Shading may alter the colonization pattern and dominance between two invasive submerged aquatic plant species

Egeria densa and Elodea canadensis are two common invasive submerged macrophytes in streams and rivers worldwide. We conducted a six-week growth experiment with monocultures of E. densa and E. canadensis and mixed communities of the two species under five different shade levels (35%, 63%, 79%, 90% and 95%). Our aim was to test the effect of shade and competition on colonization of vegetative propagules of these species in flowing waters. We found that biomass accrual in both species was greatly reduced at high shade (≥ 79%) and that E. densa individuals accrued more biomass than E. canadensis individuals at all shade levels. At the least shade level (35%), interspecific competition with E. densa stimulated biomass accrual of E. canadensis individuals, compared to those within an E. canadensis monoculture. At higher shade (≥ 63%), this facilitation response disappeared, although E. canadensis plants continued to have longer stem length and a greater degree of branching than E. densa plants at ≤ 79% shade. At the highest shade level, competition with E. canadensis reduced biomass accrual of E. densa . Hence, shade influenced the competitive interactions between these two species. At low shade, the competitiveness of E. canadensis was enhanced, but in the field E. densa appears to be the dominant species in streams, especially those with higher levels of shade and less frequent disturbance by flushing flow events.