Invasive Phragmites australis management outcomes and native plant recovery are context dependent

Abstract

The outcomes of invasive plant removal efforts are influenced by management decisions, but are also contingent on the uncontrolled spatial and temporal context of management areas. Phragmites australis is an aggressive invader that is intensively managed in wetlands across North America. Treatment options have been understudied, and the ecological contingencies of management outcomes are poorly understood. We implemented a 5‐year, multi‐site experiment to evaluate six Phragmites management treatments that varied timing (summer or fall) and types of herbicide (glyphosate or imazapyr) along with mowing, plus a nonherbicide solarization treatment. We evaluated treatments for their influence on Phragmites and native plant cover and Phragmites inflorescence production. We assessed plant community trajectories and outcomes in the context of environmental factors. The summer mow, fall glyphosate spray treatment resulted in low Phragmites cover, high inflorescence reduction, and provided the best conditions for native plant recruitment. However, returning plant communities did not resemble reference sites, which were dominated by ecologically important perennial graminoids. Native plant recovery following initial Phragmites treatments was likely limited by the dense litter that resulted from mowing. After 5 years, Phragmites mortality and native plant recovery were highly variable across sites as driven by hydrology. Plots with higher soil moisture had greater reduction in Phragmites cover and more robust recruitment of natives compared with low moisture plots. This moisture effect may limit management options in semiarid regions vulnerable to water scarcity. We demonstrate the importance of replicating invasive species management experiments across sites so the contingencies of successes and failures can be better understood.

Do common assumptions about the wetland seed bank following invasive plant removal hold true? Divergent outcomes following multi‐year Phragmites australis management

“…We hypothesized that imazapyr would negatively influence seedling emergence, due to its longer residence time in the soil (Tu et al, 2001). However, both seed bank and above-ground vegetation communities were similar in imazapyr-and glyphosate-treated plots (this study; Rohal et al, 2019b); perhaps imazapyr did not limit emergence from the seed bank because soil moisture reduced residence time (Wang et al, 2006) or soil concentrations were below seed mortality thresholds (Morash & Freedman, 1989).…”

Suppression of established invasivePhragmites australisleads to secondary invasion

“…The glyphosate-based herbicide was equally as effective across the water depth gradient along which dense P. australis naturally occurred (10 -48 cm). In semiarid regions, drier sites may result in less successful herbicide-based P. australis suppression, as water stress limits the translocation of the herbicide in the plant (Rohal et al 2019a). Our results suggest that, in an area with adequate moisture for P. australis, the actual water depth does not affect how well the herbicide works as water depth does not inhibit adsorption by plant leaves and translocation into rhizomes.…”

We Can Better Manage Ecosystems by Connecting Solutions to Constraints: Learning from Wetland Plant Invasions

“…Even research that evaluates control techniques focuses primarily on ecological responses and rarely considers human aspects to decision making (Kettenring and Adams, 2011;Hulme, 2014;Matzek et al, 2015). For instance, although late summer or early fall is optimal timing for systemic herbicide application efficacy to control many common wetland invaders (Adams and Galatowitsch, 2006;Rohal et al, 2019a;Bansal et al, 2019), treatment application timing is often governed by seasonal labor availability, which peaks earlier in the summer. Studies are needed that evaluate a range of treatment and management options to allow practitioners to weigh trade-offs in effectiveness vs. other constraints (e.g., Jardine and Sanchirico, 2018;Matthews et al, 2020).…”

“…Training deficiencies among managers may also limit effective use of information provided by researchers (Walsh et al, 2019). For example, the recognition of linkages among wetland complexes has led to the conclusion that wetland plant invasions should be managed on a landscape scale (Matthews et al, 2009b;Rohal et al, 2019a). However, landscape-level management plans are rarely implemented (Trammell et al, 2018), possibly reflecting limited training in landscape ecology among practitioners who are hired for field operations skills, rather than other expertise.…”

“…Ultimately, the use of EBIPM Researchers initiated collaborations by surveying practitioners regarding their management goals, strategies, and challenges (Rohal et al, 2018). Relevance of the intended research for practitioners was maximized by incorporating practitioner insights and site-based knowledge and by conducting experiments at longer temporal and larger spatial scales than would have been possible without access to management machinery (Cranney, 2016, Rohal et al, 2019a, Rohal et al, 2019b. Management effectiveness was improved because new knowledge and treatment recommendations were incorporated into ongoing management (with enhanced buy-in from practitioners who had seen which treatments were working firsthand).…”