A Plant Friend Most Would Pass By

Green leaves and red stems just above center are the plant of interest.

“When I discovered a new plant, I sat down beside it for a minute or a day, to make its acquaintance and hear what it had to tell.” John Muir, in Explorations in the Great Tuolumne Cañon, 1873.

John Muir—our great naturalist and conservationist—often wandered alone in the Sierra Nevada of California, passionately admiring and studying the plants. The vast wilderness enraptured him, but it also brought on feelings of loneliness at times. Perhaps that’s why he referred to plants as “friends” (1).

Muir came to mind last June, as I got to know a new plant friend near South Pass, at the southern end of the Wind River Mountains. For days I wandered through expansive sagebrush gardens bright with displays of pink, yellow, blue and white spring flowers. But I had to ignore them. I was being paid to search for rockcresses (genus Boechera, formerly Arabis)—small thin drab easily-overlooked plants.

But I didn't mind too much. I love the sweeping landscapes and granite blobs of the South Pass area. So does my field assistant.

Strolling with eyes glued to the ground, I pondered my fate—consigned to survey a challenging and esthetically unremarkable plant, the russeola rockcress (Boechera pendulina var. russeola). It is neither showy nor rare. Even worse, it no longer exists according to the latest treatment of the genus Boechera. So or course the more I thought about it, the more I became enamored of this plant! (I’m biased towards underdogs)

“But we know that however faint, and however shaded, no part of it is lost, for all color is received into the eyes of God.” John Muir (unpublished Pelican Bay Lodge manuscript)

The flowers of the all various rockcresses are small, white-to-purple, and 4-petaled. They’re useless for identifying plants to species. Mature seedpods (siliques) and basal leaves are required. One of the first things I learned from the russeola rockcress was that it can be recognized by the combination of pendulous siliques, reddish stems (hence russeola), and ciliate-margined but otherwise bare basal leaves. This probably sounds impossible to spot at the scale of these plants (a few inches tall at most), but the power of a well-developed search image is astonishing. Let’s have a look.

Below is a specimen from the Rocky Mountain Herbarium, University of Wyoming. Note the tiny white flowers. The normally pendulous arrangement of the siliques was distorted with pressing. The reddish color of the stems disappeared with drying.

The next photos show just how un-photogenic russeola rockcress is in the field, in part because the area is often windy. Still, the distinguishing features are obvious once one gets to know them. And fortunately, russeola prefers sparsely vegetated microsites.

Green leaves with ciliate margins (coarse hairs), reddish stems, pendulous blurry siliques.

Handlens is 2.5 cm long (1 in).

Sometimes the shadows are more obvious than the seedpods that cast them.

Distinctive upright green basal leaves at pencil tip, with reddish stem waving in the wind.

Ciliate-margined leaves; up close those hairs look gnarly! (click on image to view)

Though not rare overall, the russeola rockcress is uncommon in the South Pass area, which makes hunting for it fascinating—why does it only grow where it does? Muir pondered the same question; for him, learning why plants grow where they do was learning a bit more about the marvelous work of God.

After four long days of searching, I had learned that russeola (we're now on a first-name basis) is indeed restricted to rock. But it doesn’t grow in rock, i.e., not in crevices. That’s the habitat of the littleleaf rockcress, Boechera microphylla. Russeola prefers pockets of gravelly soil (decomposed granite) that develop in low, almost ground-level exposures of rock at the base of the big granite blobs.

Collection of littleleaf rockcress from South Pass area. Note upright very thin siliques.

Low rock outcrops such as this are prime targets for russeola survey.

Russeola grows on coarse granitic soil that develops in pockets in the low outcrops.

Russeola generally prefers less vegetated areas, such as this low ridge (dike?).

What's going on here?!

Dropseed rockcress, Boechera pendulocarpa (not to be confused with "pendulina"), frequently occurs with russeola on the same microsites. It’s the gray plant in the middle of the photo above, along with two russeola plants. But dropseed rockcress is a less picky plant, and it grows in a variety of habitats.

Though we’re now good friends, russeola has not revealed all of its secrets. I looked at a LOT of what appeared to be perfectly good habitat, but russeola wasn’t there. I wasn’t disappointed, or even surprised. After all these years I know that plants often don't grow everywhere it seems they could. In fact we should expect that, for a plant has to get to those perfect places—a seed has to land there. The great plant ecologist Henry A. Gleason made this clear, yet we often forget, perhaps driven by the human need to predict.

“Does a plant always grow in every habitat suitable to it and over the whole extent of the habitat? The answer is emphatically no. As previously stated, plants attain their range by migration. Possibly this plant is on its migratory way and has not yet arrived. … Possibly it has only recently arrived and has not yet had time to spread over the whole extent of the habitat. Possibly it is meeting with such strenuous competition from other plants that only a few individuals have a chance to grow.” Gleason and Cronquist, The Natural Geography of Plants, 1964 (2)

Russeola grows on bare gravelly soil at the base of this small outcrop.

Finally, some readers may wonder why was I paid to look for a plant that’s neither showy nor rare, nor even recognized by “experts.” I will try to explain. This is a long and winding tale that the faint of heart may wish to skip.

The main objective of our project was to locate additional populations of the small rockcress, Boechera pusilla, a globally-rare plant known from a single population (near South Pass). However, in the latest revision of the genus, authors Al-Shehbaz and Windham inadvertently (I think) expanded B. pusilla to include plants formerly called B. pendulina var. russeola. Does this mean that the small rockcress is no longer rare? No. It means the key and species descriptions were poorly constructed.

The authors lumped together the two varieties of Boechera pendulina—the typical one and russeola—though they acknowledged there’s no evidence they are conspecific. As a result (unintended), their key and descriptions do not address the plants we call russeola. Russeola material now keys to small rockcress (the very rare one), but only because there is no better match. As we discovered during post-field season herbarium study, russeola rockcress and small rockcress clearly are different. Hoping to eliminate the confusion, we collected and preserved leaves for DNA analysis, but Al-Shehbaz and Windham declined the offer, explaining they had insufficient funding to add another sample.

Thus russeola’s taxonomic status remains in limbo (3). But who cares?! The plants certainly don’t. Whatever we call them, these plants are real, and I’m happy to have made their acquaintance.

A dense “stand” of russeola—not a common situation.

Notes

(1) Muir’s passion for plants and his botany adventures are wonderfully recounted in Nature’s Beloved Son; rediscovering John Muir’s botanical legacy (Gisel and Joseph 2008).

(2) The first 10 chapters of The Natural Geography of Plants, including the discussion of why plants are restricted in distribution, were written by Gleason. Arthur Cronquist completed the book (second half) at Gleason’s request.

(3) Other experts recognize the russeola rockcress as a valid taxon. For example, our material clearly keys to and fits descriptions of Boechera pendulina var. russeola in Vascular Plants of Wyoming 3rd ed. (Dorn 2001) and Rollins’s 1982 treatment of North American Arabis (Boechera).

Yermo--a flamboyant and mysterious rare plant

Wyoming high desert with red indian paintbrush, blue larkspur, and lots of yellow flowers (in foreground; click on image to view). But where's yermo?

I walked slowly, zig-zagging uphill, scanning the ground, checking everything yellow—yellow flax, yellow wild buckwheat, various DYCs (damn yellow composites). But no yermo. Had I missed it? Was my attention flagging in the heat and wind? “Go slow, look carefully” I said, and wandered back downslope but further north. Still nothing. Uphill again. Near the top I stopped ... and burst out laughing. There it was—spectacular! Yermo can't be overlooked.

Yermo’s astonishing flamboyance.

Few Wyoming rare plants are as showy as yermo. It really stands out in the high desert setting, where aridity and wind favor low profile plants. Yermo is tall by local standards—up to a foot in height. The leaves are large, soft and fairly green, while most of its neighbors have dull tough drab little leaves, often covered in hair to keep precious moisture from evaporating.

Yermo in the ‘hood.

Yermo isn’t totally without desert adaptations. Up close, one sees the leaves are leathery and look a bit waxy. It has a substantial taproot that surely helps during hard times.

Yermo xanthocephalus, from the original description of the species (Dorn 1991).

In 1991, Bob Dorn, who knows the Wyoming flora well (he wrote Vascular Plants of Wyoming), was searching for a rare Phlox when he “encountered a very unusual plant”—as he wrote in his description of the new species (Dorn 1991). In fact, Yermo xanthocephalus wasn’t just a new species; it was unusual enough to be a new genus!

Yermo translates to “wilderness” or “wasteland”—an uninhabited place; xanthocephalus means “yellow head.” Thus the official common name is desert yellowhead, but "yermo" is used just as often, if not more.

Yermo is a member of the sunflower or daisy family, the Asteraceae. Like all members, it has composite heads of flowers. In this case there are just 4-6 flowers per head.

Five heads with just a few flowers each (the two lower heads haven't opened yet).

The distinctive keeled yellow bracts that enclose each small head are more obvious in bud.

Rosettes of leaves are young plants. Hopefully they'll produce flowering stalks in some future year.

There aren’t many yermo plants in the world. All of them grow in a tiny area of high desert, on the order of 35 square miles, in central Wyoming. For almost 20 years, botanists thought yermo was restricted to a single site. “We have searched far and wide for additional plants,” wrote Scott and Scott in 2009, “especially at sites with similar geological, geomorphological, and climatological characteristics, with no avail.” But then …

In 2010, a new population was discovered, on an escarpment “that had been intensively surveyed for many miles” except for a gap of about a mile and a half (Heidel et al. 2011). Plants will do that—defy human analysis and prediction. They may be completely absent from what we’ve identified as potential habitat, and then when we’re about to give up, we find them where they "shouldn’t be!”

Yermo mid photo, growing where no one looked until 2010.

Prior to 2010, many botanists visited yermo, and carefully studied its habitat. It was described as level to gently-sloping sparsely-vegetated outwash at the base of eroded slopes. Plants were more dense in concave areas or depressions, possibly sites of snow accumulation. Compared with adjacent sagebrush areas, yermo soils were slightly finer, more alkaline, and less able to retain moisture.

Original yermo site, where it grows on sparsely-vegetated outwash (click on image to view). WYNDD photo.

Everyone assumed yermo was an extreme habitat specialist. If there were more populations, they would be on similar sites. But yermo fooled the botanists. In 2010, it was found about five miles from the original population, not on sparsely-vegetated outwash but rather on the upper slopes of an escarpment, with bluebunch wheatgrass and junegrass, diverse forbs, and low shrubs. When I visited this site, I estimated vegetative cover to be 25-50%—not sparse. In addition, soil analysis has shown that the two sites differ significantly for at least 9 of 17 parameters measured (Heidel et al. 2011). Obviously we need to be more openminded.

The original population contains on the order of 10,000 plants—with several areas of concentration, scattered plants in between, and outliers. The second population contains 400 plants, in seven small isolated patches. Do these represent dispersal events? Is yermo spreading and colonizing new habitat?

In addition to its unusual robustness and limited inconsistent distribution, yermo is odd in another way. Many of our rare plants are difficult to distinguish from their close relatives; the South Pass rockcress, which I wrote about recently, is a prime example. But not yermo. Not only is it easy to recognize, it looks like nothing else! We have few clues as to its close relatives; we can only speculate.

Discoverer Bob Dorn considers yermo most closely related to several eastern North American species. He suggested it’s a relic of warmer wetter times when this part of Wyoming was forested—about 20 million years ago. That's when the sediments and ash that make up the rocks where yermo grows were deposited. With climate change—drying and cooling—forests retreated eastward, replaced by grassland and then desert. But it’s unlikely that yermo has been around that long. For one thing, diverse environments have intervened over the last 20 million years, including periglacial permafrost during Pleistocene times. Generally, there’s no reason to associate the age of a species with the age of the rock where it grows.

Still, yermo may be a relic species, just more recent. Equally possible, given how little we know, it may be a truly “new” species—the product of recent catastrophic evolution. Plants can make evolutionary leaps, for example through hybridization or genome duplication (multiple times even!). The resulting new species may look quite different from the parents.

Maybe when genome sequencing and analysis become really cheap, we can compare the full genomes of yermo and candidate relatives and figure all this out. I sure hope so! I would love to know yermo’s story. Why is it only here? Is it rare because it’s a holdover from a different time and environment? Or is it rare because it evolved recently and hasn’t had time to spread? Who are its parents? What’s its future? Will it persist, disperse seeds, and colonize new sites for botanists to find?

In 2002, Yermo xanthocephalus was listed as a Threatened species by the US Fish and Wildlife Service due to rarity and potential threats. Discovery of the second population in 2010 did not significantly change its status.

Sources

Dorn, RD. 1991. Yermo xanthocephalus (Asteraceae: Senecioneae): A new genus and species from Wyoming. Madroño 38:198 – 201.

Heidel, B., Fertig, W., Blomquist, F., and Abbott, T. 2008. Wyoming's Threatened and Endangered Species: Yermo xanthocephalus (desert yellowhead). Wyoming Bureau of Land Management, Cheyenne, WY. In collaboration with Wyoming Natural Diversity Database.  PDF

Heidel, B., Handley, J., and Andersen, M. 2011. Distribution and habitat requirements of Yermo xanthocephalus (desert yellowhead), Fremont County, Wyoming. Report prepared for the USDI Bureau of Land Management - Wyoming State Office by the Wyoming Natural Diversity Database - University of Wyoming, Laramie, WY.  PDF

Scott, RW, and Scott, BJ. 2009. Yermo xanthocephalus Dorn, a research report. Prepared for Bureau of Land Management. Central Wyoming College Herbarium and Scott Environmental Resources, Inc. in cooperation with Wyoming Natural Diversity Database. Riverton, WY. PDF

Plants & Rocks: South Pass Rockcress, South Pass Granite

The lineup.

At the southern end of the Wind River Mountains near South Pass—where thousands of travelers on the old Oregon Trail crossed the Continental Divide “with no toilsome ascents”—granite mounds rise above rolling sagebrush grassland like irregular lumps of clay. This is a great place to hang out if you like to conjure up the far distant past, for they mark the southwest edge of North America 2.5 billion years ago.

Late Archean South Pass granite; botanist (center) and field assistant (lower right) for scale.

But that wasn’t why I was wandering around these outcrops … slowly walking, walking, walking ... staring at the ground. I was searching for plants that grow nowhere else in the world. One has to look hard to find them. They’re inconspicuous, small, drab, and have no flowers this time of year. Not that flowers would help—they’re also inconspicuous, small, and drab.

The South Pass rockcress (aka small rockcress), about 10 cm tall; drawing by Isobel Nichols (source).

Thirty years ago, the South Pass rockcress (Boechera pusilla) was one of the targets in my first rare plant survey project. It was known from a a single location, which was only vaguely specified: “in cracks and crevices of huge metamorphosed [actually igneous] rocks off Wyoming State Highway 28, 39 miles southwest of Lander,” collected in 1981.

Boechera is part of the mustard family (Brassicaceae). It’s a large genus of subtly-different species—109 in North America, and at least 25 in Wyoming (experts disagree on classification). The group is thought to be actively-evolving (source), which may explain why rockcresses are tough to identify, and why field botanists are happy to ignore them. But I couldn't.

The flowers are small, with little variation among species, so we depend on mature fruit—siliques—for identification. Those of the South Pass rockcress are relatively broad (to 2 mm) and arch downwards but not sharply so.

Boechera pusilla, with spreading-descending relatively-broad siliques (seed pods).

One also has to examine the hairs on the basal leaves, visible with a 10x hand lens (click on images below to enlarge). Are they sparse, and simple to few-branched (South Pass rockcress)? Or are they dense, and branched multiple times (other species in the area)?

Sparse simple-or-forked hairs on basal leaves of Boechera pusilla.

Dense fine dendritic hairs on basal leaves of Boechera pendulocarpa (dropseed rockcress).

These are tough decisions but unavoidable because at least four other rockcresses grow in the South Pass area. Some of them hybridize, making intermediates. It’s a mess.

[Boechera used to be part of Arabis] “The taxonomic complexity of Arabis, in the broad sense, is legendary … most of the problematic taxa come to reside in Boechera. A rare confluence of hybridization, apomixis, and polyploidy makes this one of the most difficult genera in the North American flora [emphasis added] (source).

Maybe you can imagine the agony of a young field botanist trying to find this rare rockcress among common ones. My strategy was to collect all the different rockcresses I saw, from multiple locations, and send them to the late Reed Rollins at Harvard University, the expert for the group. It was Rollins who collected the specimen from “huge metamorphosed rocks” in 1981. My collections came back from Harvard with only one labeled Boechera pusilla, the South Pass rockcress. Was it from the same location as Rollins’s 1981 specimen? We’ll never know, but in any case, this rockcress was obviously rare.

In fact, it's rare enough to be a Category 1 candidate for Federal listing under the Endangered Species Act. To avoid listing, the Bureau of Land Management has taken steps to protect it, including closing a four-wheel-drive road to the site, and funding regular monitoring.

We don't mind that we now have to walk to see the South Pass rockcress!

Yours truly, counting South Pass rockcress plants in 1988, the first year of monitoring. WYNDD.

I returned to the single known South Pass rockcress site this year, to help with monitoring. Now I have 30 years of experience searching for rare plants, many of which are difficult to distinguish from common relatives (why is that?—another of life’s unanswered questions). It was a lot easier to recognize our target. After examining the various rockcresses at the site, I was comfortable making identifications in the field. A cheat sheet helped:

Modified from Heidel 2005.

The first step in a rare plant survey involves visiting known sites to develop a search image for the target species—a pattern that will really grab your attention. In this case it was the small cluster of green leaves below one or several stems with dangling siliques (pods). If I spotted such a cluster, I then glanced at the width and position of the siliques (relatively broad, spreading or descending). Finally I got down at plant level with my hand lens, and looked for the distinctive hairs on the basal leaves.

Armed with this search image, I went hunting. At the single known site, the South Pass rockcress grows at the base of a large outcrop of South Pass granite, in low rocky habitat with pockets of gravely soil, pretty much at ground-level (see monitoring photo above). I drove around until I found similar sites, and then slowly criss-crossed potential habitat, eyes glued to the ground. At the third site, I was stopped dead in my tracks by a cluster of green leaves below dangling siliques. The hairs on the basal leaves confirmed it—this was a new location for the South Pass rockcress!

South Pass rockcress grows among low rocks mid photo; main outcrop visible behind on right.

Microhabitat is gravely soil in pockets and crevices.

At knife-tip: cluster of green basal leaves, and three stems with drooping siliques. Not a photogenic plant!

Now the hard question—how much more is out there? How rare is the South Pass rockcress? There are many similar outcrops in the South Pass area … yet I had come up with only one specimen in all my collecting in 1986 … yet I didn’t really know what I was looking for then … and is it really restricted to just South Pass granite??! Obviously more work is needed.

The new site is exciting, but also discouraging. The population is really small, and with widely-scattered individual plants. Three of us, botanists all, went back two days later and it took us 15 minutes to find a plant. In all, we found only 11 after a thorough search of about 0.25 ha (half an acre). If this little rockcress sometimes (or often?) grows in small sparse populations, survey will be tough indeed.

Potential habitat: South Pass granite above rolling sagebrush grassland.

Finally, for the taxonomy geeks among us:

L to R: Boechera pendulocarpa, B. microphylla, B. pusilla.

Might the South Pass rockcress be a recently-evolved hybrid, rare because it hasn’t had time to disperse far? We know it’s an allotriploid (two sets of chromosomes from one parent, one from the other). Could it be the offspring of occasional crossing between the littleleaf and dropseed rockcresses? They’re common at both known sites, and …

“The sexual diploid species are relatively distinct from one another, but they hybridize wherever they come into contact [italics added]. Through apomixis and polyploidy, the hybrids become stable, self-propagating lineages. … for any pair of sexual diploid species (e.g., AA and BB), this process can yield different intermediates, including AB apomicts and both possible apomictic triploids (AAB and ABB). … Under these circumstances, even the most distinctive sexual diploid progenitors can become lost in a seemingly continuous range of morphological variability” (source).

Sources

Al-Shehbaz, IA, and Windham, MD. Boechera in the Flora of North America. http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=104152&key_no=2 (accessed 27 June 2016).

Heidel, B. 2005. Status of Boechera pusilla (small rockcress) in Wyoming. Prepared for the Bureau of Land Management. Wyoming Natural Diversity Database, Laramie, WY. https://www.uwyo.edu/wyndd/_files/docs/reports/wynddreports/u05hei06wyus.pdf

Marriott, H.J. 1986. A report on the status of Arabis pusilla, a Candidate Threatened species. Prepared for the US Fish and Wildlife Service by the Wyoming Natural Diversity Database, Laramie, WY.  Available here upon request.