Legacy effects of Phragmites australis and herbicide treatments can reduce survival but do not prevent native plant establishment

Introduced Phragmites australis represents a widespread threat to North American wetlands. Management agencies invest millions of dollars annually to manage the species, mostly by applying herbicides, to mitigate and prevent negative impacts. The often temporary reduction of P. australis rarely increases native diversity, and long-term legacy effects of P. australis and repeat herbicide treatments are unknown. We used a coordinated management program targeting mostly small P. australis populations in the Adirondack Park in New York State, United States, to assess the potential for such legacy effects. We planted individuals of three native species as sentinels into treated and untreated, uninvaded reference wetlands after unassisted succession had occurred in treatment areas. Sentinel plants survived and grew in treated areas, suggesting legacy of P. australis and its management did not permanently limit establishment of native plants. However, responses were variable among sentinel species, with negative or neutral impacts on survival rates and neutral or positive effects on growth. Species-specific responses and large variation in survival rates between sites and sentinels indicate that site factors are a dominant influence on survival and growth. Importantly, as treatment frequency increased, survival of one sentinel species decreased significantly, indicating a potential for long-term negative impacts of repeated herbicide applications. Additional replanting after P. australis treatment and initial unassisted plant succession may enable more diverse plant communities to return, including species not able to recruit from local seed banks or seed sources. However, it remains unclear if active transplanting will enable more long-term suppression of P. australis.