The Monthly Fern: Bracken—dreadful or delightful?

"Shelter" by Colin.

Come my sweet and let us lie in

Some idyllic wooded glade

And let us stay til merry-made

Amid the Bracken.

For April, the South Dakota fern-a-month series features the world's most widespread fern—Pteridium aquilinum, Bracken (aka Pasture Brake, Eagle Fern, Helecho Macho, and more). Thought to be native to the Northern Hemisphere, it's now widely naturalized and known from all continents except Antarctica. In South Dakota, it grows in the Black Hills in the western part of the state (1).

Bracken is said to thrive in a variety of habitats—woodlands, fields, old pastures, thickets, disturbed soils, burned areas, and marshes. But sources vary on this. For example, some say it's intolerant of wet soil and shade; others say it can grow well in all but very alkaline soils. In any case, Bracken forms extensive colonies of robust plants to 1.5 m or more tall, from deep rhizomes to 20 feet long. The large triangular leaves (fronds) are twice or thrice pinnately compound (2- or 3-times divided into leaflets). When fully grown, the blades often bend to horizontal, shading much of the ground (source).

Leaf division is an important character in fern identification, but can be hard to understand and explain. However I will try. Below is a thrice pinnately compound Bracken leaf. It's divided into 9 large segments (one terminal), which are divided into many narrow segments, which are divided (or nearly so) into small ultimate segments. "Pinnately compound" means segments line up on each side of an axis (rachis). More here.

Bracken frond by Olegivvit (labels added).

As in most ferns, Bracken's spores are borne on the underside of leaves in clusters called sori (remember?). In Bracken, sori are continuous along leaflet margins. In youth, the sori are covered; with maturity, leaflet margins unroll, exposing mature sporangia (spore shooters).

In Bracken, young sori are protected under rolled leaflet margins, sometimes with tiny membranous indusia (flaps, click image to view); Zharkikh photo.

In this frond, leaflet margins have unrolled and lines of brown sori are visible; Zharkikh photo.

Brown "beads" are sporangia; each contains many minuscule spores ready to be "shot"; Zharkikh photo.

My first encounter with Bracken was in the Bear Lodge Mountains in the northwest Black Hills. That was at least 40 years ago, but the memory remains vivid. In a stand of tall quaking aspen, Bracken's horizontal fronds formed a lovely lacy ground cover. It was an idyllic setting, and still comes to mind when I think about Pteridium aquilinum.

That memory prompted me to search for a bit of English poetry about Bracken (it has close ties to moorlands). But it was rarely mentioned, and never in an idyllic setting (2). Perhaps poets know of Bracken's reputation. It's not a particularly nice fern, and there are many reasons to dislike it. The horrors that follow were provided by Robbin Moran, a man who who loves ferns!

Bracken grove with conifers and ferns and little else; Charlesblack photo.

The common objection to Bracken is that it's weedy—in fact a noxious invader. With its vigorous growth and colonial habit, it swamps (with litter) or shades out other species. Eradication is difficult due to its deep extensive rhizomes.

Other dangers lie hidden. Bracken is filled with nasty stuff, ostensibly for defense against insects and other herbivores. It contains at least two kinds of insect hormones, which cause uncontrolled molting and death in any insect that eats it. It also contains thiaminase, an enzyme that breaks down vitamin B1. This makes Bracken hazardous to livestock, which often find it palatable. Overconsumption will cause thiaminase-induced staggers (treatable with vitamin B1, thiamine).

And there's more. Bracken is rich in tannins and therefore bitter-tasting, which is good. For if consumed, for example in the absence of other forage, tannins inhibit enzymes critical to cellular metabolism. Bracken also produces a deadly chemical weapon, hydrogen cyanide, in response to tearing of leaf tissue, thereby deterring or killing the perpetrator.

Gosari, a popular Korean dish of Bracken fiddleheads; Hyeon-Jeong Suk photo.

Bracken fiddleheads (young shoots, also called croziers); Phil Gayton photo.

After reading of Bracken's many hazards, I was surprised to learn that people happily eat its fiddleheads. But this is dangerous too. Though cooking removes tannins and thiaminase, carcinogens remain, and increased rates of stomach and esophageal cancers have been reported where fiddleheads are popular, for example in Japan, Korea, and Britain.

The main carcinogen is the compound ptaquiloside (3). It occurs throughout Bracken plants, but is highly concentrated in young growth, in spring and early summer. Humans take up ptaquiloside mainly by eating fiddleheads, but there are other sources—airborne spores, milk and meat from affected animals, and contaminated ground and surface water where Bracken grows (source).

Now that I know of Bracken's nefarious ways, do I feel foolish about my early love affair? No, for it also offers delights, as the Radnoshire Wildlife Trust in Wales explains: "Bracken can be an important habitat in its own right.  It supports over 40 species of invertebrate, forming an important part of the diet for 27 of these while 11 are found only on bracken. It is an important breeding habitat for moorland birds ... and reptiles and mammals benefit from its shelter." It's also a great candidate for areas in reserves and gardens where nothing else will grow. Just keep an eye on it!

Finally, Bracken is beautiful, in fact so beautiful that its lovely lacy fronds make a woodland irresistible. Even Robbin Moran agrees. "The grove seems so peaceful and idyllic" he writes, though he knows that in the shadows there lurks a femme fatale.

"Afternoon light" by Colin.

PS Last month I promised to include the fern life cycle in April's Monthly Fern post. But being overwhelmed by Bracken's dark side, I've postponed "the bugbear of botany students" until May. You're off the hook for now! But this also means you must wait for an answer to the burning question: "How many average-sized fern spores does it take to fill a can of Coke?" Stay tuned.

Notes

(1) I'm surprised Bracken hasn't been reported from eastern deciduous forests in the far eastern counties of South Dakota. Looks like good habitat to me, and it grows in Minnesota not all that far away.

(2) In the absence of suitable poetry, I wrote my own (I promise I won't do it again). I did find a poet named Bracken, as well as Bracken, a literary magazine: "Bracken is green and lush, coarse and delicate, drinks from the earth, and spreads underground, more root than frond. Bracken is understory, invades, takes over, shades and protects. We seek poetry and art that will root, tender and tough, in us."

(3) At least one of Bracken's insect hormones also is a carcinogen (source).

Sources

Moran, RC. 2004. The Natural History of Ferns. Timber Press.

Royal Horticultural Society. Advice—Bracken.

Stone, J. A successful fern, or a case for control. Radnorshire Wildlife Trust (blog).

Peculiar Eruptive Mountains on the Colorado Plateau

The La Sal Mountains rise 8000+ feet above the Colorado Plateau (source).

In 1875, two geologists employed by the US government were studying mountains in southeast Utah. They worked 90 miles apart, each one in an isolated cluster of peaks rising above the mostly horizontal Colorado Plateau. They found the same strange type of structure and the same kinds of igneous rocks, and in their reports published two years later, they reached the same conclusions.

Albert Charles Peale was an employee of the War Department, specifically the Geological and Geographical Survey of the Territories led by Ferdinand Vandeveer Hayden. His party—one geologist, two topographers, two packers, and a cook—was surveying the Grand River District in western Colorado and eastern Utah (1). They spent a week in the Sierra la Sal, "which afforded magnificent opportunities for work" and then headed south. But hostile locals ("Indian trouble") brought field work to a sudden end. In their hasty exit, "all [rock] specimens had to be abandoned."

Grove Karl Gilbert was an employee of the Department of the Interior, specifically the Geographical and Geological Survey of the Rocky Mountain Region led by John Wesley Powell. On his descents of the Colorado River, Powell had seen an unmapped cluster of peaks to the west, which he named the Henry Mountains (2). They looked volcanic—domed, with dark lava-like rock on the top. Volcanology was a young science then, and geologists were debating whether volcanos were elevated craters or built from accumulated lava. So Powell sent Gilbert to the Henrys to "determine the facts" (Hunt 1988). He and his party stayed two months, more than enough time to answer the volcano question.

Peale worked in the La Sals, Gilbert in the Henrys; map based on data from National Atlas, labels added.

That winter Peale wrote up his findings, but two years would pass before Geological Report on the Grand River District was published (3). He estimated they had surveyed 6000 square miles of which "the greater part ... is plateau in character, the Sierra la Sal being the only mountain group." It was an isolated cluster of about 30 peaks arranged in three "eruptive centers". Peale was emphatic about origins: "there can be no doubt of the eruptive character of the mountains... porphyritic trachyte has been pushed up through the sedimentary layers which now dip away from the mountains" (Peale 1877a).

Peale called the La Sals "eruptive mountains of a peculiar type ... igneous and yet non-volcanic" (1877b). They were non-volcanic because lava didn't reach the surface. But neither were they plutons emplaced deep underground. Instead, magma had stopped somewhere in between, deforming the overlying rocks. The intruded rock was exposed much later by erosion. There was no name for this type of structure, so he described it in detail, pointed out its peculiarity, and left it at that.

Sections across Sierra la Sal showing tilted sedimentary strata on intruded trachyte (Peale 1877a, cropped).

From the Sierra la Sal, Peale studied the Henry Mountains off to the west. He knew John Wesley Powell (Gilbert's boss) thought they were volcanic—"the summits of these mountains mark in reality the level of former valleys down which the volcanic material flowed" (Powell 1875, quoted by Peale). But even from ninety miles away Peale could see that was incorrect. "l am inclined to class the Henry Mountains with the Sierra la Sal and Abajo [Mountains], as their outline is similar ..."

From his vantage point in the heart of the Henrys, Gilbert "agreed" with Peale (unknowingly). The peaks were neither elevated craters nor accumulated lava nor even volcanic. In fact they were a novel type of structure, as he warned his readers:

"If the structure of the mountains be as novel to the reader as it was to the writer, and if it be as strongly opposed to his preconception of the manner in which igneous mountains are constituted, he may well question the conclusions in regard to it while they are unsustained by proof. I can only beg him to suspend his judgment until the whole case shall have been presented." (Gilbert 1877)

Gilbert gave the novel structure a name—laccolite—thereby making the Henry Mountains the type locality for laccoliths (today's term). He distinguished them from volcanic eruptions, where lava reaches the surface and accumulates. "The lava of the Henry Mountains behaved differently ... it stopped at a lower horizon, insinuated itself between two strata, and opened for itself a chamber by lifting all the superior beds."

Gilbert's sections across the familiar Mountain of Eruption (volcano) and the novel Laccolite.

Like Peale, Gilbert had to wait two years for publication of his findings. He finished his monograph the winter after his second season in the Henrys. "It was at once put in type, and in anticipation of a speedy issue the current year [1877] was marked on the imprint..." But the many illustrations caused delays. Geology of the Henry Mountains was finally bound and distributed in 1879.

By that time a wealth of information about igneous mountains had accumulated, prompting Gilbert to prepare a second edition (1880). It differed from the first mainly in the addition of an Appendix: Recently Published Descriptions Of Intrusive Phenomena Comparable With Those Of The Henry Mountains. At the end of the section about Peale's findings in the La Sals, Gilbert concluded, "All of these features are paralleled in the Henry Mountains and they leave no reasonable doubt that the structures are identical."

I visited the Henry Mountains in 2012, accompanied by the spirit of Grove Karl Gilbert. I camped at Starr Springs as he had, and hiked to the spectacular south face of Mount Hillers, "revetted by walls of Vermilion and Gray Cliff sandstone" as he explained.

South face of Mount Hillers—steeply tilted sandstone on flanks, intruded trachyte on crest (Jack Share).

Vermillion sandstone "tilted almost to the vertical".

Since then, I've been keen to visit more of the peculiar eruptive mountains on the Colorado Plateau. Last September I finally did, spending a week in the Sierra la Sal.

La Sals upper right; snow highlights 3 clusters of peaks. Upheaval Dome upper left. (Google Earth)

Peale's three eruptive centers live on, though they're now called intrusive centers ("eruptive" means volcanic). But these are special intrusions—emplaced at depths intermediate between volcanos (surface) and plutons (deep). They now have their own descriptor—hypabyssal, aka subvolcanic (but still laccoliths).

Two hypabyssal intrusion-cored peaks: Castle Mountain (left) retains a cap of sedimentary rock; La Sal Peak (right) is trachyte (Ross 1998, cropped).

The three intrusive centers of the La Sal Mountains are conveniently named northern, middle, and southern (4). All normally are accessible via the paved Loop Road, but road construction kept me in the northern one. From a very nice small primitive campground, I hiked to see what I wanted to see—the distinctive features of these peculiar eruptive mountains.

It was a short walk to Castle Valley Overlook with views of the Colorado Plateau. The Plateau doesn't look horizontal, but the rock layers are. The spectacular landforms—towers, buttes, rims. deep winding canyons—are erosional. True uplifts like the La Sal Mountains are uncommon. Gilbert called them "disturbances in a region of geological calm."

Looking northwest near Castle Rock Overlook; in the valley bottom left of center is Round Mountain, a small intrusion perhaps connected to the La Sals.

Let's head on down and see what we can see.

Among Peale's important observations were tilted sedimentary strata that "now dip away from the mountains". He concluded they were pushed up and tilted by rising magma. The hike provided good views of steeply tilted sedimentary rocks.

Nearly vertical beds of reddish sedimentary rocks below trachyte slopes of Grand View Mountain (left); high peaks visible on horizon just right of center.

With part of the Loop Road closed, the high peaks weren't easy to access. So the next day we hiked up a rough dirt road to view trachyte. It's common above the flanking sedimentary rocks, forming steep slopes and discouraging travel as Peale noted. "The only difficulty met with in the study of this interesting region is the great amount of debris that has accumulated ..."

Some kind of outcrop (could be rhyolite) beyond steep slope of "debris".

Trachyte with a dark xenolith—country rock broken off and carried up by magma. 

Fall colors on trachyte.

Mount Peale, a large laccolith and high point of the La Sals (Suffusion of Yellow).

Peale was hesitant to identify the igneous rock of the La Sals, without specimens to give to petrologists for "critical examination". But it looked very much like rock he had seen in similar intrusions in Colorado. So he assigned it to a general category—porphyritic trachyte. It seems trachyte was the accepted name for shallow intrusive rocks low in silica in Peale's time (see Appendix in Gilbert 1880). Now it may be trachyte or diorite, depending in part on whom you ask (5). Being very much a 19th century naturalist at heart, I will stick with trachyte.

On the other hand, everyone agrees the rock is porphyry—visible crystals (phenocrysts) in a fine-grained matrix of trachyte. This is a very cool rock, with an interesting history. As the magma rose it gradually lost heat, eventually dropping to a temperature where hornblende and plagioclase formed crystals. This changed the composition of the remaining molten magma, and when it stopped c. 6–10 km below the surface, it rapidly crystallized to form the trachyte matrix (Ornduff et al. 2006; see also Fractional crystallization).

Porphyritic trachyte mementos from Henry Mountains (left) and La Sal Mountains.

As I drove away from the La Sals, I thought a lot about the pioneering geologists of the American West. Like me, they were inspired by geology and the beauty of the landscapes, but their geotripping was very different. Travel (route-finding required) and camping were much more challenging. And where specifically should they go? (no guidebook). However they had the promise of discovery, which surely made up for all the hardships!

AC Peale and two unidentified men, probably during the
Geological & Geographical Survey of the Territories (Smithsonian Archives).

Notes

(1) The Grand River was the section of the Colorado above the confluence with the Green. Its name was changed in 1921.

(2) Powell named the cluster of peaks for Joseph Henry of the Smithsonian Institution, who helped secure funding for Powell's exploration of the Colorado River.

(3) Publication of Peale's report was delayed through no fault of his own. As his boss, FV Hayden explained, it was caused by "the great increase of labor incident to the International Exposition at Philadelphia", labor that would have gone toward preparation of reports. In general, the regular Reports of the Geological and Geographical Survey of the Territories were inadequate for sharing discoveries. In 1874, a new publication—Bulletins— was created to "publish without delay ... new or specially interesting matter". Peale had an article in Bulletin No. 3, about the peculiar eruptive mountains of Colorado and adjacent Utah, including the La Sals (1877b).

(4) Some sources refer to the La Sal intrusive centers as composite plutons or coalesced intrusions.

(5) Ross (1998) reported that La Sal igneous rocks were 59–71% Si02 (silica), and called them trachyte based on "the Total Alkali-Silica classification of LeBas and others". Wilson and others (2016, based on reports from 1953, 1959, and 1992) reported that igneous rocks of the Henry Mountains were 58–63% SiO2, and called them diorite. (Thanks to Mike for taking a stab at trachyte vs. diorite.)

Sources

Bartlett, RA. 1962. Great Surveys of the American West. Norman, OK: University of Oklahoma Press.

Fillmore, R. 2011. Geological evolution of the Colorado Plateau of eastern Utah and western Colorado. Univ. Utah Press.

Gilbert, GK. 1877. Report on the Geology of the Henry Mountains. GPO. BHL.

Gilbert, GK. 1880. Report on the Geology of the Henry Mountains. 2nd edition. GPO. Google Books PDF. Appendix p 153–161 contains added material about igneous mountains.

Gould, LM. 1927. Geology of the La Sal Mountains, Utah Papers of the Michigan Academy of Science, Arts and Letters Vol. 7: 55-106. HathiTrust

Hunt, CB. 1958. Structural and igneous geology of the La Sal Mountains, Utah. USGS Professional Paper 294-1. PDF

Ornduff, RL, Wieder, RW, Futey, DG. 2006. Geology Underfoot in Southern Utah. Mountain Press Publishing. (see Vignette 32, Intruders in a sedimentary domain)

Peale, AC. 1877a. Geological report on the Grand River District, in Hayden, FV. Ninth annual report of the United States Geological and Geographical Survey of the Territories (p. 31–101). BHL

Peale, AC. 1877b. On a peculiar type of eruptive mountains in Colorado. Art. XVIII in US Geological and Geographical Survey of the Territories Bulletin No. 3: 551–564. BHL

Powell, JW. 1875. Exploration of the Colorado River of the West. Geographical and Geological Survey of the Rocky Mountain Region (Henry Mts p. 200-203).

Ross, ML et al. 1998. Geology of the Tertiary intrusive centers of the La Sal Mountains, Utah; influence of preexisting structural features on emplacement and morphology in Laccolith complexes of southeastern Utah; time of emplacement and tectonic setting. USGS Bull. 2158: 61-83. PDF

Wilson et al. 2016. Deformation structures associated with the Trachyte Mesa intrusion, Henry Mountains, Utah, Implications for sill and laccolith emplacement mechanisms. J. Structural Geology 87: 30-46. free online