South Dakota's Leguminous Trees (& a vine)

A legume is a fruit type specific to the pea family. When dry, it splits on two sides to release seeds.

Continuing my journey through the South Dakota sylva, this month I'm following (learning about) our four leguminous trees—members of the Legume or Pea Family (Fabaceae). However I wanted to include all three subfamilies and their interesting flowers, so I'm throwing in a vine as well (1).

I will start with the largest—the pea subfamily or Faboideae, represented by two tree species in SD.

Siberian Peashrub—some love it, some hate it; MWI.

The Siberian Peashrub, Caragana arborescens, is native to Siberia and parts of China and central Asia. It probably was introduced by settlers on the prairies of North America, where it continues to be recommended for shelterbelts (windbreaks). It's easily grown in harsh habitats, is a nitrogen-fixer, and provides wildlife habitat. But sometimes it grows too easily and is a noxious weed, for example where it takes over prairie via shading, or provides cover for predators adjacent to habitat for prairie birds, considered one of the most threatened groups of birds in North America (Carter & Johnson 2022).

Like most leguminous trees, Siberian Peashrub has pinnately compound leaves—divided into leaflets arranged in pairs. Usually the base of the leaf stalk is armed with a pair of spines.

Jamie Nielsen, U. Alaska Fairbanks (labels added).

Being a member of the Faboideae, Peashrub flowers have petals arranged as in typical pea flowers: a broad ± upright banner, 2 lateral wings, and 2 lower petals united to form a keel. Hidden inside the keel are 10 stamens and a pistil, making the flowers bisexual.

A "typical pea flower" with 2 petals removed to show stamens and pistil; source.

Siberian Peashrub flowers; MWI (labels added).

Seeds are contained in legumes (aka pods), as is the case for almost all members of the Fabaceae (hence the old family name Leguminosae). Shape and size can vary greatly among genera and species.

Siberian Peashrub legumes; MWI.

Black Locust in full fall color; MWI.

Black Locust, Robinia pseudoacacia, may have originally been restricted to the Allegheny Mountains in the eastern US, but it has been widely planted, becoming naturalized and obscuring its original range. Like the Siberian Peashrub it has pinnately compound leaves often with a pair of spines at the base of the stalk. But the Black Locust has an odd number of leaflets (note the terminal leaflet below), whereas Peashrub has an even number.

Black Locust leaves can reach 30 cm in length; MWI.

Black Locust belongs to the Faboideae, so its flowers are typical pea flowers—with banner, wings and keel. They're arranged in elongate pendulous clusters, and are both fragrant and lovely.

Black Locust flowers—note greenish patch at base of banners; MWI.

Black Locust legumes are flat, to 10 cm long, and reddish or purplish brown with age. Seeds often are mottled with purplish brown and black spots.

Black Locust legumes late in the season; MWI.

Black Locust's mottled seeds; MWI.

Kentucky Coffeetree is SD's only known native leguminous tree (Vern Wilkens photo).

The Kentucky Coffeetree, Gymnocladus dioicus, is native to the eastern and central US, including the southeast corner of SD. It's one of two SD trees in the Caesalpinoideae. As is common in this subfamily, the leaves are twice compound—both leaves and leaflets are pinnately divided.

A single Coffeetree leaf, twice compound except for a pair of simple leaflets at the base; MWI.

Some members of the Caesalpinoideae have flowers with specialized petals, somewhat like those of the Faboideae. In others the flowers are regular, with undifferentiated petals. SD trees fall into the latter group.

Kentucky Coffeetree flowers have 5 slender petals and even narrower sepals; MWI.

As the species name—dioicus—indicates, flowers are unisexual with male and female flowers on separate trees (dioecious). Rarely flowers are bisexual.

Legumes of the Coffeetree are large and leathery, with very hard seeds nestled in sticky toxic pulp. Native Americans and early settlers roasted and ground the seeds for a drink reminiscent of coffee (no caffeine). But the seeds are highly toxic prior to roasting, so don't eat them straight from the tree!

Kentucky Coffeetree legumes, to 15 cm long; MWI photo.

Seeds c. 1 cm long; they're highly toxic unless roasted, and the "coffee" isn't all that great; MWI.

Thornless Honey Locusts (var. inerme) are popular in landscaping; MWI.

The other SD tree in the Caesalpinoideae is the Honey Locust, Gleditisia triacanthos. It's native to North America but probably not to SD; however widespread planting and escapees have obscured its original range. Leaves can be once or twice compound, even on the same tree.

One Honey Locust leaf, this one twice compound; MWI.

Trunks of wild Honey Locusts (not cultivars) have downright nasty thorns to 20 cm long (2). These are often 3-branched, explaining the species name, triacanthos (3 thorns). Fortunately there are thornless varieties. These are popular for landscaping, especially where shade is needed quickly. They transplant easily, grow rapidly, and can tolerate harsh site conditions.

Yikes!! MWI.

Like Coffeetree flowers, those of Honey Locust are regular and unisexual, with male and female flowers on different trees (dioecious), or sometimes on different branches of a single tree (monoecious). Rarely flowers are bisexual, for example in the photo below.

Honey Locust flowers, both female and bisexual; MWI.

Honey Locust legumes are long, to 40 cm, and often curl distinctively when mature. Inside are very hard seeds in sticky pulp. In contrast to that of Coffeetree, this pulp is sweet! In fact it's the "honey" of the common name. Both wildlife and livestock find it tasty.

Nicely arranged Honey Locust legumes; MWI.

As promised, I'm including a non-tree to illustrate flowers of the third legume subfamily, the Mimosoideae. Nuttall's Sensitive Brier, Mimosa nuttallii, is a sprawling or climbing subshrub (woody only at base) native to the central US including SD.

In its youth (growth of the season), Nuttall's Sensitive Brier shows the beauty of prickles! ANPS.

These briers are heavily armed with curved prickles, allowing them to climb if support is available nearby (otherwise they sprawl on the ground). Humans find the prickles quite painful to touch, yet Nuttall's Sensitive Brier is desirable forage for wildlife and cattle, and can be wiped out with overgrazing.

The leaves are twice-compound, and the ultimate segments are quite slender, especially when touched, for they fold longitudinally in response! They are quite sensitive, closing in just a few seconds. They later relax until the next time they are tickled, when they again quickly fold (more about Mimosa's muscles here).

Two twice-compound leaves of Nuttall's Sensitive Brier, with segments mostly closed; ANPS.

Typical of the subfamily, the flowers of Nuttall's Sensitive Brier are tiny. But they are spectacular! Arranged in very dense clusters, the numerous flowers open simultaneously (flowers of the other subfamilies open progressively). Furthermore, each little flower has 8–12 hot pink stamens to 12 mm long. In other words, the beautiful balls of flowers are mostly stamens (yellow tips are anthers).

Flowers in each head open simultaneously, but heads bloom progressively (upwards); ANPS.

Mimosa nuttallii head from below (stem removed); note tiny flowers with outrageous stamens! ANPS.

Like the rest of the plant, the legumes are quite prickly. They're much larger than the flowers, to 12 cm long. Obviously only a few flowers in a cluster can produce legumes. There isn't room for more.

Mimosa nuttallii legumes are ridged, with prickles along the ridges; ANPS.

And so we come to the end of this month's tour. For those readers still with me, thanks for being a traveling companion! Hopefully next month I will have photos of SD trees in person (should that be "in plant"?). April was too rainy. May looks promising.

Notes

(1) Recently the three long-standing subfamilies of the Fabaceae were split, shuffled and rearranged in ways still being debated. For more see Fabaceae, Phylogeny and Taxonomy in Wikipedia; and The Legume Phylogeny Working Group. 2017. A new subfamily classification of the Leguminosae based on a taxonomically comprehensive phylogeny. Taxon 66:44–77.

(2) For the botanically curious, the sharp-pointed structures on the trunk of the Honey Locust technically are thorns rather than spines because they are modified shoots (able to branch). See Wikipedia's wonderful article devoted to the intricacies of thorns, spines and prickles.

Sources

Arkansas Native Plant Society. 2018. Know Your Natives – Sensitive Brier. Photos CC BY_NC 3.0 (non commercial use with attribution). An excellent website.

Johnson, WC, and Knight, DH. 2022. Ecology of Dakota Landscapes. Yale University Press.

MFI. Minnesota Wildflowers; a field guide to the flora of Minnesota. A terrific source of information and photographs for the many plants shared with South Dakota.

This is my contribution to the May gathering of tree followers, kindly hosted by The Squirrelbasket.

South Dakota Maples—Sugar, Silver, & a Trickster

Sugar Maple in Sica Hollow, northeasternmost South Dakota.

For April's tree-following report I'm sticking with my strategy of following (learning about) South Dakota trees—helpful in preparing our guide to trees and shrubs of the state. This month I chose maples, of which there are three.

South Dakota is not known for its trees. It's largely a prairie state, though much of what was prairie is now fields of corn, wheat, soybeans, hay, sunflowers and more. Most people who visit (i.e., tourists) head straight for the Black Hills in the far west, which are mostly covered in coniferous forest (that's why they look black).

But far eastern South Dakota also is forested. It's part of the immense Eastern Deciduous Forest (EDF) ecoregion, which covers the eastern third of the United States and extends into southeast Canada and northeast Mexico.

Globally, the North American EDF is one of the largest parts of the Temperate Broadleaf Forest Biome (yellow green). Source.

Authorities frequently claim that the EDF reaches its western limit in Minnesota, but they're wrong. It continues into South Dakota where it's best developed in the southeast and northeast corners of the state.

South Dakota land cover. Dark green units in the southeast and northeast are large stands of Eastern Deciduous Forest (modified from Johnson & Knight 2022).

EDF thrives where summers are warm and moist and winters are cold, as is the case in easternmost South Dakota (summers are increasingly dry going west). As the name indicates, the dominant trees lose their leaves each winter. These include oak, elm, basswood (linden), hackberry, and maples.

Eastern Deciduous Forest in northeast South Dakota, with a thick understory of young Sugar Maples hoping to reach the sky.

Of the three maple species in South Dakota, two are native to the far east—Sugar Maple and Silver Maple. However Silver Maple is widely planted, and escapees are occasionally found in the wild as far west as the Black Hills. The botanists who described and named these maples long ago chose very similar scientific names: Acer saccharum and A. saccharinum (thanks, guys, I hope I can keep them straight!). Both mean sugary, referring to the sap (Saccharum is the genus name for sugarcane). However the Sugar Maple's sap is far sweeter (Plant Finder, Missouri Botanical Garden).

These two maples are similar in appearance, enough so that some effort is needed to tell them apart. Both have what we think of as typical maple leaves—palmately lobed, like the fingers of a hand. Fortunately the details differ, which is more easily shown with photos than explained with words. But we intend to do both.

Leaves of Sugar Maple are more shallowly lobed with broader sinuses (gaps) between the main lobes. The sinuses usually are less than half the overall leaf length and are rounded at the bottom.

Sugar Maple, Acer saccharum (Minnesota Wildflowers Information).

Leaves of Silver Maple are more deeply lobed, and with narrower sinuses between the main lobes. The sinuses usually are at least half the length of the leaf and are more pointed at the bottom.

Silver Maple, Acer saccharinum (MWI).

Both trees develop furrowed ridged bark with maturity, but that of the Silver Maple is more flaky, enough to make it look shaggy ... or so they say. Do you have tips for telling these two maples apart? If so, please leave a Comment below.

Sugar Maple, Acer saccharum (MWI).

Silver Maple, Acer saccharinum (MWI)—a bit shaggy?

Now for those readers about fall off their chairs with curiosity ... there is indeed a third maple in South Dakota. But it's a trickster.

Yes, this IS a maple!

It's a maple with lobes so deep that they're leaflets.

This is Acer negundo, the Boxelder—also known as Ashleaf Maple (ash has similar leaves), Manitoba Maple, Érable à feuilles composées, and Poison Ivy Tree (not poisonous but leaflets are similar in shape). Instead of palmately lobed leaves typical of maples, it has pinnately compound ones—divided into paired leaflets with one more at the tip.

In contrast with Sugar Maple and Silver Maple, which are uncommon in South Dakota and limited to the far east, Boxelder is common and widespread across the state, and apparently not terribly picky about habitat. Some call it a weed.

The Boxelder I followed in 2018—a bit of wildscaping for this warehouse.

This is my contribution to the monthly gathering of tree-followers kindly hosted by The Squirrelbasket. More news here.

Sources cited

Johnson, W. Carter, and Knight, Dennis H. 2022. Ecology of Dakota Landscapes; past, present, and future. Yale University Press; Biodiversity Institute, U. Wyoming. ISBN 978-0-300-25381-8.

South Dakota Tree-Following—Prairie Crabapple

Pyrus ioensis. 1913, Curtis's Botanical Magazine. Source.

This month's tree-following report features another tree from South Dakota, part of my effort to learn more about the state's trees. There's a practical reason for this—I'm writing species descriptions and selecting photos for an online Guide to Trees, Shrubs, and Woody Vines of South Dakota.

Having recently worked my way through the challenging Rose Family (see post about the diverse and complicated fruits), a Rosaceaous tree was the obvious choice. I picked one new to me—Prairie Crabapple, Malus ioensis (formerly Pyrus ioensis).

I also wanted to showcase photos from Minnesota Wildflowers (now includes all plants not just wildflowers). It will be a major source of photos for our guide as our states share many species. Almost all their photos are free for non-commercial use (that would be us!), there are tens of thousands to choose from, and the photographers are botanists who know what's needed for identification. The Prairie Crabapple is a fine example (photos below are from the website).

Prairie Crabapples grow as shrubs or trees to 6 m tall, and can form dense thickets from root suckers.

Malus ioensis in bloom, hence the pinkish tinted crowns.

Prairie Crabapple leaves by Katy Chayka, creator and driving force of Minnesota Wildflowers.

With maturity, bark develops irregular ridges or plates that peel away to reveal reddish inner bark.

Prairie Crabapple flowers are typical of the Rose Family, with five showy-but-simple petals surrounded by five sepals. Inside the petals are numerous pollen-producing stamens surrounding a pistil containing ovules awaiting fertilization to become seeds.

Flower bud showing fuzzy sepals. These help with id.

Botanically speaking this crabapple is "armed", in this case with short branches that become sharp-tipped. Technically these are thorns, which are derived from branches or shoots (vs. spines which develop from leaves, and prickles which develop from the outer layer of a stem or branch; there's a Wikipedia article devoted to this topic).

Thorn developed from short flowering shoot.

Fruits of crabapples are pomes, from Old French "pome" meaning apple. Those of Prairie Crabapple are only about 2.5 cm across. They are "edible but barely so" according to Katy.

I'm looking forward to seeing Prairie Crabapple in the wild, of course! But there's a problem. Though multiple sources report it for South Dakota, I found NO specimens in a search of SEINet, the online portal to digital herbaria across the country. And the South Dakota Natural Heritage Program, which tracks rare plants, lists it as "Reported for woodlands of e SD, no vouchers yet found." I may be off on a treasure hunt once spring comes.

USDA Plants shows Malus ioensis in Lincoln, Clay, and Codington counties in South Dakota. Unfortunately no evidence or sources are provided (arrows added).

This is my contribution to this month's gathering of Tree Followers, kindly hosted by The Squirrelbasket. Once again—if you're looking for a good time, I invite you to join us!

Sources

Minnesota Wildflowers. Malus ioensis (Prairie Crabapple).

Flora of North America, Malus ioensis.

Fruits of the Rose Family—a Cornucopia!

How many Rosaceous fruits are in this painting? Answer at end of post. (Still Life with Fruit by Severin Roesen, 1852; Smithsonian Open Access)

The Rosaceae is a large cosmopolitan family of plants, with c. 3000 species scattered across every continent except Antarctica. It's best represented in the Northern Hemisphere, mainly in temperate habitats. This is a family much loved by humans. Gardeners have grown the "queen of the flowers" (the rose) for at least 5000 years, and we've enjoyed the diverse delicious fruits even longer.

Members of a plant family, even a large one, are supposed to be similar and often they are. For example, flowers of the vast majority of species in the Rose Family (excluding ornamentals) have five showy-but-simple petals surrounded by five green sepals. Inside the petals are numerous pollen-producing stamens surrounding one-to-many ovaries, each of which contains one or two ovules awaiting fertilization.

A typical Rosaceous flower: 5 sepals (green tips visible), 5 petals, many stamens (anthers at tips), and a cluster of numerous ovaries. (Wild Strawberry, Fragaria virginiana. Matt Lavin photo)

Unlike the flowers of the Rose Family—so similar and so simple—the fruits are diverse and often complex, in fact notoriously so. For centuries botanists have tried to subdivide the family based on fruit type. They've always failed.

By definition, fruits are mature ovaries containing mature ovules (seeds). But there can be so much more—thick flesh, plumose tails, tough skins, rock-hard coats, accessory structures, and aggregation. The Rose Family includes all of these! Though this diversity frustrates plant taxonomists, the rest of us can enjoy it :)

It would be foolish to try to cover the full range of Rosaceous fruit diversity. Readers would begin to fall away less than halfway through. Instead here are some favorites, starting with complicated and delicious types, and finishing with one that's quite simple and not at all tasty, but spectacular in its own way.

But first ... do you know which Rosaceous fruit type is most widely consumed by humans?

Our favorite fruit in the Rose Family is a pome, from Old French "pome" meaning apple (source).

Malus domesticus, the domestic apple tree, probably originated in the mountains of Central Asia. Now it's represented by thousands of cultivars grown in temperate regions worldwide. The apple itself is a pome, which botanists define as a fruit with "a central core containing multiple small seeds, which is enveloped by a tough membrane and surrounded by an edible layer of flesh." Technically speaking, the thick fleshy layer is an accessory structure.

Slicing an apple in half lengthwise shows how evolution elaborated on the basic seeds-in-mature-ovary structure in creating the pome.

Granny Smith apples. Photo by benjamint444, labels added.

The pome is unique to the Rose Family, but not just to apples. Pears and quinces also bear pomes, as do many native species. Their pomes are small, but look closely—they are indeed tiny apples.

Pomes of four species of hawthorne (Crataegus), c. 1 cm diameter; by Nadiatalent.

Another popular fruit type with a delicious accessory structure is the drupe, also known as stone fruit.

Domestic cherry—one of many delicious drupes in the Rose Family (source unknown). 

The genus Prunus includes what appears to be a diversity of fruits: cherries, plums, peaches, nectarines, apricots, and almonds! But in fact they all are drupes, defined as "fruit in which an outer fleshy part surrounds a single shell (the pit or stone) with a seed (kernel) inside" (source). [Unlike pomes, drupes are not limited to the Rose Family. For example, olives and dates are drupes.]

A sliced peach reveals tasty flesh surrounding the pit. DG Passmore, 1895, National Agricultural Library.

The almond tree, Prunus dulcis, also bears drupes. The almond itself is a seed (source; labels added).

Drupes are well represented in the wild, for example our many species of wild cherries.

Harvest time! Chokecherries, Prunus virginiana, by Matt Lavin.

"Thousands of drupes readied to make chokecherry whatever" says Matt. Would that be jelly? wine?

Now we advance to a higher level of complexity—a fruit that develops from multiple ovaries of a single flower. A tasty but controversial example is the strawberry (actually not a berry but that's not the controversy).

Wood Strawberry, Fragaria vesca (Jakob Sturm, 1798, BHL via Flickr). Question added.

Strawberry flowers are typical of the Rose Family (see above). But after fertilization, things get interesting. The many ovaries, each containing a single ovule, mature to become achenes—dry one-seeded fruits (g/G in illustration). At the same time the flower base (receptacle) grows, becoming a red mass of yummy flesh with the achenes embedded on its surface (f in illustration).

Herein lies the controversy: What is the fruit of the strawberry? Is it the fleshy globe adorned with achenes, or the achenes themselves? Some botanists rage over this, insisting achenes are the true fruits (in Wikipedia for example). Others think this silly, and simply refer to strawberries as aggregate fruits (my preference).

Young strawberry, with styles still present on maturing achenes (Olivier via Flickr).

Wild Strawberry, Fragaria virginiana (JW Frank via Flickr).

As promised, the final fruit is simple but beautiful—a single achene, specifically that of Mountain Mahogany (genus Cercocarpus). These shrubs and small trees of the arid American West are members of the Rose Family. But no matter how long one stares at their little flowers, it's hard to see roses.

Flowers of Birchleaf Mountain Mahogany (C. betulifolia) are just 5 mm across and have NO petals. The sepals form a cup with many stamens surrounding a single pistil (Joe Decruyenaere via Flickr).

A Mountain Mahogany flower may be humble, but not the resulting fruit. The pistil and its ovule become an achene with a long persistent feathery style, ready to fly with the wind. En masse, these "seed tails" transform the plant that bears them.

Fruits of Mountain Mahogany (C. montanus) preparing to launch; seed tails to 8 cm long (Matt Lavin).

Mountain Mahogany transformed by seed tails. Achenes can be spectacular! (C. ledifolius, Matt Lavin)

Now we return to where we started—"How many Rosaceous fruits are in Roesen's painting?" The answer is "many" (exact number depends on your opinion regarding aggregate fruits). I found pomes (apples), drupes (peaches, plums, cherries, maybe nectarines), aggregates of drupes (blackberries), and aggregates of achenes (strawberries). Did I miss anything?

This has been quite a long post, I agree. But I would be remiss if I were to omit Robert Frost's thoughts on the subject.

Sources (in addition to links in post)

Rosaceae in Flora of North America, which includes a list of fruit types:

"... achenes aggregated or not, follicles aggregated or not, drupes aggregated or not, aggregated nutlets, pomes, aggregated drupelets, or capsules; sometimes involving accessory organs, for example, hypanthium, torus."

Haywood, VH. 1978. Flowering Plants of the World. Oxford University Press.

Judd, WS, et al. 2002. Plant Systematics, 2nd ed. Sinauer Associates, Inc.