Responses of three invasive alien aquatic plant species to climate warming and plant density.

Climate warming may impact plant invasion success directly, as well as indirectly through changes among interactions within plant communities. However, the responses of invasive alien aquatic species to plant density and rising temperatures remain largely unknown. We tested the effects of plant density and neighbour plant identity at different temperatures to better understand the performance of a community of invasive species exposed to climate warming. A microcosm experiment was conducted with three invasive aquatic plants species-Elodea canadensis, Egeria densa and Lagarosiphon major-, at mono and polycultures with low and high plant density, at 16 °C, 19 °C and 23 °C. The results clearly demonstrated that rising temperature influenced, either as a single parameter or as a combined factor, at least one of the measured traits of the three invasive species. Leaf area of E. densa, root number of L. major and growth of E. densa and L. major were influenced by temperature, plant density and neighbour identity. Plant density influenced all traits with the exception of leaf area of E. canadensis and lateral branch production of E. densa. Neighbour identity had no effect on growth rate and leaf area of E. canadensis, on lateral branch and roots production of E. densa and on leaf area of L. major. These findings establish that rising temperature could enhance competition or facilitation among E. canadensis, L. major and E. densa and could cancel the beneficial effects of the presence of a neighbour species; however, the magnitude of this effect was strongly dependent on plant density. Rising temperature due to climate change will likely play a crucial role in interactions between invasive species within plant communities and in the further spread of these invasive aquatic plants.