To examine whether disturbed communities have characteristics that make them more susceptible to invasion than the non-disturbed survey sites we included stands disturbed by natural blowdown and also timber harvest sites. In the summer of 2011, we decided to add an experimental aspect to four of the survey sites that contained large areas of dense buckthorn infestation (College of St. Benedict, St. Joseph; Afton State Park, Afton; Warner Nature Center, Marine on St. Croix; and Hyland Lake Park Reserve, Bloomington). This allowed us to investigate invasive species removal management as another disturbance ‘treatment’. Invasive removal as a type of disturbance has received little attention, but since it is a management strategy employed by a variety of public and private agencies, the efficacy of the approach deserves attention.
At each of the four selected sites we established grid consisting of 12 six by six meter invasive species removal plots (in a four by three arrangement) with a six-meter buffer around each plot. Within each plot, we established three one-meter radius subplots at 0°, 120°, and 240° from the center of the main plot (2 m from the center). In each of these subplots, we identified and estimated percent cover of all herbaceous species. In addition, we identified, measured the diameter, and counted all the stems of the tree and shrub species. Environmental variables were also recorded for each six-meter square plot (slope, aspect, leaf litter depth, percent bare soil, light levels).
Design of buckthorn removal experiment
Close up of one plot
In each plot, we used three common techniques to remove the invasive biomass (plus a control treatment where nothing was done). Plots were randomly assigned to a different treatment types. The three buckthorn removal treatments were weed-wrenching, basal bark herbicide application, and biomass removal using the cut-and-paint technique. Treatments were applied at each of the four sites during October and early November, 2011 after surveying all vegetation in each plot.
Buckthorn removal techniques: left panel, manual removal; middle panel, basal bark application of herbicide; right panel, cut and paint application of herbicide
In addition to the buckthorn removal study, we initiated a seed addition pilot study to investigate the possibility of using covers crops to reduce buckthorn regeneration after removal. These post-removal strategies to improve native species regeneration have to deal not only with getting the native species back, but also with preventing the germination of buckthorn. A cover crop could provide shade and take up available space, thus preventing germination of buckthorn from the seedbank. However, it is unclear how planting will interact with the variety of buckthorn removal methods. We expected that a high cover of seeded species would prevent the germination of buckthorn relative to control conditions (i.e., no cover crop planted) Also we expected a perennial cover crop to be more successful at resisting buckthorn germination compared to an annual species. Within each six by six meter plot, six 1m squared subplots were set up, three in one randomly determined corner and three in another corner.
Plot planted with wild rye
Two different species were selected for planting: Virginia wild rye (Elymus virginicus), a perennial species and oats (Avena sativa) an annual. Within each set of three plots, the plots were randomly assigned to three cover crop treatments: control (no cover crop), oats, or rye and seeding was carried out six months after buckthorn removal. Each planting plot was surveyed before seed application for percent cover of vegetation, percent bare soil, and number of buckthorn seedlings and resurveyed in August 2012, May 2013, and August 2013.