Abiotic and Landscape Factors Constrain Restoration Outcomes Across Spatial Scales of a Widespread Invasive Plant

Abstract

The natural recolonization of native plant communities following invasive species management is notoriously challenging to predict, since outcomes can be contingent on a variety of factors including management decisions, abiotic factors, and landscape setting. The spatial scale at which the treatment is applied can also impact management outcomes, potentially influencing plant assembly processes and treatment success. Understanding the relative importance of each of these factors for plant community assembly can help managers prioritize patches where specific treatments are likely to be most successful. Here, using effects size analyses, we evaluate plant community responses following four invasive Phragmites australis management treatments (1: fall glyphosate herbicide spray, 2: summer glyphosate herbicide spray, 3: summer imazapyr herbicide spray, 4: untreated control) applied at two patch scales (12,000 m 2 and 1,000 m 2 ) and monitored for 5 years. Using variation partitioning, we then evaluated the independent and shared influence of patch scale, treatment type, abiotic factors, and landscape factors on plant community outcomes following herbicide treatments. We found that Phragmites reinvaded more quickly in large patches, particularly following summer herbicide treatments, while native plant cover and richness increased at a greater magnitude in small patches than large. Patch scale, in combination with abiotic and landscape factors, was the most important driver for most plant responses. Compared with the small plots, large patches commonly had deeper and more prolonged flooding, and were in areas with greater hydrologic disturbance in the landscape, factors associated with reduced native plant recruitment and greater Phragmites cover. Small patches were associated with less flooding and landscape disturbance, and more native plants in the surrounding landscape than large patches, factors which promoted higher native plant conservation values and greater native plant cover and richness. Herbicide type and timing accounted for very little of the variation in native plant recovery, emphasizing the greater importance of patch selection for better management outcomes. To maximize the success of treatment programs, practitioners should first manage Phragmites patches adjacent to native plant species and in areas with minimal hydrologic disturbance.

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

“…Applied scientists increasingly recognize that restoration outcomes are highly influenced by uncontrolled spatial and temporal contingencies in addition to management decisions, though this concept has been infrequently applied to invasive species driven restorations (Grman, Bassett, & Brudvig, 2013; Rohal et al, ). This acknowledgment has led to a call to compare restoration outcomes from similar approaches across sites and to interpret the variability to improve prediction in restoration and inform restoration planning (Brudvig et al, ).…”

“…The ecological contingencies of treatment effectiveness for invasive removal and native plant recovery are poorly understood (Flory, ; Kettenring & Adams, 2011; Rohal, Cranney, & Kettenring, ). Contrary to our expectations, we did not observe a significant influence of nutrients on Phragmites cover.…”

“…These findings suggest drought represents an important restoration threshold, a point at which the likely outcome of restoration does not justify the costs. Herbicide‐based restoration is thus likely to be constrained in arid wetlands where water scarcity is an ever‐increasing issue (Meyerson et al, ; Rohal et al, ).…”

“…The ecological contingencies of treatment effectiveness for invasive removal and native plant recovery are poorly understood (Flory, 2010;Kettenring & Adams, 2011;Rohal, Cranney, & Kettenring, 2019 is an ever-increasing issue (Meyerson et al, 2010;Rohal et al, 2019).…”

Suppression of established invasivePhragmites australisleads to secondary invasion

“…With the current distribution and level of establishment of P. australis in North America, most control projects must focus on suppression, containment, and asset protection rather than aim for complete eradication. As the size of invaded area increases, it becomes increasingly difficult to achieve eradication (Quirion et al 2017) or to recover native vegetation communities (Rohal et al 2019b). The speed at which P. australis begins to re-colonize a treated area means that longterm monitoring and follow-up treatment are essential components of any control project.…”