Phalaris arundinacea seedling establishment: effects of canopy complexity in fen, mesocosm, and restoration experiments

Phalaris arundinacea L. (reed canary grass) is a major invader of wetlands in temperate North America; it creates monotypic stands and displaces native vegetation. In this study, the effect of plant canopies on the establishment of P. arundinacea from seed in a fen, fen-like mesocosms, and a fen restoration site was assessed. In Wingra Fen, canopies that were more resistant to P. arundinacea establishment had more species (eight or nine versus four to six species) and higher cover of Aster firmus. In mesocosms planted with Glyceria striata plus 1, 6, or 15 native species, all canopies closed rapidly and prevented P. arundinacea establishment from seed, regardless of the density of the matrix species or the number of added species. Only after gaps were created in the canopy was P. arundinacea able to establish seedlings; then, the 15-species treatment reduced establishment to 48% of that for single-species canopies. A similar experiment in the restoration site produced less cover of native plants, and P. arundinacea recruited more readily. Results suggest that, where conditions are favorable for native plant growth, even species-poor canopies can inhibit P. arundinacea establishment from seed, but when disturbances create gaps, species-rich canopies confer greater resistance to invasion.Key words: diversity, establishment, fen, invasion resistance, species richness, wetlands.