An evaluation of sedge meadow restoration strategies intended to limit Phalaris arundinacea reinvasion by altering light and nitrogen availability

Invasive species removals often have undesired effects on ecosystems. For example, eradicating Phalaris arundinacea from invaded sedge meadows creates conditions that favor reinvasion over native species establishment. Preventing Phalaris reinvasions is necessary to restore native vegetation to these ecosystems. To determine if reducing light, by sowing native cover crops, or nitrogen, by incorporating soil sawdust amendments, will prevent Phalaris reinvasion, we conducted a factorial study in 2 experimental wetlands with controlled hydrology. First year seedling establishment and growth, and second year biomass were measured on 10 native perennial species and Phalaris seeded under 3 levels of cover crops (5-species, 1-species or absent) in soils with or without sawdust amendments. Phalaris and perennial seedling establishment decreased by 89% and 57%, respectively, under light-limiting canopies of the 5-species cover crop. In plots with sawdust, Phalaris seedling establishment decreased by 59% despite sawdust’s short-lived effects on nitrogen depletion. Surprisingly, competition from perennial species reduced Phalaris seedling establishment as much as cover crops regardless of sawdust treatment. The only benefit of incorporating sawdust into plots where perennial species were sown was decreased Phalaris growth. Data of second year biomass and first year seedling establishment and growth revealed similar trends, suggesting that competitive outcomes were determined after one growing season. Our results showed that reducing light by sowing cover crops is counterproductive to restoration goals because cover crops prevent native species establishment. In addition, establishing perennial species quickly is more important and practical than reducing nitrogen in preventing Phalaris reinvasions in sedge meadow restorations.