Combined effects of shading and clipping on the invasive alien macrophyte Elodea nuttallii

Abstract

The response of macrophytes to herbivory and mowing in freshwater ecosystems is ambiguous. A recent study based on a plant growth model and a meta-analysis of field studies suggested that submerged macrophytes are more susceptible to plant removal by herbivory when additionally shaded by periphyton. Here, we test whether such synergistic effects of shading and herbivory or mowing can be experimentally confirmed. We used Elodea nuttallii, one of the alien invasive species of the highest concern in Europe. It is a preferred species by herbivorous water birds and is subject to mowing as a common management measure. We exposed E. nuttallii to four levels of clipping, with and without additional shading. Our results show a synergistic effect between shading and clipping for final shoot height of E. nuttallii and an additive effect for biomass, as enhanced branching compensated for shoot biomass losses by clipping in shaded treatments. Our results indicate that shading may help to maintain a lower canopy height of E. nuttallii when grazed by water birds or mown, but does not synergistically reduce total plant biomass. We conclude that the response of aquatic macrophytes to combined stress by shading and herbivory or mowing can differ among growth parameters and among species and is important to consider in aquatic plant management.