Andrew Kaul is a Restoration Ecologist in the Center for Conservation and Sustainable Development at the Missouri Botanical Garden. Mike Schuster is a Researcher in the Department of Forest Resources at the University of Minnesota.
Removing invasive shrubs is a critical step in ecological restoration projects in many degraded forests and woodlands across the Eastern US. Invasive shrubs quickly spread and outcompete native plants, which leads to declines in plant species diversity and ecosystem functioning. By suppressing these aggressive non-native species, restoration efforts can promote the regeneration of native vegetation that provides habitat and food sources for local wildlife. Removing invasive shrubs also allows more sunlight to reach the forest floor, enabling the regeneration of native tree seedlings and understory plants. Establishing an understory community is necessary to conduct effective management with prescribed burns in fire-adapted systems.
Rhamnus cathartica, also known as “common buckthorn” or “European buckthorn”, is one of the most aggressive invasive species in Eastern North American forests. This buckthorn species is a shrub/small tree originally native to Europe and Western parts of Asia, brought to North America in the 1800s, and planted as a hedge-forming species. Buckthorn biology and control have been studied extensively in recent years due to its significant ecological and economic impacts, particularly in the Midwest US.
Many aspects of its growth and reproduction make buckthorn a successful invader.Buckthorn is dioecious (male and female flowers occur on separate individuals) and femalesproduce copious quantities of small berry-like fruits called drupes (like a cherry) that are widely dispersed by birds. Buckthorn can thrive in many soil and light conditions, being especially tolerant to low-light environments caused by shading from other trees and shrubs. It also has unique phenology, holding its leaves late into fall. Like many invasive shrubs, its dense branching physiognomy leads buckthorn to form thickets that shade out native vegetation, decreasing diversity of plant species in invaded forests. In addition to effects on plants, buckthorn invasion has also been linked to changes in soil chemistry and may increase soil erosion through reducing the cover of understory plants.
Many of the same traits that make buckthorn a good invader also make it very difficult to remove and control when restoring natural areas. It produces extensive root systems that store nutrients, so it can re-sprout vigorously when cut. This means removal requires repeated control efforts over multiple years. The most common control methods used to manage buckthorn include mechanical and chemical treatments. Mechanically, young seedlings can be hand-pulled and saplings can be removed with a weed wrench. More mature plants are often girdled, cut, or mowed to remove most of the biomass, and then application of herbicide is necessary, otherwise, stems will survive and quickly re-sprout within only a few weeks. Herbicide can also be applied to basal bark without cutting, but this method is less effective on large individuals.
Even with persistent effort, complete buckthorn control is rarely successful because it can quickly recolonize areas if not thoroughly removed. Fortunately, there is some good news about the feasibility of buckthorn management. It was previously believed that buckthorn seeds could form persistent “banks” in the soil, with seeds surviving for up to 6 or 7 years, waiting for the right set of conditions to trigger germination. However, our recent study showed that buckthorn seeds actually germinate in one to two years, with 97% germinating in the first year. This is a critical discovery for the management of buckthorn, as it indicates that after removing large individuals, management methods to suppress seedling establishment are key to preventing reinvasion. Moreover, if seedlings are suppressed for the first couple of years, then buckthorn control is possible. Because these fleshy-fruited invaders are so readily dispersed by birds, complete eradication is unlikely, but continued management can keep densities low enough to not impact native plant communities.
Our research group at the University of Minnesota led by Peter Reich has been studying buckthorn for several years, funded by multiple grants from the Minnesota Invasive Terrestrial Plants and Pests Center (MITPPC). Recent work in our group has focused on how to suppress buckthorn regeneration after initial removal of large individuals. The Cover it up! project investigates which methods of revegetation are most effective for restoring the ground-layer with native species that can prevent buckthorn recruitment and growth. Various experimental introductions of native plants have included direct seeding shrubs, direct seeding trees, adding herbaceous seed mixes with variable ratios of grasses to forbs, and treatments with combinations of functional groups. For example, we combined sowing the Standard Cover It Up seed mix of 34 native grasses, and forbs with planting bare-root plants from other functional groups – trees, shrubs, ferns, or sedges.
Across several experiments, one of the most important results has been that the extent to which revegetation treatments are effective in suppressing buckthorn, is mostly explained by their ability to rapidly establish vegetation, preempt space, and shade out buckthorn seedlings. Two of the most successful strategies include 1) planting native tree species and 2) seeding a mix of native Elymus spp. grasses (wild ryes) and wildflowers to establish an herbaceous understory. The native trees used in this experiment included species like Sambucus canadensis (elderberry), Abies balsamea (balsam fir), and Acer saccharum (sugar maple), planted immediately after clearing-out buckthorn. Planting woody native species can reduce buckthorn regrowth by up to 80%, and establishing a grassy herbaceous community can reduce regrowth by 77%.
We recently published A Guide to Forest Understory Revegetation, which makes several science-based recommendations for invasive shrub management based on our research. Revegetation should occur as soon as possible after initial removal of large individuals. Additionally, restoring sites via revegetation will be most effective in areas with at least 10% open canopy so that sufficient light is present for native seedlings to establish. If opening the canopy is feasible by selectively removing some trees, this will facilitate a greater cover and diversity of herbaceous plants in the understory. When conducting revegetation with herbaceous species, planting a native seed mix with a high proportion of cool season (C3) grasses is ideal. These species grow well in shadier environments, establish rapidly, are inexpensive, and produce dense vegetation that can carry a fire to facilitate management with prescribed burns. Shade-adapted wildflowers such as Ageratina altissima (white snakeroot) or Hydrophyllum virginianum (Virginia waterleaf) should be included in the seed mix in order to add value to pollinators and other insects. When conducting revegetation with woody species, planting native tree species can be highly effective in excluding buckthorn, but revegetation through bare-root plantings has its drawbacks, being more expensive and labor intensive. This method would work well if implemented in smaller areas, especially where there are few deer. In general, revegetation plansshould prioritize reducing light reaching the ground where buckthorn seedling are growing.Counterintuitively, this can be achieved by opening the tree canopy to facilitate establishment of herbaceous cover in the understory.
In addition to studying methods of revegetation to suppress buckthorn seedlings, our research group is also investigating novel methods for removal of mature buckthorn plants. In June 2024, we initiated an experiment testing the efficacy of critical period cutting to kill large buckthorn without the use of chemicals. This method was pioneered by Friends of the Mississippi River (FMR) as a strategy for removing buckthorn without using herbicide, which is prohibited in Minneapolis parks. This method involves using a saw or loppers to cut off the top of a buckthorn plant about 1.5 m from the ground early in the growing season. Subsequently, a couple times throughout the growing season, each buckthorn is revisited and stripped (picked off easily by hand) of all re-growth. This process is then repeated the following year, if necessary. Cutting the stem at chest height rather than near the ground has multiple benefits of 1) reducing risk of stumps as a tripping hazard, 2) relocating previously cut stems, and 3) stripping re-sproutsthat usually occur at the end of the stem, which is near chest height.
Given FMR’s success implementing the critical period cutting method, our experiment is examining when this method is most effective in killing buckthorn depending on the size of the individual or its light environment. We are also quantifying the minimal effort required to ensure buckthorn mortality by examining survival of buckthorn over two years of stripping re-sprouts, with 4, 6, or 10 total removals. To test how effective this method is for controlling other invasive shrub species, we are working with collaborators in Wisconsin, Maine, and Missouri, who are also conducing this experiment on the invasive shrubs Frangula alnus (glossy buckthorn) and Lonicera maackii (bush honeysuckle). These are also serious woody invaders of forests throughout the Midwest and our research aims to improve forest restoration outcomes at smaller scales when herbicide or other larger-scale methods are not possible.
If you are interested in learning about these projects in greater depth, you can read the guide to forest understory revegetation here or learn more about ongoing research in the Cover It Up! project here. If you have any questions, feel free to contact Mike (schuster@umn.edu) or Andrew (akaul@mobot.org).