Wetland plant responses to varying degrees of purple loosestrife removal in southeastern Ontario, Canada

This study examined the response of native vegetation to varying levels of purple loosestrife (Lythrum salicaria) removal and determined its impact on subsequent loosestrife germination and establishment in a southern Ontario wetland during 1993 and 1994. The study site was treated in 1991 with varying dosages of triclopyr amine herbicide, the triethylamine salt formulation of triclopyr [[(3,4,6-trichloro-2-pyridinyl)oxy] acetic acid]; 4.0, 8.0, and 12.0 kg active ingredient (a.i.)/ha. Different levels of adult loosestrife control were achieved. The lowest density of adult loosestrife was observed in the 8.0 and 12.0 kg a.i./ha treatment plots where root kill was most effective. Most new adult loosestrife plants in the 8.0 and 12.0 kg a.i./ha treatment plots established from seed. The highest densities of loosestrife seedlings and grass (Gramineae) species were observed in the treatment plots with the lowest number of adult loosestrife plants (12.0 kg a.i./ha). Sedge species (Carer spp.) did not differ between treatment levels during 1993 and 1994. Loosestrife seedling densities decreased from 1993 to 1994, suggesting that increased native plant species can slow the rate of loosestrife reestablishment from the seed bank. Native vegetation seems to replace adult loosestrife for a limited time following herbicide application; however, without subsequent treatment, loosestrife will slowly reinvade a wetland.