Glyphosate redirects wetland vegetation trajectory following willow invasion

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AbstractAimsAerially applied glyphosate is an economic tool to deal with large areas of invasive plants. However, there are few studies investigating non‐target effects or rates of reinvasion, particularly over multi‐year time frames. The aims were to evaluate the effectiveness of aerial application of glyphosate for control of dense stands of the invasive grey willow Salix cinerea, and determine the vegetation trajectory over the subsequent 2 yr.LocationWhangamarino Wetland, Waikato, New Zealand.MethodsA before–after control–impact (BACI) experiment was conducted in a Ramsar‐listed wetland in New Zealand. Effects on S. cinerea cover, canopy light interception and non‐target damage were monitored over a 7.1 ha experimental area prior to, and for 2 yr following, aerial application of glyphosate. Vegetation classification, ordination and species richness analyses were undertaken to describe community‐level effects.ResultsAerial application of glyphosate to an established willow canopy was effective in reducing cover to <5% on average for up to 2 yr post‐spray when assessed using 100 m2 vegetation survey plots. Smaller 1 m2 plots were more sensitive for detecting willow reinvasion, which was noted from 1 yr post‐spray. Collateral damage to non‐target sub‐canopy species was generally minimal, except for the native tree fern Dicksonia squarrosa, which showed marked reductions in cover and no recovery over the study period. Species richness was higher in sprayed plots post‐spray and a shift towards a native Carex‐dominated sedgeland community was detected.ConclusionsAerial application of glyphosate to a dense canopy of mature willow was effective in reducing the cover and dominance of this invasive wetland tree species. Minimal collateral damage occurred, facilitating recovery and expansion of a native sedgeland community. The risk of secondary invasion did not eventuate, although exotic species richness spiked in the year following spraying. Sedgelands are susceptible to willow reinvasion via seed, so longer‐term trajectories will diverge depending on management intensity. Using currently available tools, the management options are either repeated cycles of herbicide application to redirect the trajectory from reinvasion to short‐statured sedgelands, or intensive initial management to establish an alternative, more resilient trajectory to native wetland forest.