Suppression of reed canarygrass by assisted succession: a sixteen-year restoration experiment.
Abstract
Assisted succession could enable long-term restoration where successional trajectories stall due to competition from invasive plants. Many invasives are shade-intolerant; therefore, interventions reducing light availability should suppress invasion and re-establish successional processes. However, given how ubiquitous nonlinearities are in ecology, restoration success also depends on identifying critical system thresholds, for example invader abundances below which regeneration of desired species is possible. We report the successful use of assisted succession to restore a swamp forest invaded by Phalaris arundinacea (reed canarygrass; hereafter Phalaris), initiated by a high-density planting of woody species to outcompete Phalaris by reducing light availability. We established five pre-planting treatments in a Phalaris near-monoculture in Wisconsin, USA: herbicide-only, herbicide+plough, herbicide+burn, mow+herbicide and control. In 2003 we planted 22 tree and shrub species at high densities, then in 2019 we censused the site to: (1) screen for long-term differences among treatments, (2) evaluate long-term effects of our interventions on community composition and (3) characterize the critical thresholds that enable invader suppression and restoration success. Vegetation responses and light availability across our four pre-planting invader removal treatments did not differ. Late fall glyphosate application suppressed Phalaris long enough that a dense canopy of native woody species could establish and eventually out-shade it. Overstory tree and shrub densities of 0.071/m2 suppressed Phalaris to 50% cover, but, due to nonlinearities, much higher densities were needed to reduce light availability and thus Phalaris cover enough to shift the system from being invader-dominated. Compositional similarities between juvenile woody species and the overstory suggest a long-term restoration success. Synthesis and applications. Invasive species management and the restoration of target plant communities can be aided by assisting successional trajectories that have stalled. We document a restoration strategy for forests invaded by shade-intolerant invaders that is both effective and economical, as only a simple site preparation and single planting effort is required. Establishing a dense canopy of woody species in this way can break the feedbacks maintaining invader dominance and re-introduce feedbacks enabling long-term ecosystem recovery. We also illustrate the value of identifying critical thresholds influencing the abundance and impact of key invasive species.