Tree Following: Killer Cottonwood in Utah

On a hot afternoon several weeks ago, I pulled into Hittle Bottom Campground on the Colorado River in southern Utah. After parking in the only site with shade, I opened the windows, put screens in place, inserted $10 in the payment envelope, and started for the pay station. But I was stopped in my tracks by a Killer Tree, right next to our campsite!

The area around it had been cordoned off with orange caution tape, but I checked carefully to make sure we were safe. Indeed everything was fine. We were out of range of falling limbs.

Click on image to view caution tape, marked by arrows.

The big cottonwoods that grow along lowland rivers in North America—Populus deltoides and P. fremontii —are infamous for dropping large dead branches. As the Colorado AAA has observed:

No one writes poems about “under the spreading cottonwood tree” because it can actually be dangerous to sit under a cottonwood in high winds due to breaking branches.

The technical term for this is "dieback".

Some cities (Denver for example) ban these cottonwoods in part because of dieback. They grow fast (to six feet per year!) and are relatively short-lived, so falling limbs will be a problem. And they grow roots toward and into reliable water sources such as city water and sewer lines! This is an impressive adaptation for the trees but a problem for us (source).

The Hittle Bottom Campground has no water aside from the river, so managers don't worry about cottonwood roots invading plumbing. But dieback is a problem, hence the caution tape. Of course I wanted photos, so I risked my life so in the absence of imminent danger (the day was calm) I stepped over the tape to commune with the Killer Tree.

Zig-zag form due to lost branches.

The bark was especially photogenic, even with tape.

This is Fremont's Cottonwood, named to honor the famous explorer and surveyor John Charles Fremont. However the honor probably celebrates another of his achievements, one less widely known—botanical discovery! Fremont was not a taxonomic expert but he knew how to collect plants. And collect he did—on the order of two thousand specimens. Among these were at least 165 species new to science, some 40 of which were named in his honor. For more about Fremont's botanizing, see JC Fremont was here.

On the afternoon of March 30, 1846, Fremont and his party "encamped on Deer Creek, another of these beautiful tributaries to the Sacramento [River, in California]. Mr. Lassen, a native of Germany, has established a rancho here ...". They stayed for five days, during which time Fremont collected plants, including a cottonwood. He suspected it might be a new species, as he had noted the previous year when he was in southern Utah (no collection was made or survived).

Fremont's 1846 specimen from "Deer Creek at Lassens" (Gray Herbarium). He collected both male (above) and female (below) flowers, demonstrating knowledge and care in collecting plants.

Typical of field botanists at that time, Fremont relied on experts for identification. He sent many of his collections to the leading American plant taxonomist—Asa Gray at Harvard. Gray often passed along western specimens to his colleague, Sereno Watson, who was more familiar with that flora. It was Watson who named and described Populus fremontii.

Sereno Watson (Wikimedia). A colleague described him as "tall, very erect, [with] good features, a high-bridged nose, and a carefully tended beard of great length and whiteness. Almost to the end of his life he walked with a brisk elastic step suggesting physical energy remarkable for a man of his years."

In his 1875 paper, Watson distinguished P. fremontii from its close relatives "especially by the remarkably developed torus" (now called floral disc). He also noted that young growth tended to be somewhat hairy. In contrast, the similar P. deltoides has smaller floral discs and young growth is not hairy.

A century later, James Eckenwalder, expert on the genus Populus, reached the same conclusions, recognizing the larger floral disc and often hairy young growth as distinguishing features for Fremont's Cottonwood (see also his treatment in Flora of North America).

Populus fremontii from Sargent's 1896 Silva of North America; added enlargement shows female flower with floral disc—thought to have evolved from petals and sepals.

Fremont's Cottonwood with capsules and young leaves (TreeLib, J Morefield photo).

Mature leaves and bark, Fremont's Cottonwood (TreeLib).

These last photos are included in part to thank Blake and Nathan Willson for their wonderful Tree Library website—"a digital platform for teaching and studying trees with a focus on promoting awareness and understanding of trees and their global importance to the environment."

Fremont's Cottonwood, Rio Grande, New Mexico. "Trees are our silent partners, sensing us as we move about, providing shelter, offering us beauty, and nurturing and protecting the earth." (TreeLib)

Sources, in addition to links in post

Eckenwalder, JE. 1977. North American cottonwoods (Populus, Salicaceae) of sections Abaso and Aigeiros. J. Arnold Arboretum 58:193–208 [P. fremontii p. 198-200] BHL.

Fremont, JC. 1887. Memoirs of my life: including in the narrative five journeys of western exploration during the years 1842, 1843-4, 1845-6-7, 1848-9, 1853-4 Internet Archive.

Sargent, CS. 1896. The silva of North America: a description of the trees which grow naturally in North America exclusive of Mexico. Vol. 9 (P. fremontii p.183 ...) BHL.

Watson, S. 1875. Revision of the genus Ceanothus, and descriptions of new plants ... Proceedings American Academy of Arts & Sciences. Vol. 10 (P. fremontii p. 350) BHL.

This is my October contribution to the monthly gathering of Tree Followers, kindly hosted by The Squirrelbasket.

South Dakota Trees: Black Walnut

Black Walnut with spring leaves—its "yearly trick of looking new" (Philip Larkin, The Trees). Union Grove State Park, South Dakota.

Walnuts from a previous year, still unopened. Is extreme toughness adaptive?

Continuing my quest to get to know South Dakota's trees, I chose Black Walnut (Juglans nigra) for this month's tree-following. It's native to the state but barely, probably just in the far east. This is another deciduous hardwood that's common in the midwestern and eastern USA, but rare in South Dakota. Humans have long contributed to the spread of these species, so it can be difficult to pin down their "native" range.

Black Walnut trees can reach 150 ft in height. On open sites the crown is large and rounded. In forested habitat it's smaller, atop a tall mostly unbranched trunk. 

Black Walnut is popular in landscaping; Victoria, British Columbia (TreeLib).

Juglans nigra was once "a dominant and majestic canopy species of primeval midwestern and southeastern forests." But with clearing for agriculture, and harvest for railroad ties, gunstocks, log cabins, furniture, ship-building and more, it became much less common (GoBotany). The venerable giants—to 6 or 7 ft in diameter—are gone. Even so, Black Walnut wood is widely available and quite popular—prized for its dark grain (more information at The Wood Database).

Black Walnut in cross section; photo by Roger Culos.

Black Walnut bowl and photo by Joe Nestlerode.

Juglans nigra leaves are alternate, but often cluster at branch tips and appear whorled (MWI).

Black Walnut usually can be recognized by its leaves, which are long (to 6 dm) and pinnately compound with 8–23 leaflets (terminal one often reduced or lacking). Leaflets are lanceolate, to 15 cm in length, and have serrate margins.

Carl Linnaeus, who named and described the species in 1753, included leaf features in the polynomial name: Juglans foliolis lanceolatis argute serratis—a Juglans with lanceolate leaves with serrate margins. Did Linnaeus mistake leaflets for leaves? Perhaps there was no such distinction in his day.

Black Walnut in Linnaeus's Species Plantarum (1753). Note "nigra" in fine print on right.

In the image above, "nigra", "alba" and "regia" on the page margin hint at a revolution underway. The long descriptive polynomial names were being replaced with binomials (still in use). Species Plantarum is considered the first major botanical work to use binomials consistently. This is where Juglans foliolis lanceolatis argute serratis; exterioribus minoribus became Juglans nigra L. (L. refers to Linnaeus, the authority).

Juglans nigra L. from The Linnaean Herbarium, with permission. Linnaeus did not cite a type specimen in his description, so this one was selected as the lectotype (Reveal et al. 1987).

Like all walnut species, Juglans nigra has unisexual flowers and trees bear flowers of both sexes (i.e., are monoecious). Flowers are inconspicuous—small, yellowish to greenish, with no petals and only tiny sepals. Males form catkins (elongate pendulous clusters) to 10 cm long, with numerous flowers. Female flowers occur in little clusters of 1–4 in leaf axils (where the leaf stem attaches to the branch).

Catkins of male flowers, each with numerous stamens (MWI).

Three female flowers in a leaf axil, each with 2 feathery stigmas (TreeLib).

A female flower has a single pistil. With fertilization, it matures to become ... well ... that depends on whom you ask.

 Juglans nigra fruit, fibrous covering partly removed. Is this a nut? drupe? pseudodrupe? (Plant Image Library)

In their treatments of the Black Walnut, early botanists just described the fruit. The first to do so may have been Mark Catesby, an English naturalist who studied the flora and fauna of the American colonies. In his Natural History of Carolina, Florida and the Bahama Islands (1731) he wrote:

"The thickness of the inner shell requires a hammer to break it. The outer shell is very thick and rough on the outside. The kernels are very oily and rank tasted; yet, when laid by some months are eat by Indians, squirrels, etc." [Kernels are seeds, also called nutmeats.]

Illustrations in Catesby's book "were etched by the author"; plants and animals often were paired, e.g., Black Walnut and American Redstart (relative sizes of fruits and bird are correct) (BHL).

In his 1819 American Sylva, François-André Michaux (featured in this post) provided a bit more detail:

"The husk is thick, and ... when ripe it softens and gradually decays. The nut is hard, somewhat compressed at the sides, and sulcated [with narrow grooves]. The kernel, which is divided by firm ligneous partitions, is of a sweet and agreeable taste, though inferior to that of the European Walnut." 

Black Walnut, by the great botanical illustrator Pierre-Joseph Redouté, in Michaux 1819 (BHL). Male flowers upper left; fruit with green husk lower right next to brown nut.

However it wasn't long before botanists were attempting to assign fruits to defined types. Asa Gray, the eminent Harvard professor of botany, included a system of fruit types in his 1868 Lessons in Botany and Vegetable Physiology. Now 156 years later, it's still in use because botanists haven't come up with anything better. This is not for lack of trying (see Judd et al. 2002, for example).

So what did Professor Gray call the walnut? Well, actually ... he hedged!! In his New Manual of Botany. A handbook of the flowering plants (1908) he called it a "a kind of dry drupe". Dry drupe? But Professor, you defined a drupe as having a fleshy outer part and hard inner part, both derived from the pistil. You provided the cherry, plum, and peach as familiar examples.

As Professor Gray demonstrated, the walnut is not easily categorized. And it's not alone—more than a few fruits confound us in this way.

Parts of a walnut: husk, shell (nut), nutmeat (seed). Wild Harvests.

Some botanists take a strict approach: a fruit must develop from a pistil. This is where the fruit of the walnut—specifically the husk—causes problems. "[It] superficially resembles a drupe, with a hard 'stone' surrounded by a soft, often fleshy husk. The husk, however, is not part of the fruit wall (it develops from the involucre and calyx), and the fruit is actually a nut." (Juglandaceae in Flora of North America).

The Wikipedia Walnuts article mostly agrees, but calls them "accessory fruit because the outer covering [husk] of the fruit is technically an involucre and thus not morphologically part of the carpel [pistil]; this means it cannot be a drupe but is instead a drupe-like nut."

Other botanists are more broad-minded. A fruit with a (relatively) soft outer part and hard inner part can be a drupe, no matter the origins of the parts. Some avoid the controversy altogether by calling the walnut fruit a "drupe or nut" or explaining that it's a nut "but some experts call it a drupe."

That's more than enough discussion of disputed terminology. Let's turn now to something for which there is widespread and probably unanimous agreement. The Black Walnut is a tough nut to crack, in fact one of the toughest!

Cracking a Black Walnut Appalachian style (Blind Pig & the Acorn).

Black Walnut nutmeats are available commercially thanks to "high-tech" processing (video here). But there are alternatives. For those who aspire to collect, clean and crack Black Walnuts themselves, a wealth of helpful information is available online. The most common approach is a hammer, as Mark Catesby recommended nearly three centuries ago. Iowa State University Extension and Outreach explains:

"The hammer method involves placing the nut, pointed end up, on a hard surface and striking the point with the hammer until it weakens and splits into sections along its axis ... shattering of the kernels is often a problem. Shattering can be reduced by soaking the nuts in water for 1 or 2 hours before cracking. The soaking process allows the kernels to absorb enough moisture to become somewhat flexible, resulting in larger kernel pieces."

"Once split, use a pick or plier to remove the kernels inside." (ISU Extension & Outreach)

And for those of us with less patience, John Sankey includes a variety of tools on his Black Walnut Crackers webpage, with tips on use.

The Duke Black Walnut Nutcracker is highly-recommended ($78 on Amazon).

Sources, in addition to links in post

Catesby, M. 1731. Natural history of Carolina, Florida and the Bahama Islands. Volume 1 (Black Walnut p 67). "The illustrations were etched by the author from his own drawings and hand colored under his direction." Catesby also paid for printing. BHL

Gray, A. 1868. Gray's lessons in botany and vegetable physiology. NY: Ivison, Blakeman, Taylor & Co. (BHL).

Gray, A. Circa 1908. Gray's new manual of botany. A handbook of the flowering plants and ferns of the central and northeastern United States and adjacent Canada. NY: American Book Co. BHL

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

Linnaeus, Carl. 1753. Species Plantarum 2, p. 997. BHL.

Michaux, F-A. 1819. The North American sylva, or A description of the forest trees of the United States, Canada and Nova Scotia. BHL

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

Reveal, JL, et al. 1987. On the identities of Maryland plants mentioned in the first two editions of Linnaeus' Species plantarum. Huntia 7:209–246. PDF