes was highest at the elevated climatic conditions. Growth at 20‰ salinity resulted in declined aboveground growth, lower transpiration rates (E), stomata conductance (gs), specific leaf area, photosynthetic pigment concentrations, and a reduced photosynthetic performance. The negative effects of salinity were, however, significantly less severe at elevated CO2 and temperature than at the ambient climatic conditions. The Delta-type P. australis had higher shoot elongation rates than the EU-type P. australis, particularly at high salinity. The Delta-type also had higher maximum light-saturated rates of photosynthesis (Asat), maximum carboxylation rates of Rubisco (Vcmax), maximum electron transport rates (Jmax), triose phosphate utilization rates (Tp), stomata conductance (gs), as well as higher Rubisco carboxylation-limited, RuBP regeneration-limited and Tp-regeneration limited CO2 assimilation rates than the EU-type under all growth conditions. Our results suggest that the EU-type will not become dominant over the Delta-type, since the Delta-type has superior ecophysiological traits. However, the projected rise in atmospheric CO2 and temperature will alleviate the effects of salinity on both phenotypes and facilitate their expansion into more saline areas.

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The common reed (Phragmites australis) is a clonal wetland grass with high genetic variability. Clone-specific differences are reflected in morphological and physiological traits, and hence in the ability to cope with environmental stress. The responses to progressively increasing salinity of fifteen distinct Phragmites australis clones reveal genotype-related strategies of salt avoidance and exclusion. The salinity-induced inhibition in shoot elongation rate and photosynthesis varies widely between clones. The differences can be partially attributed to their geographic range, but not correlated to ploidy level. Thus, the genetic background is a major factor influencing the salinity tolerance of distinct Phragmites australis clones.

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Compared with non-invasive species, invasive plant species may benefit from certain advantageous traits, for example, higher photosynthesis capacity and resource/energy-use efficiency. These traits can be preadapted prior to introduction, but can also be acquired through evolution following introduction to the new range. Disentangling the origins of these advantageous traits is a fundamental and emerging question in invasion ecology. We conducted a multiple comparative experiment under identical environmental condition with the invasive haplotype M lineage of the wetland grass Phragmites australis and compared the ecophysiological traits of this invasive haplotype M in North America with those of the European ancestor and the conspecific North American native haplotype E lineage, P. australis ssp. americanus. The invasive haplotype M differed significantly from the native North American conspecific haplotype E in several ecophysiological and morphological traits, and the European haplotype M had a more efficient photosynthetic apparatus than the native North American P. australis ssp. americanus. Within the haplotype M lineage, the introduced North American P. australis exhibited different biomass allocation patterns and resource/energy-use strategies compared to its European ancestor group. A discriminant analysis of principal components separated the haplotype M and the haplotype E lineages completely along the first canonical axis, highly related to photosynthetic gas-exchange parameters, photosynthetic energy-use efficiency and payback time. The second canonical axis, highly related to photosynthetic nitrogen use efficiency and construction costs, significantly separated the introduced P. australis in North America from its European ancestor. Synthesis. We conclude that the European P. australis lineage was preadapted to be invasive prior to its introduction, and that the invasion in North America is further stimulated by rapid post-introduction evolution in several advantageous traits. The multicomparison approach used in this study could be an effective approach for distinguishing preadaptation and post-introduction evolution of invasive species. Further research is needed to link the observed changes in invasive traits to the genetic variation and the interaction with the environment.