Growth, reproduction and functional trait responses of three freshwater plant species to elevated carbon dioxide.
Rising atmospheric CO2 concentration is one of the most important ways human activity is contributing to global change. It has led to climate-related changes that affect physiological performance, phenology, and distributions of species globally. Elevated [CO2] enhances growth in many plant species through increased photosynthetic rates, improved water and nitrogen use efficiency, and altered functional traits. In this study, we investigated the growth, reproduction and functional trait responses to elevated [CO2] of two free-floating ferns, the native Azolla filiculoides (Azollaceae) and the exotic invasive Salvinia molesta (Salviniaceae), and a submerged native angiosperm, Vallisneria spiralis (Hydrocharitaceae). The species were grown in monocultures under ambient (~400 ppm) and elevated (~600 ppm) [CO2] in a greenhouse experiment. We found that A. filiculoides had enhanced relative growth rate and reproduction under elevated [CO2]. However, its root to shoot ratio was unaffected while its specific leaf area was reduced under elevated [CO2]. In contrast, S. molesta and V. spiralis did not differ in their growth, reproduction or functional trait responses between the CO2 treatments. We conclude that freshwater plant responses to elevated [CO2] may vary across species, with functional differences potentially driving differential responses as exemplified by the lack of response of the CAM physiology species Vallisneria spiralis and the strong response of the nitrogen-fixing A. filiculoides compared to the non nitrogen-fixing species in this study. Future work should focus on the impact of elevated CO2 on plant growth in the context of environmental conditions such as pH, temperature and light availability in freshwater systems.