Monday, June 10, 2024

South Dakota Tree-Following: le Chêne à gros acorn

"An Oak, whose leaves are as large, and which bears such large fruits, is well suited to attract the attention of lovers of foreign cultures, and to find a place in the parks and gardens of a large extent." FA Michaux, 1810 (translated from French). TreeLib photo.
In 1785, 15-year-old François-André Michaux traveled to America with his father, André Michaux, Royal Botanist to Louis XVI. They had been provided a commission and instructions to collect unusual plants and useful trees. The latter were especially valuable, as European countries were running out of wood. Like fossil fuels today, wood was critical for transport (e.g., large armadas of wooden ships!) and in manufacturing (charcoal for iron smelters, glassworks, and more).

Michaux the younger returned to France in 1790, as did his father six years later. Together they went to work on what would become the great Histoire des Arbres Forestiers de l’Amerique Septentrionale. It was published in 1810 in French by François-André alone, as his father had died in 1802 (subsequent references to "Michaux" in this post are to the younger). It would be published in English as The North American Sylva (1).

Quercus macrocarpa, Over Cup White Oak (today's Bur Oak). Michaux 1810; BHL.
Among their discoveries was an oak with exceptionally large acorns, the basis for the species name given by Michaux—macrocarpa. The common name at that time, Over Cup White Oak, refers to the sizable cup that sometimes nearly covers the nut. Unfortunately the wood was "inferior in quality to that of true white oak" (2).

Quercus macrocarpa is one of nearly 100 oak species in North America north of Mexico (3). While it's easy to recognize a tree as an oak (with leaves), trying to distinguish among oak species can be very frustrating. Many oaks hybridize, acorns often are lacking or immature, and the tiny flowers are similar among species and not useful in identification. Botanists generally rely on leaves and twigs. But leaves are variable, even on a single tree, so one must collect a representative selection of mature sun leaves (not shade) for identification. Twigs with mature buds can be helpful (FNA).

Male catkins (flower clusters) of Bur Oak, tiny male flowers in insert. Oak flowers are unisexual, and trees are monoecious (with flowers of both sexes). MWI photo.
In South Dakota however, we have it easy. Bur Oak is the only oak species native to the state. This lack of diversity is disappointing but it does make life simpler. And we will take a close look at our oak anyway. Nothing wrong with enjoying a tree.

Michaux considered the Bur Oak "a very beautiful tree ... its foliage appeared to me to be very thick and quite dark green. Its leaves, larger than those of other species which grow in the United States, often are 40 centimeters in length, and 20 centimeters at their widest part. They are crenellated at their summit ... and cut very deeply in their lower two thirds."

A classic Bur Oak leaf: crenulate at the tip, deeply lobed below, fiddle-shaped overall.
Gazing up through Bur Oak canopies is a lovely way to spend an afternoon.
All oaks have clusters of terminal and lateral buds at the tips of twigs, which helps in recognizing leafless oaks in winter. Older twigs of Bur Oak often have corky ridges.
Terminal and lateral buds clustered at tip of a Bur Oak twig. MWI photo.
Corky ridge on an older Bur Oak twig. Nature Manitoba.

An acorn is by definition the fruit of an oak; illustration from Cronodon
The fruit of the Bur Oak is, of course, an acorn—a nut in a cup. Those of the Bur Oak are usually distinctive: "The acorns, oval in shape, are also [like the leaves] larger than those of all other species of Oak trees found in North America ... These acorns are contained, up to two thirds of their length, in a thick, unequal cup, and whose edges are lined with loose and flexible filaments" (Michaux 1810).

The overlapping scales of the cup of the Bur Oak are bumpy (tuberculate). Those near the rim have elongate tips, which make the cup look fringed—the basis for another common name, Mossy-cup Oak. But the fringe may be absent, as Michaux noted. "Sometimes, however, when these Oaks are found in the middle of dense forests, or the summers are not very hot, these filaments [elongate tips] do not appear, so that the edge of the cup is completely plain, and appears as if folded internally."

Sometimes the cup nearly encloses the nut. Bur Oak acorns in Missouri. Grey Wanderer photo.

When I arrived at Newton Hills State Park in far eastern South Dakota last month, I secured a site in a small area for tent campers in an oak opening (Bur Oaks prefer open habitat). Having forgotten my trip to eastern Nebraska long ago, I was surprised by the stature of the Bur Oaks. Being a westerner, it's not what I'm used to.

Canopies composed of beautifully sinuous branches.
Another campground, more oaks.
Everything about them was photogenic, including the bark.
Quercus macrocarpa; from USGS digitized "Atlas of United States Trees" by Elbert L. Little, Jr. Source. This is a 1971 map; Bur Oak is now known to grow farther west in northeast Wyoming. 
Bur Oak has a large range, from New England through the Midwest and south nearly to the coast in Texas. Its western limit is near the Hundredth Meridian as far north as Nebraska. But it grows farther west and north in the Dakotas, Wyoming, Montana and Manitoba. In fact it's the most northerly of our native oaks, being the most cold tolerant (Nelson et al 2014). Less hospitable habitat may be reflected in stature (4). These are the Bur Oaks I know—scrappy little trees or even shrubs.

Bur Oaks in front of Ponderosa Pines, northeast Wyoming. Matt Lavin photo.
Western Bur Oak acorns are smaller than those of eastern trees; note thumb for scale. Matt Lavin photo.


(1) The American Sylva has a complicated history, with at least a dozen known editions in multiple printings and formats (Constantino 2018). Most recently (2017), the New York Botanic Garden published 277 color plates from the Sylva in The Trees of North America: Michaux and Redouté’s American Masterpiece.

(2) Not everyone agrees with Michaux. According to the Flora of North America, "Wood of Q. macrocarpa is similar to that of Q. alba [White Oak] and produces one of the best and most durable oak lumbers."

(3) North American oaks are divided into three sections: white, red, and golden. Bur Oak belongs to the white oaks, characterized by leaves with no bristles on the tips or lobes, acorns produced every year (if conditions allow), and acorn cups with tuberculate (bumpy) scales (Nelson et al. 2014).

(4) Smaller Bur Oaks in the west and northwest are sometimes recognized as Quercus macrocarpa var. depressa (see Tropicos for the name's origins). In a study of Q. macrocarpa in the Black Hills and parts of New Mexico, Maze (1968) saw evidence of past hybridization with the more southern Q. gambelii; ongoing hybridization is unlikely because today's species are not sympatric. Maze recognized hybrids based on leaf and cup morphology, not tree stature. However one hybrid was described as being shrubby. In Flora of North America, the smaller form is considered an endpoint in clinal (continuous) variation from east to west, but more study may support recognition of var. depressa.

Sources (in addition to links in post)

Costantino, Grace. 2018. Exploring the First American Silva. Biodiversity Heritage Library Blog.

Maze, J. 1968. Past hybridization between Quercus macrocarpa and Q. gambelii. Brittonia 20:321–323.

Michaux, François-André. 1810. Histoire des arbres forestiers de l'Amérique septentrionale. Paris, L. Haussmann. (Quercus macrocarpa p 34–35 and Plate 3). BHL

Nelson, G, et al. 2014. Trees of eastern North America. Princeton U Press.

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

Tuesday, May 14, 2024

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.


(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.


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.