See the paper: Common Buckthorn’s (Rhamnus cathartica) Influence on Minnesota Soil Properties in Contrast to Native Black Cherry (Prunus serotina)

Background

Buckthorn is a type of plant that spread from Europe in the mid-1800s. It was used in gardens and hedges for its aesthetic appeal, but spread quickly and began taking over forests. Because buckthorn is non-native, it has no natural predators to control its population. Birds are able to help buckthorn spread rapidly because they eat the berries and disperse seeds over long distances. 

Buckthorn changes an ecosystem in many ways. It grows to be a medium sized shrub or small tree, and has a dense network of leaves that block light from reaching the forest floor. This prevents many smaller plants from accessing sunlight to photosynthesize, often causing them to die. Buckthorn also keeps its leaves much longer into the fall than other plants, extending the time that it has to grow. Because of this, forests that have a lot of buckthorn are losing species diversity. 

While there is a lot known about forest-wide impacts of buckthorn presence, not much is known about how buckthorn changes soil chemistry and composition. A St. Olaf student in 2016 sought out to investigate this. 

The Study

Kari Smerud (Class of 2017, B.A. Biology) chose twelve sites to study in the Natural Lands. Six sites were adjacent to buckthorn, and six sites were adjacent to black cherry, which is a native plant to Minnesota that occupies a similar niche in the environment as buckthorn. 

From these samples, Smerud looked at soil density, percent organic matter, soil moisture, earthworm presence, soil respiration (how much CO₂ is expelled from the soil), and nutrient levels. 

The Findings

Smerud found that there was no significant difference in levels of nitrate and ammonium in the soil, but that there was greater soil respiration at sites near buckthorn. This indicates that there is higher microbial activity in soils around buckthorn due to the dense leaf litter that is accumulated from buckthorn plants. There were also more earthworms at sites with buckthorn as compared to sites with native black cherry, suggesting that buckthorn supports invasive earthworm populations. High microbial activity is also interactive with high earthworm presence around buckthorn. 

These changes in soil properties allow buckthorn to thrive, while harming native plant species. The study suggests that there are implications for conservation. Smerud suggests that forest managers should assess the changed soil state after clearing an area of buckthorn, and be selective when choosing which native plants to restore to an area, or consider ways to return the soil composition to its original state. 

Relevance

Buckthorn is a persistent invasive species in Minnesota, and a primary focus of Natural Lands management. Three common methods of management are prescribed burning techniques, manual removal, and herbicide application, each with their own tradeoffs. The Natural Lands practices all three of these tactics. Explore here to learn from the Minnesota DNR how you can help support native plants.

Citation

Smerud, K. 2016. Common Buckthorn’s (Rhamnus cathartica) Influence on Minnesota Soil Properties in Contrast to Native Black Cherry (Prunus serotina). St. Olaf College Local Ecology Papers.