Herbivory, mowing, and herbicides differently affect production and nutrient allocation of Alternanthera philoxeroides

This glasshouse study examined the effect of three damage types on plant growth and nutrient allocation of the invasive aquatic plant, alligator weed (Alternanthera philoxeroides). The damage included: repeated leaf removal, a single application of herbicide, and one-time shoot removal. Damage types were meant to simulate the effects of insect herbivory, chemical, and mowing/grazing, respectively. Response variables included plant biomass and both the concentration and abundance of nutrients. Complete shoot removal and herbicide treatments caused an initial decline in growth rate, followed by several weeks of increasing rates and finally a second decline during the fourth week. Plants from control and repeated leaf removal treatments showed a steady increase in growth rate from the treatment application to the final harvest, but control plants were accumulating biomass three times faster than repeated defoliation plants by the fifth week (9.7 and 3.5 g week^-1, respectively). Not surprisingly, all treatments led to lower total cumulative biomass 5 weeks after treatment application (mean 30.8 g) when compared with controls (49.0 g). However, despite the repeated leaf removal and complete shoot removal treatments removing similar quantities of biomass (mean 8.0 and 7.5 g respectively), repeated removal of leaves produced less total biomass (26.2 g) and led to less cumulative above ground biomass (20.1 g) than the other treatments (mean total = 33.1 g, mean above ground = 25.7 g). Repeated leaf removal also produced less below ground biomass (6.1 g) than the shoot removal treatment (8.5 g) and had the greatest negative effect on nitrogen and potassium abundance in plant tissues after 5 weeks. In addition, it reduced the amount of phosphorous to a lower level than herbicide treated or control plants. These results indicate that repeated leaf removal was the treatment most effective in reducing biomass and depleting nutrients in A. philoxeroides plants.