Suppression of established invasivePhragmites australisleads to secondary invasion

Abstract

Invasive Phragmites australis (European Common Reed) is rapidly spreading throughout North American wetlands, with negative impacts on wildlife and native plants. The removal or suppression of P. australis is desired to provide an opportunity for native vegetation and wetland fauna to recover. In Ontario, managers applied a glyphosate-based herbicide to >400 ha of P. australis in ecologically significant Great Lakes coastal marshes, representing the first time this tool was used over standing water to suppress P. australis in Canada. Using a replicated Before-After-Control-Impact monitoring design, we 1) evaluated the efficacy of glyphosate-based herbicide at suppression P. australis along a water depth gradient and 2) assessed the recovery of the vegetation community for two years after treatment in relation to local reference conditions. We found that herbicide reduced live P. australis stem densities by over 99% the first year after treatment and about 95% the second year post-treatment. Treatment was equally effective along the entire water depth gradient (10 – 48 cm). The initial ‘suppression’ focused management was successful, but sustained monitoring and ‘containment’ focused follow-up treatment will be required to prevent reinvasion. Two years after treatment, the vegetation community does not resemble reference conditions. Although some treated plots initially increased in similarity to the vegetation communities typical of reference areas, many plots where P. australis was suppressed are on a novel trajectory comprising a vegetation community dominated by non-native Hydrocharis morsus-ranae. Secondary invasions represent a major challenge to effective recovery of native vegetation after P. australis control.

Wetland Restoration inTypha-Dominated Braddock Bay of Lake Ontario

“…Hydrocharis morsus-ranae is an undesirable invasive species on the New York State DEC 2014 Prohibited and Regulated Invasive Plants list (NYSDEC 2014), and it has spread rapidly within the disturbed restoration areas at Braddock Bay. Robichaud and Rooney (2020) found H. morsus-ranae as a secondary invader when stands of P. australis that had been suppressed by herbicide treatment.…”