Individual study: Evaluation of a large-scale invasive plant species herbicide control program in the Berkshire Taconic Plateau, Massachusetts, USA
McAlpine L. & Porder S. (2009) Evaluation of a large-scale invasive plant species herbicide control program in the Berkshire Taconic Plateau, Massachusetts, USA. Conservation Evidence, 6, 117-123
In the late 1990's, The Nature Conservancy (TNC) documented a rise in five invasive plant species, barberry Berberis thunbergii, bittersweet Celastrus orbiculatus, garlic mustard Alliaria petiolata, buckthorn Frangula alnus, and honeysuckle Lonicera morrowii on the periphery of the relatively intact and uninvaded 14,600 ha Berkshire Taconic Plateau in Massachusetts (USA). The Plateau comprises an ecologically significant block of forest. In response, TNC began a large-scale herbicide-based control program on approximately 3,600 ha of land with the goal of reducing invasive cover to less than 10%. Our objective was to evaluate the efficacy of this effort, but this was hampered by a dearth of untreated control sites and pretreatment data on invasive species cover. Four sites (three treated, one untreated) on the plateau periphery similar in understory vegetation, overstory cover, slope, and proximity to a hiking trail were surveyed and compared. Across each site, native and invasive plant percent cover within 44, 1m² plots was measured, and native and invasive presence absence recorded on an additional 2,000m² area. All five target invasives were present at all four sites 5-years post-treatment. In two of the treated sites, invasive percent cover significantly exceeded the 10% goal, largely due to the abundance of garlic mustard. Without garlic mustard, all the sites (including the untreated one) had < 10% invasive cover. Surprisingly, the high level of invasive cover did not have a significant negative impact on native cover (native species richness was not quantified), although a hypothesized negative relationship was invoked as justification for the herbicide treatment.
Given the difficulty in finding comparable treated and untreated sites after herbicide application, we suggest 1) that quantitative data on invasive abundance be gathered prior to a control program and 2) that treated and untreated plots be allocated to monitor outcomes. Without this, determination of effectiveness is difficult and likely to be inconclusive.