Suppression of reed canarygrass by assisted succession: A sixteen-year restoration experiment

Assisted succession could enable long-term restoration of invaded areas where successional trajectories have stalled due to competition from invasive species. Many invasives are shade-intolerant, therefore interventions that reduce light availability should suppress invasion and simultaneously re-establish successional processes. However, restoration success also depends on identifying critical system thresholds, e.g., 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 the invader by reducing light availability. We established five pre-planting treatments in a Phalaris near-monoculture in Wisconsin, USA: herbicide-only, herbicide+plow, herbicide+burn, herbicide+mow, and control. In 2003 we planted 23 tree and shrub species at high densities, then in 2019 we censused the site to: (1) evaluate the effect of our interventions on community composition, (2) document trends in community change over time, and (3) determine light availability thresholds that influence community composition. We found no differences among pre-planting invader removal treatments. Late fall glyphosate application suppressed Phalaris long enough that a dense canopy of native woody species could establish and eventually out-shade it. Overstory 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. Regeneration of the woody species we had initially planted suggests long-term restoration success. Synthesis and applications. An empirical understanding of long-term community dynamics can help manage invasive species and restore target plant communities. We show a cost-effective restoration strategy for forests invaded by shade-intolerant invaders that arrest succession. Our data indicate that establishing a dense canopy of woody species through assisted succession can 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.