Nonlinearities in Assisted Succession to Suppress Reed Canarygrass: A 16-Year Restoration Experiment
Upper Midwest Invasive Species Conference. 12-14 November, 2024. Duluth, Minnesota, USA.
Long-term forest restorations often fail where successional trajectories are stalled by competition from invasive plants. Many invasives are shade-intolerant, therefore interventions reducing light availability should suppress invasion and re-establish forest succession. However, given how ubiquitous nonlinearities are in ecology, restoration success also depends on identifying restoration target thresholds, e.g., invader abundances below which regeneration of desired species is possible. We report the successful use of assisted succession through the attainment of critical light thresholds to restore a swamp forest invaded by reed canarygrass (Phalaris arundinacea).
In 2003, we planted 22 tree and shrub species at high densities into a reed canarygrass near-monoculture after establishing five pre-planting treatments (four removal treatments plus controls). 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 critical thresholds that enable invader suppression and restoration success.
Outcomes across our four removal treatments did not differ. Late fall glyphosate (Roundup, 0.7% active ingredient solution applied using a boom sprayer, application rate of ~7L/hectare) suppressed reed canarygrass long enough that a dense canopy of native woody species could establish and eventually out-shade it. Overstory woody densities of 0.071/m2 suppressed reed canarygrass to 50% cover, but, due to nonlinearities, much higher densities were needed to reduce light availability and thus reed canarygrass enough to shift the system from being invader-dominated. Compositional similarities between juvenile woody species and the overstory suggest long-term restoration success.
Our results show that restoration of target forest communities can be aided by assisting successional trajectories that have stalled. Establishing a dense canopy of woody species 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 abundance and impact of key invasive species.
