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 m2 and 1,000 m2) 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.
Highlights
Invasive plants can reduce the abundance and diversity of native plant communities, and can markedly alter ecosystem functions and services, making invasive plant management a restoration priority (Hejda et al, 2009)
In 2015 and 2016, 1 and 2 years after the final herbicide treatments, Phragmites cover was reduced at a greater magnitude in the small patches compared to the large patches across all treatments, but the difference was only significant in the summer glyphosate treatment (2015 QM = 4.85, P = 0.03; 2016 QM = 4.57, P = 0.03; Supplementary Table S2) due to wide confidence intervals, indicating large amounts of site variability
Native perennial cover significantly increased in small-scale patches across all treatments in most years in 2014–2016, and the increase was consistently at a greater magnitude than large patches, though the difference was only significant in the summer glyphosate treatment in 2015 (2015 QM = 3.79, P = 0.05; Supplementary Table S3)
Summary
Invasive plants can reduce the abundance and diversity of native plant communities, and can markedly alter ecosystem functions and services, making invasive plant management a restoration priority (Hejda et al, 2009). Clearly understanding the constraints to restoration success is important in order to prioritize sites for invasive species management that are most likely to have successful outcomes (i.e., effective removal of the invader and native plant recovery), or to plan for the additional efforts needed to overcome thresholds at sites that have a high degree of impairment (Suding, 2011). Local abiotic factors can influence the effectiveness of management tools, the likelihood for native plant recovery, and the competitive dynamics between native and invasive species (Diez et al, 2009). To add a further layer, the spatial scale of a managed patch can have implications for restoration outcomes (Holl and Crone, 2004; Morrison et al, 2010), but this has been relatively unexplored (Brudvig, 2011)
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