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.