Invasive plant species are prevalent in southern Wisconsin. Two common non-native invasive shrubs are buckthorn (Rhamnus spp.) and honeysuckle (Lonicera spp.). Landowners and managers allocate a lot of time and money to controlling these shrubs, which can spread quickly and create dense thickets that block sunlight from reaching the ground. In turn, native shrubs, grasses, and wildflowers are often not able to survive.
Common techniques for controlling invasive shrubs include hand-clearing and forestry mowing, often followed by herbicide or prescribed fire treatments, or both. While hand-clearing and forestry mowing both remove mature shrubs, the ground layer conditions after these treatment – such as woody debris left by the forestry mower, and possibly soil compaction – could affect the re-establishment of native and non-native understory species. In 2021, researchers Tim Kuhman (Edgewood College), Brad Herrick (Arboretum), and Christy Lowney (Arboretum), along with students from Edgewood College, designed a study to assess the effectiveness of different management techniques for controlling woody invasive shrubs, as well as the success of native herbaceous species at re-establishing following each management technique.
Five sites with high densities of invasive shrubs were selected in Lost City Forest. At each site, one 20-by-20-meter control plot and four 20-by-20-meter treatment plots were established, for a total of five 20-by-20-meter plots per site (n=25). Treatment plots were randomly assigned a management treatment of 1) forestry mowing, 2) forestry mowing followed by prescribed fire, 3) hand-clearing, or 4) hand-clearing followed by prescribed fire. Within each 20-by-20-meter plot, a grid of 25 one-square-meter vegetation sampling quadrats was established (n=625).
In summer 2021, pre-treatment data on the trees, shrubs, understory vegetation, leaf litter, woody debris, and light availability were collected in each plot. During the fall of 2021, treatment plots were hand-cleared by volunteers during Saturday morning restoration work parties led by volunteer restoration team leaders and by Arboretum land care staff. Forestry mowing was done by land care staff.
In spring 2022, measurements of woody debris will be collected and the prescribed fire treatments will be completed. Follow-up data collection will be completed in summer 2022 to assess changes in vegetation, leaf litter, woody debris, and light availability following each management technique, including the no management control plots. Findings from this study will inform future management and restoration of oak savannas at the Arboretum and throughout the Midwest.
—Christy Lowney, Arboretum research specialist, and Tim Kuhman, Edgewood College associate professor