If you’ve spent time out in the woods anywhere in or around the Twin Cities, then I’m sure you’ve run into buckthorn. Common buckthorn (Rhamnus cathartica) is a shrub or small tree introduced from Europe and Asia in the late 1800’s as an ornamental and medicinal plant. It made great hedgerows and can be an excellent privacy fence…except that it has since escaped cultivation and moved into the forest, where it turns entire areas of forest into a giant privacy fence. Buckthorn grows in dense stands that are difficult for people and animals to walk through. It increases nest predation of birds, decreases amphibian populations, and changes mammal habits. It shades out understory plants and decreases flowering of native herbs and growth of tree seedlings. But all of these things happen above ground. What is buckthorn doing underground? Is buckthorn changing how nutrients cycle through our forest soils, and if so, what consequences might that have for forest productivity?
We examined these questions in an experiment at the Warner Nature Center, a private property managed by the Science Museum of Minnesota, located about 40 miles northeast of St. Paul. Buckthorn had been present at the site for about 30 years and is slowly spreading across the 380-acre property. We established two transects of 12 research plots along these invasion fronts, or natural gradients of buckthorn density – some plots were located in areas with many large buckthorn, while others were in areas with few small buckthorn. In addition, we also set up 4 plots in areas where nature center staff had manually removed buckthorn 3 and 6 years prior to the study – buckthorn had reinvaded these sites, but at much lower levels than their previous very dense levels.
At each plot, we collected green buckthorn and maple leaves off of the trees, put out large pots to catch leaf litter from buckthorn, maples, and oaks in the fall, and collected soil samples. We then measured nitrogen, phosphorus, calcium, potassium, and other micronutrients in multiple biotic and abiotic pools, including green leaves, leaf litter, roots, and surface soils. We also measured soil texture (how much of the soil was sand, silt, or clay) and the species and size of overstory trees in order to statistically control for these variables and be more able to attribute any differences in nutrient levels to buckthorn density.
So what did we find? In areas with more buckthorn, there was more nitrogen, calcium, and potassium in plant leaves, leaf litter, and soils; more plant roots; and higher soil pH.
What does it all mean? We found evidence that buckthorn is changing the cycling of nutrients in this system. Buckthorn has a different phenology (timing of life events) than the native plants, mainly that it leafs out earlier in the spring and holds its leaves later into the fall, meaning that it is actively growing and taking up nutrients after native plants have already gone dormant for the winter. We think it may be capturing nitrogen that would otherwise have been leached, or lost, out of the system in rainwater, leading to higher levels of available nitrogen. Since nitrogen is often a limiting nutrient in temperate forests, the retention of more of this nutrient could increase the total productivity or growth potential of these invaded forests. Other researchers have found invasive plants are often more capable of taking advantage of increases in nitrogen than native plants. This means that buckthorn’s increase in soil nitrogen may feedback to further buckthorn invasion, or encourage the invasion of other nonnative species, such as honeysuckle or garlic mustard.
Higher levels of calcium in buckthorn leaf litter could also have impacts on invasion by European earthworms. Earthworms require calcium for bodily functions and therefore may prefer to eat buckthorn leaf litter high in calcium and nitrogen instead of less nutrient-rich oak or maple leaf litter. The presence of dense buckthorn stands could feed high populations of earthworms, allowing them to continue to spread, and further changing forest soils.
But it’s not all bad news – we did find a bit of hope in the fight against buckthorn. In our plots where the nature center staff had previously removed buckthorn, we did not observe legacy effects of buckthorn. The soil and leaf litter collected from those plots had levels of nitrogen and other nutrients similar to those collected from other plots with low levels of buckthorn. This suggests that removal of buckthorn can undo the effects of buckthorn on forest nutrient cycling.
Overall, we found evidence that buckthorn is affecting conditions underground in addition to its well-known aboveground impacts on our forests. This is further impetus to try to prevent new invasions of buckthorn into forest stands where it is not yet present, and good incentive to try to manage new infestations before they get out of control. Happy buckthorn busting!
For more information about this research, as well as related projects, please visit:
This research was conducted with Kevin Mueller, Alex Roth, Tim Whitfeld, Sarah Hobbie, and Peter Reich. Funding was provided by the Legislative-Citizen Commission on Minnesota Resources (LCCMR, M.L. 2010, Chp. 362, Sec. 2, Subd. 6c ‘Healthy Forests to Resist Invasion’) and the National Science Foundation’s Integrative Graduate Education and Research Traineeship program (DGE-0653827).