Growth and morphology in relation to temperature and light availability during the establishment of three invasive aquatic plant species

Invasive freshwater plants are currently spreading rapidly and this is likely to continue with further changes in global climate resulting in changes in physical and chemical conditions in freshwaters. We studied the effect of summer temperature (20 °C, 25 °C and 30 °C) and light availability (25% and 50% of incident light availability) on shoot establishment in terms of growth rate, photosynthesis, and morphology of three invasive aquatic plants (Elodea canadensis, Egeria densa and Lagarosiphon major) in order to assess their interspecific competition. Light availability had an overall stronger effect on growth rate and plant morphology than temperature in the three species. Growth rate increased three-fold from low to high light, and low light reduced belowground biomass, increased stem length, and reduced branching and lateral spread. Photosynthetic rates were the only parameter for which temperature had an equal or stronger effect than light availability. The results show that E. canadensis has the most competitive establishment of the three species in both high and low temperature and light availability. E. densa is most competitive in warm water compared to colder water, whereas the opposite pattern is present for L. major which is most competitive in colder water. In conclusion, we suggest that that E. densa will dominate warmer, shallower waters, whereas L. major will dominate in colder, clear-water lakes, while E. canadensis continues its established role as a pioneer species that is quickly replaced by the two taller species after their arrival.