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.

City Trees Green Up Earlier

 A recent study found that trees in cities leaf out in earlier in spring than those in rural areas.  This post explains what happens as deciduous (leaf-shedding) plants enter and leave dormancy, and how a shift in timing could have ripple effects.

It is December, and the trees are, in their way, asleep. They are dormant, quieted by the declining temperatures and longer nights that signal a time of harsh conditions.

They will have a long nap. Last summer many deciduous trees began covering their buds – next year’s hope of growth – in protective scales. In autumn the leaves gave it up. The chlorophyll, proteins and sugars in their blades broke down or withdrew into branches, trunks and roots to be stored over winter.

As their substance retreats, leaves become liabilities. Through stomates, small pores on the surfaces of their blades, leaves continue to lose water that can’t be replaced from frozen soil. To prevent desiccation, the leaves are cut off.  Invisible lines of cells, called abscission layers, form on the leaves’ petioles, like tear-off lines that mark where they will separate from the trees. Some leaves offered a colorful sendoff and fell. Others, those with abscission layers not quite complete, are still hanging on, rattling in the winter wind.

Although they are dormant now, the trees are primed to renew their growth in spring. After enough cold days have accumulated and as days grow longer, they will begin to stir. As in all aspects of plant growth, timing is everything. If buds break too early, say in an unusually warm February, new growth would likely be damaged by a returning freeze. To avoid this, day length acts as a check. Even if buds have been adequately winter-chilled and temperatures then rise, short days (long nights) are a sign that winter isn’t over, and growth will not resume.

Timing is important not only to avoid freezing, but also to attract pollinators. Insect-pollinated trees and shrubs, especially those that are native here, have long relationships with native insects. Time of flowering may coincide with time of insect emergence and vice versa, each benefiting from the presence of the other. If plants flower earlier than normal, their pollinators may not yet have emerged, or if it's too cold, they may not be active. 

If timing of life events -- phenology -- depends on external cues, what happens when temperature and light are altered? Does a tree’s phenology change when environmental indications change, such as in warmer and artificially brighter cities?

Yes, according to a recent study that looked at satellite and phenological data around the globe (1). According to the study, on average, spring green-up occurs six days earlier in cities compared to rural areas, due mostly to warmer urban temperatures.  When photoperiod – daylength– is factored in, the effect is greater. Urban trees exposed not only to warmer temperatures but also to lights on streets, parking lots, billboards and other artificial sources leafed out an average nine days earlier than rural trees. It's thought that night length, normally a check on early leaf-out, is shortened by city light, and the trees are “tricked” into resuming growth in artificially warmer and brighter conditions. 

The study raises several questions, especially about climate change.  If a warming climate causes trees to green up earlier even in the countryside, would rural darkness limit how much earlier they resume growth? In other words, without city lights, would winter's long nights continue to serve as a check on how early the trees leaf out?

Also, could warm urban winters, made even warmer by climate change, prevent city trees from accumulating enough cold exposure to leaf out early, even if nights are artificially short and spring-like? 

If trees do leaf out and flower early, will allergy season also start earlier? How will insects adapt to the change? Will their phenology shift, too?

These questions can be answered by part by continuing to observe the phenology of plants and animals. Both citizens and scientists are important in that effort. By recording when trees, shrubs and other plants leaf out, flower and form seeds, they contribute to an understanding of what triggers these life events and how their timing might be shifted by environmental changes. 

To learn more about phenology and how to help observe seasonal changes, open the Phenology tab.  

References

(1) Meng, Lin. 2021. “Green with phenology.” Science Vol. 374, Issue 6571 (November 25, 2021): 1065-1066. DOI: 10.1126/science.abm8136

Dr. Meng’s study is also discussed in a National Public Radio interview at https://www.npr.org/2021/11/29/1059861862/climate-change-and-city-lights-are-tricking-trees-into-growing-leaves-too-soon