Can Red Elderberry Outcompete Common Buckthorn?

Red elderberry, Sambucus racemosa, breaks it buds in early spring. Its phenology makes it a potential
competitor with common buckthorn, Rhamnus cathartica. 

One of the frustrations of removing common buckthorn (Rhamnus cathartica), is that it keeps coming back. Cut stems that aren’t treated with herbicide will sprout multiple shoots, and in areas where buckthorn has been removed, more sunlight is available to support the growth of sprouts and seedlings.

Controlling buckthorn then requires repeated visits to cut, re-treat or pull up the plants. Buckthorn seeds remain viable in the soil for up to five years, so several trips are necessary to remove seedlings and young plants. Even after the buckthorn seed bank is exhausted, nearby stands provide additional sources. Birds that eat the fruits can drop seeds into the treated area, turning buckthorn control into an ongoing project.

Buckthorn seedlings thrive where higher light intensity reaches the
forest floor.

Recognizing these challenges, scientists at the University of Minnesota are looking at a new way to manage this invasive plant. Instead of investigating mechanical or chemical controls, their research, called the Cover It Up study, asks whether native plants can thwart recolonization by exploiting buckthorn’s weakness: shade intolerance.

One of the plants in their study is red elderberry (Sambucus racemosa), a common understory shrub. Contrary to the perception that buckthorn leafs out earlier and retains leaves later than any native plant, elderberry is one of the earliest plants to resume growth in spring – even earlier than buckthorn. It also holds its leaves well into fall, rivaling buckthorn as the understory plant with the latest senescence.

That extended phenology suggests that both buckthorn and red elderberry are shade-avoidant, not shade tolerant. In fact, co-principal investigator Michael Schuster and his colleagues found that buckthorn growth is linked to light availability in spring and fall, but not in summer (1). Schuster and others also think that forests with a diverse understory can better resist invasion, because species with extended phenologies, like red elderberry, can block light from reaching buckthorn during those critical seasons (2).

Phase 2 of the Cover It Up study began in 2020. This expanded part of the research enrolled citizen scientists across Minnesota to remove buckthorn, establish experimental plots and sow seeds of native grasses, sedges, wildflowers, shrubs and trees. Their aim is to see what techniques can best prevent buckthorn recolonization in different parts of the state.

Phase 2 will conclude this year, and although it’s closed to new volunteers, anyone interested in following the research can subscribe to the quarterly project newsletter.

For more information about the Cover It Up study, including a list of species included in the Phase 2 seed mix, visit the project website at https://coveritup.umn.edu/. The seed list is under the Resources tab.

To learn how to identify buckthorn and how it harms ecosystems, visit these sites:

University of Minnesota Extension Service
Minnesota DNR
Minnesota Department of Agriculture
Woody Invasives of the Great Lakes Collaborative

A January podcast from To Know the Land features Michael Schuster discussing the Cover It Up research.  To listen, click here.  

Finally, to learn how to identify red elderberry, see the Minnesota Wildflowers page for that species.

References

(1) Schuster MJ, Wragg PD, Williams LJ, Butler EE, Stefanski A, Reich PB. 2020. Phenology matters: Extended spring and autumn canopy cover increases biotic resistance of forests to invasion by common buckthorn (Rhamnus cathartica). Forest Ecology and Management 464. https://doi.org/10.1016/j.foreco.2020.118067.

(2) Schuster MJ, Wragg PD, Reich PB. 2021. Phenological niche overlap between invasive buckthorn (Rhamnus cathartica) and native woody species. Forest Ecology and Management 498. https://doi.org/10.1016/j.foreco.2021.119568.