Effects of submergence and eutrophication on the morphological traits and biomass allocation of the invasive plant Alternanthera philoxeroides

Submergence and eutrophication are increasingly common in riparian zones as a result of climate change and artificial disturbance. This study aimed to test the effects of submergence level and eutrophication degree on a worldwide invasive riparian plant, Alternanthera philoxeroides. A greenhouse experiment was conducted in which the new ramets of A. philoxeroides were treated with three submergence levels (0, 7, and 13 cm) combined with five nutrient concentrations (1000-, 100-, 50-, 10-, and 1-fold dilution of modified Hoagland solution). The results showed that submergence severely inhibited the growth of A. philoxeroides, whereas a great increase in nutrient concentration promoted its growth. Submergence and increasing nutrients both reduced biomass allocation to the roots in A. philoxeroides, which primarily increased investment into stems upon submergence or leaves when not submerged. A. philoxeroides also showed significant phenotypic plasticity, elongating its stems by increasing internode length rather than number to cope with submergence. These results suggest that A. philoxeroides could actively adapt to submergence and mild eutrophication would not promote its growth, while extremely high nutrient concentrations in water would be beneficial for its adaptive growth. Our study contributes to understanding the causes of the invasiveness and wide distribution of A. philoxeroides in disturbed riparian zones where submergence and eutrophication often co-occur.