COS 287-6 - Suppression of Reed Canary Grass by Assisted Succession: a 16- Year Restoration Experiment.

Abstract: Assisted succession, an intentional effort of revegetation by seeding and planting, can accomplish restoration as a long-term goal of remediating an invaded area. Phalaris arundinacea is an aggressive grass invader of North American wetlands as it is able to shift a dense vegetation to a complete monospecific stand. Phalaris is a shade intolerant species, therefore restoration efforts that include changes in light availability could potentially suppress the invasion. We report a successful long-term restoration strategy for a swamp forest invaded by Phalaris through the assisted succession of woody plants that outcompete the grass by limiting light availability. We established five pre-planting treatments in a Phalaris monoculture in Wisconsin, USA: herbicide+plowing, herbicide+burn, herbicide+mowing, herbicide-only, and control. In 2003, we planted 23 species of native trees and shrubs across all treatments. Sixteen-years later, we estimated the total amount of light availability above the herbaceous level using hemispherical photos and censused the planted area to: (1) evaluate the effect of our interventions on community composition, (2) understand trends in community composition over time (successional dynamics), and (3) determine light availability thresholds that influence community composition.



We found no differences among the pre-planting Phalaris removal treatments. The application of glyphosate in late fall was sufficient to suppress Phalaris density in combination with the establishment of native woody species. The highest density tree species in the overstory were Ash, Tamarack and Aspen. The highest density shrub species were Dogwood, Elderberry, Nannyberry and Slender willow. Overall, native woody species were able to generate a dense canopy that reduced light availability, which favored the suppression of Phalaris and enabled regeneration of canopy species. The target of light availability required to achieve 50% reduction of Phalaris cover was 35.29±3.4% light (estimate ± S.E.). Stem densities of 0.07 per m² in the overstory and 0.19 per m² in the sapling layer were capable of meeting the goal of 50% Phalaris suppression. Phalaris density was regulated by the reduction of light availability via the increased density, richness and diversity of planted woody species, which was enabled by reducing Phalaris competition prior to planting with a single herbicide treatment. We show a long-term and a cost-effective restoration strategy in a swamp forest invaded by a shade intolerant invasive grass. Our data suggest that the establishment of woody species through assisted succession provides a dense canopy that contributes positively to the recovery of ecosystem feedbacks.