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. 

Flower Parts for Plant ID

Large-flowered Trillium, Trillium grandiflorum

Thanks to the four biomes that meet here, Minnesota hosts a diversity of plant life. According to the most recent MNTaxa plant checklist, the state is home to at least 2,250 species and varieties of vascular plants (1). Of those, about 94% are flowering plants, and most guidebooks focus mostly or entirely on that group. To help identify these plants, many wildflower guides rely on flower parts – their presence or absence, their number, their appearance –to arrive at a plant's name.

It’s useful, then, to know how flowers are put together. This post introduces basic flower structure, beginning with the names of flower parts and some common variations and then introducing some terms for flower clusters, called inflorescences. 

Flower parts

In a model flower, parts are organized in four whorls. From outermost to innermost, they are sepals, petals, stamens and one or more pistils. In some flowers, pistils are composed of joined parts, called carpels (seed leaves), to form a compound pistil. In others, the pistils are composed of only one carpel, so they’re called simple pistils. The diagram below shows a simple pistil.

All the sepals together form the calyx, from a Greek word describing a husk or a case for a bud. Many flower buds are enclosed in and protected by a calyx before they open. Similarly, all the petals together form the corolla, from a Latin word meaning wreath or crown.

 

Each of the flowers below has all four parts. Sharp-lobed hepatica has flowers with multiple stamens, multiple simple pistils, five petals, and five sepals. Large-flowered trillium has three sepals, three petals, six stamens, and one compound pistil formed of three joined carpels. Three stigmas emerge from the top of the pistil.

The photo of wild geranium below shows two flowers in different stages of development. In the left flower, stamens are at peak maturity and are releasing pollen. The pistil in this flower is immature and hidden by the stamens. In the right flower, the stamens are past peak and are withering. The pistil, however, is in its prime, with five, curved stigmas ready to accept pollen.

Because the stamens and pistils mature at different times, the flower can’t pollinate itself. This difference in timing, called dichogamy (dy-COG-amee), favors cross-pollination and mixing of genes, creating more diverse -  and perhaps more successful – offspring. 

Tepals

Tepals are petals and sepals that look alike. They are especially common among plants in the lily family and its close relatives. Tulips and day lilies are two garden favorites that have tepals. Native white trout lily and blue-bead lily also have tepals.

Regular (actinomorphic) and irregular (zygomorphic) flowers

When viewed face on, regular flowers look like wheels or stars: Their parts are evenly distributed all the way around. More than one line can be drawn through the center of the flower to create similar halves. Such flowers are also called radially symmetric or actinomorphic, which means star-shaped.

In contrast, irregular flowers have only one plane of symmetry: Only one line can be drawn across their faces to produce similar halves. Irregular flowers are also called bilaterally symmetric or zygomorphic. “Zygo” is a Greek prefix meaning pair.

Complete and incomplete flowers

If flowers have all four parts – sepals, petals, stamens and pistils – they’re complete. If they’re missing one or more of these parts, they’re incomplete. Wild strawberry, for example, has complete flowers. The flowers of Canada anemone, however, have no petals and are incomplete. In that plant, sepals are the large, colorful parts that attract pollinators.

Perfect and imperfect flowers

These terms refer to the reproductive parts of a flower, the pistils and stamens. A perfect flower has both parts, whereas an imperfect flower has only one. A staminate flower has only stamens; a pistillate flower has only pistils.

If a species has imperfect flowers and the staminate and pistillate flowers are on the same plant, the species is monoecious (mon-EE-shus), meaning one house. If the species has staminate and pistillate flowers on different plants, it is dioecious (dy-EE-shus), meaning two houses. Sometimes dioecious plants are said to have separate “female” and “male” individuals.

Most flowers of silver maple, for example, are imperfect. Staminate and pistillate flowers are shown below. They may be on separate trees or on the same tree, so the plants can be dioecious or monoecious, respectively. Occasionally, a tree may also have perfect flowers.

Wind-pollinated flowers

Wind pollinated flowers don’t rely on insects to visit them, so they lack showy petals and sepals. Stamens and stigmas, however, are often numerous and obvious when the flowers mature. Silver maple, shown above, is wind pollinated, as are willows and aspens. Grasses are also wind pollinated. Two  prairie grasses, big bluestem and Indian grass, are shown below.

Inflorescences

Some plants, like tulips or roses, produce flowers singly. Others produce flowers in clusters called inflorescences.

An aster, for example, isn’t one flower but many tiny ones, all clustered on a flat, rounded, or conical receptacle. The flowers in the center are called disk flowers. Those around the edge, often bearing petal-like rays, are called ray flowers. This arrangement, called a head inflorescence, is typical of plants in the aster or sunflower family.

Diagrams and photos of a head inflorescence and other common types are below. In the diagrams, black circles represent flowers. Different sizes of circles indicate that some flowers in an inflorescence mature sooner than others. The larger the circle, the earlier it opens. If all circles are the same size, they mature at the same time.

 

Mixed inflorescences

Adding more challenge, some plants have mixed or combined types of inflorescences. Rough Blazing Star, for example, has heads arranged in a spike, and Showy Goldenrod has heads arranged in a panicle.

References

(1) MNTaxa: The State of Minnesota Vascular Plant Checklist. Minnesota Department of Natural Resources. Accessed March 30, 2022, online at https://www.dnr.state.mn.us/eco/mcbs/plant_lists.html.

(2) Minnesota Wildflowers: A field guide to the flora of Minnesota. Online at minnesotawildflowers.info.

Leaf Morphology for Plant ID

 

The last two posts introduced leaf arrangement and leaf divisions, two features used in many guides to help identify plants. This post goes deeper into the weeds to introduce leaf morphology – the shape and structure of leaf blades, stalks and edges.

Leaves are tremendously variable in shape and structure, so there are many terms to describe them. It’s impractical – and overwhelming – to cover all of them here, so this post introduces only those that are commonly used in technical keys. To learn more, see the resources at the end of the post.

Leaf parts

Before diving into morphology, it’s helpful to know the names of leaf parts. They are marked below on the simple leaf of Common Lilac, Syringa vulgaris. Most of the terms also apply to the leaflets of a compound leaf. (See the previous post for a tutorial on simple and compound leaves.)

Leaf apices

Dozens of terms describe leaf apices, but four are especially common: acute, acuminate, mucronate and obtuse. 

Leaf bases

Leaf bases are also diverse. Common terms for their shapes are acute, obtuse, oblique, cordate, truncate and sagittate. 

Leaf margins

Leaf margins that are continuous – not toothed, notched or lobed– are called entire. Margins that aren’t entire are variously shaped and have several terms to describe them, including dentate, serrate, crenate, undulate and lobed.  

Leaf surfaces

Leaves are surprisingly diverse in surface texture. If the surfaces are smooth, they’re called glabrous. If they have a white or bluish, waxy coating that can be rubbed off, they’re glaucous. Hairy leaves have many terms to describe them, but a common overall term is pubescent.

 

Leaf attachments

Petiolate leaves are attached to a stem with a petiole, or leaf stalk. Sessile leaves lack petioles; they are unstalked and attached directly to the stems. Perfoliate leaves wrap around and are pierced by the stem. Clasping leaves, as the name implies, clasp the stem with the base of the leaf. Sheathing leaves wrap around the stem and extend down its length to form a sheath. 

Stipules

Stipules are pairs of leaf-like or thread-like appendages at the base of the petioles of some leaves. Not all species have them, but if stipules are present, their size and shape are useful for identification.

 

Leaf shapes

This is where terminology really takes off. Because leaves come in a wide variety of shapes, there are many words to describe them. Common terms are cordate, deltoid, elliptic, lanceolate, oblong and ovate. 

Adding “ob” to the beginning of cordate, lanceolate or ovate means the shape is reversed. An obcordate leaf, for example, looks upside down compared to a cordate leaf.

Expect inconsistency, intermediates and combinations

Leaves are variable even on the same plant. In the folowing photo of Japanese Lilac, Syringa reticulata, the bases of older leaves look truncate or obtuse, whereas the bases of younger leaves look acute. It’s best to look at several leaves to get a sense of what’s typical.  

 

In some cases, leaves look intermediate between two morphologies. It’s common to find combination terms for their shapes, like ovate-elliptic or lanceolate-ovate. For example, the leaves of Swamp Milkweed, Asclepias incarnata, below, are described as oblong-lanceolate or lanceolate, with an acute to acuminate apex. 

 

Test your knowledge

Here’s a description that could be found in a technical guide: Leaves petiolate, lobed, margins coarsely serrate to dentate, blades glabrous, base broadly cordate, apex acute to acuminate. Which of these leaves best matches this description? Scroll down for the answer.

The answer is Riverbank Grape. Wild Ginger has entire leaves that are not lobed, although the leaf base makes them appear so. Tall Bellflower has an acute leaf base and a margin that is not coarsely toothed.

 

More resources

Plant Identification Terminology: An Illustrated Glossary, by James G. Harris and Melinda Woolf Harris. Second Edition. Spring Lake Publishing, Spring Lake, Utah, 2001. 206 pp.

Botany Primer: Understanding Botany for Nature’s Notebook. This public-domain primer from the USA National Phenology Network covers many aspects of botany, including leaf morphology. The full citation for this reference is:

Guertin, P., Barnett, L., Denny, E.G., Schaffer, S.N. 2015. USA National Phenology Network Botany Primer. USA-NPN Education and Engagement Series 2015-001. www.usanpn.org.

Biology and botany textbooks also cover plant morphology. School and public libraries may have some on their shelves. Another choice is LibreTexts™, a non-profit collaboration that offers free online access to postsecondary textbooks. At the website, open the Explore the Libraries menu and choose Biology. Then choose Bookshelves and look for Botany.

References

(1)    Minnesota Wildflowers: A Field Guide to the Flora of Minnesota. Maintained by Katy Chayka. Accessed March 12, 2022. 

(2)    Minnesota Biodiversity Atlas. University of Minnesota, Bell Museum. Accessed March 12, 2022. 

Simple and Compound Leaves

Left: Zigzag Goldenrod, Solidago flexicaulis. Right: Black raspberry, Rubus occidentalis.

 

Plant identification guides and keys often describe leaves as either simple or compound. This characteristic is helpful in pinpointing an ID because plants tend to be consistent in the type of leaf they have: simple, compound, or a unique combination of the two.

With a little practice, it's easy to tell the two apart. Simple leaves have continuous blades, like the one in the photo of Zigzag Goldenrod above. In contrast, compound leaves have blades divided into leaflets, like the five-parted leaf of Black Raspberry, also shown above.

The leaflets of compound leaves can be arranged in one of two patterns: Pinnate or palmate. Pinnately compound leaves have leaflets arranged along a central stalk, like pinnae on a feather. Palmately compound leaves look somewhat like a hand: The leaflets radiate from a common point, like fingers on a palm. 

Pinnately compound leaves can be divided further, with primary leaflets divided again into secondary and sometimes tertiary leaflets.  Leaves that are divided two, three or more times look fern-like.

For more information about simple and compound leaves, including examples of the types described above, see this video tutorial. 

Alternate, Opposite, Subopposite and Whorled Leaves

 

Many plant identification guides group species by how their leaves are arranged on their stems. From left to right above are four common patterns:

• Alternate: One leaf per node
• Opposite: Two leaves per node
• Subopposite: Almost opposite
• Whorled: Three or more leaves per node.

Looking for the leaf arrangement is a good first step to identifying a plant. For more examples, see this video tutorial. 

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