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.

Plant Profile: Pasque Flower

Pasque Flower, Anemone patens, on April 19, 2022, at Crow Hassan Park Reserve. 

These Pasque Flowers were barely open on a cool April day, but as they expand, their bowl shapes will track the sun like tiny reflective dishes. The movement of the flowers, called heliotropism ("sun turning") keeps them warmer than their surroundings, providing an inviting place for pollinators to land.

Heliotropism is one of many adaptations Pasque Flower has to emerging and flowering early on the prairie, when conditions are unstable. Its spring phenology offers the benefit of less competition for water, light and pollinators, but it comes with the risk of late frosts and cold, windy weather that inhibits insect activity. 

Other adaptations include long hairs to blunt the effects of cold winds and chemical irritants that discourage herbivores from chomping on the first greens of the season. Crushed or chewed leaves contain protoanemonin, a molecule that irritates the digestive system. The same molecule can produce blistering rashes on the hands of wildflower-picking humans.

Where to Find Pasque Flower

Pasque Flower is a native perennial of dry prairies and open woods. It grows throughout much of Minnesota except for counties in the northeast. For a range map, see this Minnesota Wildflowers webpage. 

Pasque Flowers and Climate Change

Like other early spring perennials, Pasque Flower is especially sensitive to temperature, so this species is useful to observe for the effects of a warming climate. Around 2010, Elisabeth Beaubien and Andreas Hamann, two researchers studying the phenology of plants in the Central Parklands of Alberta, Canada, found that Pasque Flowers bloomed an average two weeks earlier than decades ago. The shift in phenology corresponded to increases in average temperature during the same period, 1936-2006. 

The two-week difference was greater than Beaubien and Hamann expected based on a thermal time model, a tool that predicts flowering time by adding accumulated degrees above a base value. They suspect increases in nighttime temperature are largely responsible for the shift. 

Their paper is here.