Researching the Perfect Buckthorn Bust

A typical buckthorn understory.Hyland Lake Park Preserve

Alex Roth, Ph.D. candidate for Natural Resources Science and Management Program, University of Minnesota

Fall is a season of change. If you look at a particular spot on the landscape today – it could be in your back yard or your favorite spot in a neighborhood park – you may find it hard to identify it as the same piece of land that you saw just two months ago. Leaves fall, greens turn to browns, and eventually everything is blanketed in white. It’s truly amazing to view the changes in the environment as we move from the colors of early fall to the first snows of November. But in many forests and natural areas around the Twin Cities, there’s one thing that doesn’t change: the presence of buckthorn. From spring through fall, buckthorn is clearly visible in forest understories, and its dark green leaves are often one of the only sources of green left on the landscape at this time of year.

The fact that buckthorn is one of the last species to drop its leaves is one of the many reasons buckthorn has an advantage over native shrub species. By leafing out before the spring canopy closes and holding its leaves late into the fall, buckthorn gains a photosynthetic advantage over other plants and effectively extends its growing season past most native species. Its prevalence in Minnesota forests, along with other culprits like deer and European earthworms, has led to a decline in the abundance and diversity of native understory plant species. But while the lingering green leaves may be a reminder that buckthorn has taken over many of our forest understories, they also make it easier to identify and remove buckthorn in the fall. This idea - understanding the biology and ecology of a species and using that knowledge to identify potential management advantages - is one that can help us in the fight against invasive plant species. But in order to be successful in this fight, we also have to challenge what we currently know about invasive plant management.

Green buckthorn leaves standing out against a brown fall background. Warner Nature Center 

I have been interested in invasive species since I was young, and I am currently studying invasive plant species as part of my PhD program here at the University of Minnesota. Specifically, I focus on buckthorn, an invasive shrub native to Europe and Asia. I try to link and build upon the tenets of invasion biology commonly used to explain invasive species success. For example, a specific species may become invasive in part because it escaped natural enemies in its home range, but can we identify other processes that help to explain its success? I also attempt to consider the principle of risk in investigating these issues. Can we determine whether certain environments are more at risk to invasion and pinpoint specific characteristics at fault? Can we decrease the risk of re-invasion after invasive species removal? In the case of buckthorn, I have learned that the more we understand about the species and how it interacts with forest environments, the better we will be able to manage these risks and the species itself. However, as in most ecological topics, the situation is always more complex than it seems.

Light, litter, and diversity were factors studied in a 
buckthorn germination experiment.
Green Hall Greenhouse, UMN St. Paul Campus

For example, at a landscape scale, we have found that buckthorn propagule pressure – the number of seeds or reproductive plant parts reaching an area – plays the most important role in buckthorn’s success. Areas near forests with high buckthorn abundance are at a greater risk of becoming invaded themselves, which makes intuitive sense. However, on a smaller scale, conditions like light availability and leaf litter depth can determine the location of microsites where buckthorn will succeed. In that vein, germination has been shown to be an important filter in the colonization success of any species, and our greenhouse experiments have demonstrated the importance of light, litter, and resident plant diversity in the germination and colonization success of buckthorn. Too much light caused seed desiccation and prevented buckthorn from germinating, too much litter prevented buckthorn from reaching the soil, and diverse communities prevented buckthorn germination through their high native plant cover. But again, things are always more complicated than they first appear. We have also found that interactions with other biotic organisms can also further the success of an invader and change the role of environmental conditions in that process. For example, while a thick layer of leaf litter can prevent buckthorn germination, invasive earthworms remove this layer during the summer months, aiding in buckthorn germination but also making a slight cover of leaf litter important for preventing seed desiccation.

Our field studies have shed more light on how earthworms and buckthorn interact to facilitate each other in the invasion process. Worms create beneficial germination conditions for buckthorn, while buckthorn provides moist soils and nutrient-rich litter for worms. In order to better understand the invasion process of a species like buckthorn, it is necessary to gain a better understanding of how these and other biotic factors interact with abiotic factors, like light, nutrient, and moisture availability, across the landscape.

In the cut-and-paint removal treatment, light availability
at ground level has increased dramatically, opening the
door for re-colonization by early successional plants.
Warner Nature Center

In my mind it’s ultimately not enough to stop at understanding the processes that affect invasion; in order to make my research useful, I strive to make it applicable to ecosystem management and conservation. Once buckthorn is established at a site, we can use our knowledge of how and why it became established to implement management strategies that successfully remove the plant, all while deterring re-invasion and promoting the regeneration of native species. To that end, we conducted a multi-site removal experiment to test different removal methods and their ability to deter buckthorn re-invasion. By implementing buckthorn removal methods that specifically modify the environmental conditions known to affect buckthorn germination (light, litter, diversity, etc.) we may be able to prevent buckthorn re-colonization and encourage re-vegetation by native species.  We found that pulling the shrub out of the ground roots and all disturbs the soil and uncovers the seed bank, leading to the most re-vegetation by herbaceous species.  The other two strategies, cutting the stems and painting herbicide on the stumps, and applying herbicide to the intact buckthorn stem and leaving the standing dead biomass, resulted in less herbaceous cover but also fewer early successional and non-native species. We also employed post-removal planting treatments to examine their ability to deter the colonization of buckthorn and influence future plant communities at the sites. 

NPS biotech Jordan using a chainsaw to remove buckthorn.
Coldwater Spring

Recently, I was lucky to work with Maria DeLaundreau of the Mississippi River Fund and Neil Smarjesse with the National Park Service to implement a modified version of this buckthorn removal project at Coldwater Spring. They were both interested in how different buckthorn removal methods might affect species regeneration at Coldwater Spring. The hope is that by implementing a variety of different techniques, we can identify one or more that can encourage re-vegetation by native species and help resist future buckthorn invasion at Coldwater Spring. By identifying these techniques and working with surrounding landowners, we may be able to not only get a handle on removing buckthorn, but make sure that in doing so, we are bringing back native plant species and preventing future invasions.

Neil Smarjesse, NPS Coldwater Spring Site Manager and Biologist, applying herbicide to a cut buckthorn stump. Coldwater Spring

Alex Roth is a Ph.D. candidate in the Forest Biology, Conservation, and Management track in the Natural Resources Science and Management (NRSM) Program at the University of Minnesota. Originally from Columbus, Ohio, Alex moved to the Twin Cities to pursue his Ph.D. When he's not working, Alex enjoys anything and everything outdoors, playing soccer, and dabbling in music.

You can contact Alex with questions or comments by email at roth0487[at]