Cholla Fields of New Mexico

On my trip down the Dry Cimarron River, across the High Plains, and then on to the Cimarron proper, I often saw vegetation brand new to me but apparently common in New Mexico. From a distance I thought it was grassland with heavy shrub cover, maybe mesquite. But soon the distinctive form of the “shrubs” was obvious: thickish stems, awkward branching, no leaves. These were cacti.

Dry Cimarron valley, land of cacti and clastic plugs (mid photo).

The cactus was unfamiliar, but easy to identify: Cylindropuntia imbricata, tree cholla. It’s large, three to seven feet tall, even 10 or 15 under favorable conditions. Often there’s a short “trunk” that soon branches. Spines are sparse, revealing green jointed stems. The yellow fruits stay on the plants through the off season. The most helpful features are the tubercles—bumps on the stems bearing specialized buds (areoles) that produce spines (more here). Cactus tubercles are usually described as “small rounded projections” but those of the tree cholla are large and elongate. The sparse spines make them easy to spot.

Yellow fruit from previous year still on plants in early May.

Stems are sometimes reddish in winter.

Prickly pear cactus (Opuntia polyacantha) has “small rounded” tubercles.

Distinctive elongate tubercles of the tree cholla.

Being a veg ecologist of the American West, I was suspicious of these cholla fields. Heavy shrub cover in grassland often indicates over-grazing. Livestock almost always prefer grass to shrubs, giving the shrubs a competitive advantage that can lead to dominance. How about tree chollas? Surely no cow or sheep would eat one if grass were available.

Grassland thick with tree chollas, Dry Cimarron Valley (with ghosts of travelers past).

Tree cholla grassland on the High Plains north of Clayton.

Tree chollas, just west of Cimarron.

Back at home, I looked for tree cholla vegetation types in the US National Vegetation Classification. I found a good match: Opuntia imbricata Ruderal Shrubland (Opuntia is a synonym for Cylindropuntia). My suspicions were confirmed:

“This anthropogenic (semi-natural) shrubland results from overgrazing of native grasslands. It occurs particularly in slightly moister settings along broad swales. Infestation of Opuntia imbricata [synonym for Cylindropuntia imbricata] is variable, and cover of Opuntia in this type ranges from 25% to nearly 100%. Grass cover and composition is highly variable, depending on site and grazing regime and history. This broadly defined community grades to grassland (often with some Opuntia cover) or to other grazing-induced shrubland communities, such as those dominated by Prosopis, Acacia, and/or Mimosa” [shrubs in the pea family] (NatureServe Explorer).

Now the inevitable counter-questions: What about bison (buffalo)? There was a time not all that long ago when bison numbering in the millions roamed the American West, including New Mexico. What did these grasslands look like then? How common was tree cholla? Did bison eat it as readily as grass? The answer: we don’t know and probably never will (add these to life’s persistent questions).

But these questions are irrelevant where tree cholla has been introduced. It’s now widely naturalized—in eastern and southern Europe, northern and southern Africa, southern South America, and eastern Australia, often to the detriment of grasslands. For example, in Queensland, it threatens native semi-arid grasslands, and has been listed as a priority environmental weed (source).

Ironically, while land managers try to find a way to eradicate tree cholla (e.g., fire, chaining, chemicals), gardeners are encouraging it. “C. imbricata is one of the best chollas for desert landscaping. … It is an attractive plant in desert gardens, and cultivation is quite easy” [for the same reason it’s so hard to eradicate in pastures]. “All you have to do is plant one of the ten-twelve-inch stems in the ground. Water it after it has a chance to put out roots, in a few weeks.” Saguaroland Bulletin v.30:no.1-10 (1976)

Summer flowers (NPS).

Tree cholla is not restricted to desert gardens. It has long been grown in Europe, though not without challenges:

“It is the commonest member of the group to which it belongs in European gardens, and was first introduced to cultivation in the earlier years of the nineteenth century. In spite of this, however, it is a plant whose flowers, which are very attractive but which will only expand under the influence of direct sunshine and unfortunately do not last long, are not often seen in the United Kingdom. Sir Edmund Loder informs us that his example at Leonardslee has but seldom flowered; when it did so in 1908 this happened during a time of very hot, sunny weather, and individual flowers only remained fully open during one afternoon, from about midday till sunset.” Curtis’s Botanical Magazine v. 135; 1909

Curtis's Botanical Magazine, 1909 (courtesy Biodiversity Heritage Library).

Guest Post: The Cactus Field(s) of Sheep Mountain

[Many thanks to Larry Haimowitz for this contribution! See bio at end of post.]

Driving to Sheep Mountain.

If you’ve ever driven west from Laramie to Centennial on Wyoming Highway 130 you’re likely familiar with this scene – that’s Sheep Mountain on the left. Most of the mountain is National Forest, but is surrounded by private property, which limits public access. You’ll find a very steep trail, just out of sight in this photo, near the NW corner of the mountain. Parking is very limited and there is no sign at the trailhead. Trails from the south end of the mountain have much gentler slopes.

Hiking Sheep Mountain, you can easily see half of all the kinds of cacti that grow in Wyoming. That’s not as impressive as it sounds: there are only seven species of cacti in the state (1, 2), three of which are certainly present on the mountain, and a fourth one that might be, though I haven’t found it yet. The most common of these are Opuntia polyacantha and Pediocactus simpsonii. The third, much less abundant, is Escobaria vivipara. You won’t have to walk more than a half mile from the trailheads at either end of the mountain to come across these three cacti, though you will have to look carefully to encounter E. vivipara. The fourth cactus, Echinocereus viridiflorus, if it is present, will be found around the base and lower slopes of Sheep Mountain.

For the most part, cacti on Sheep Mountain are solitary plants, spaced well apart from each other. Several years ago, I found an area so thick with cacti that I think of it as the cactus field of Sheep Mountain, and I think there are more just like it. I first discovered that place on a cold November morning.

A very steep trail starts from the NW corner of Sheep Mountain (above). On that particular morning, my faithful dog Tookie and I followed the fence that marks the western boundary of the National Forest (below).

The higher we went, the deeper the snow got.

After a while, we reached a windswept ridgeline, free of snow, and we started up the ridge because it was so much easier than walking in the deep snow. The ground was uneven and rocky underfoot, so I was constantly looking down to be sure of my footing. After what seemed like a long climb, I noticed something odd - little round balls of spines everywhere beneath my feet! Pediocactus simpsonii, so close together that it was hard to avoid stepping on them. There it was! A cactus field, a dense colony of Simpson’s ball cactus, covering several acres, something I’ve never seen the likes of.

Wind-blasted and sunbaked, with sparse, rocky soil, this highly exposed ridgeline looks like the most unlikely place for a plant to thrive. Oddly, in richer-looking habitats, Pediocactus is thinly scattered. I suspect that these cacti don’t like “wet feet” and need at least some full sun for development. The ridgeline would have better drainage and less competition from plants that shade the cacti.

This is what the plants looked like (above). Dormant for winter, they have lost water and shriveled to the ground to protect themselves from freezing, a common strategy for winter-hardy cacti. (Photo by Zoltan Kerner.)

Cacti of Sheep Mountain: Pediocactus simpsonii (Mountain Ball Cactus, Simpson’s Ball Cactus)

In the spring, the plants imbibe water and fatten up to prepare for flowering and growth. This is what they look like during the growing season. Plants may grow as single stems, rarely more than 4 inches in diameter, or clusters of two or more stems. Simpson’s ball cactus is the most abundant cactus on the mountain, and is widespread all over Wyoming and the intermountain west.

Close-up of Pediocactus stems.

Pediocactus simpsonii is one of three kinds of “ball” cacti in Wyoming that bear their spines on tubercles (also called nipples), which are easily seen in the photo above. The other two similar ball cacti belong to the genus Escobaria (1).

In our area, the plants start to bloom at the end of April and flower through the month of May. Flowers are about an inch across. Pink is the typical color, but there are occasional plants with white flowers.

Cacti of Sheep Mountain: Escobaria vivipara (Spiny Star, Pincushion Cactus)

Locally, Escobaria vivipara blooms later in May and into June, with flowers up to 2 inches across. Like Pediocactus, it grows as solitary plants or as clusters of stems, with each stem not more than a few inches in diameter, and it is also found over much of western North America.

How to tell the ball cacti apart:  The plants are easiest to tell apart when blooming. Escobaria vivipara has larger flowers with narrower petals, but this is not always a reliable character. A more definitive character is the origin of the flower buds. Flowers appear at the top of the plant in both of these genera, but on different parts of the tubercle. In Pediocactus, the flowers form on the tubercles next to the spines. In Escobaria, the flower buds are formed in the bottoms of the valleys between tubercles, far from the spines (1).

Cacti of Sheep Mountain: Echinocereus viridiflorus (Hedgehog Cactus, Green-flowered Hedgehog Cactus)

This is another small ball cactus, easily distinguished from the other two by spines that are born on ridges (referred to as ribs) instead of tubercles. Also, the flower buds develop lower on the plant instead of at the top (Yellow arrow pointing to rib with a flower bud). The green-flowered hedgehog cactus is found in the western Great Plains to the Rocky Mountains, from Texas and New Mexico to South Dakota and Wyoming.

The closest (to Sheep Mountain) recorded distributions of this plant are in the Laramie Range a little less than 20 miles east of Laramie (specimen here), and I have also found this plant to be abundant near the old stage stop of Virginia Dale, Colorado, 30 miles to the south. I have never seen Echinocereus viridiflorus on Sheep Mountain, but I suspect it may be present on the lower slopes. If you see it, I would love to hear from you.

The plant bears greenish-yellow flowers (hence the species name viridiflorus, meaning green-flowered) about an inch across in late May or early June. Notice how difficult it is to see the top of the plant, which is to the left of the flower – when not in flower, these ball cacti are often well camouflaged.

Cacti of Sheep Mountain: Opuntia polyacantha (Plains Prickly Pear)

This is the reason you don’t want to walk around in tennis shoes on our local prairies. (Photo by Marc Beckstrom.)

Plains prickly pear is very common in grasslands all over the state of Wyoming. It is considerably less abundant on Sheep Mountain than on the prairie, but is still quite easy to find. Opuntia polyacantha has one of the widest distributions of any cactus in North America, ranging northward from Texas all the way to Saskatchewan, and westward to British Columbia and Southern California. Like many wide-ranging prickly pears, this is a species that displays extreme variability in appearance in different regions.

Opuntia polyacantha blooms in June to early July in our area. The showy flowers are around 2 to 3 inches across and are usually a shade of yellow.

And now for the Rest of the Cacti of Wyoming

The three species of Wyoming cacti that are not found on Sheep Mountain include two prickly pears (Opuntia fragilis and Opuntia macrorhiza) and one ball cactus (Escobaria missouriensis). In Wyoming, Opuntia fragilis is known from Sweetwater County; Opuntia macrorhiza is known from the Great Plains and the Black Hills in eastern Wyoming; and Escobaria missouriensis is also known from the Great Plains and Black Hills, as well as Park County (1). More information about these species is easily found by searching the USDA Plants Database.

A Final Word on Cacti in Wyoming

If you consult Dorn and Dorn (1), you will find that they include an eighth species of cactus in Wyoming, Opuntia erinaceae, but I just told you that there are only seven species. Don’t be alarmed, Wyoming hasn’t lost Opuntia erinaceae since the Vascular Plants of Wyoming was published. It’s just that more recent taxonomic work has shown O. erinaceae to be a variety of O. polyacantha (2). Those wily Botanists!

References

(1) Dorn, Robert D. and Jane Dorn. Vascular Plants of Wyoming, 3^rd ed. ©2001; pp 147-148. Distributed by the Rocky Mountain Herbarium.

(2) Lodé. Joel. Taxonomy of the Cactaceae. Published in Cuevas del Almanzora, Spain by Cactus Adventures, 2015.

About the Author

Larry Haimowitz is working on a graduate degree in entomology at the University of Wyoming, after retiring several years ago. His love of bugs, and all things in nature, goes back to his childhood on a farm. Now he carries a camera pretty much everywhere he goes, and intends to “keep on hiking, backpacking and climbing mountains until the day I die - but just a little bit slower each year.”

Tookie and Larry.

Excruciatingly Beautiful

Cacti are odd plants -- they lack leaves, photosynthesis takes place in their fat green stems, and most are covered with pain-inducing spines.  Yet they are charismatic.  We find their unusual forms striking, beautiful, even enchanting.

Echinopsis mamillosa, an easter lily or sea urchin cactus of Bolivia.  Photo by Stan Shebs.

Various species of Parodia cacti (South America).  Photo by Karelj.

“In a Sahuaro forest, Arizona”   Lollesgard Specialty Co., Tucson.  Published 1936.

Their vicious spines are lovely too -- arranged as they are in such elegant patterns.

Spines of Echinocereus viridiflorus, a hedgehog cactus.

Coryphantha vivipara, a pincushion cactus.  This one is recovering from a fire.

Cacti are not the only spiny plants in the world, though spines often prompt people to call a plant a cactus.  When I lived in northeast Wyoming, I was a resident botanist of sorts and one day Shirley asked me “What’s that cactus in my garden with yellow flowers and leaves like a watermelon?”  I was puzzled, though I should have guessed from her apt description. It was buffalo bur (Solanum rostratum) -- quite spiny but not a cactus.

Buffalo bur is a member of the Nightshade Family, like tomato, potato and eggplant.
Photo by Joseph Marcus, Lady Bird Johnson Wildflower Center.

Only in the cactus family (Cactaceae) are spines arranged in clusters.  These develop in leaf axils -- where the leaves join the stem.  But wait -- cacti don’t have leaves!  It turns out they do, but most are tiny and soon fall off.

Clusters of spines on a prickly pear cactus pad (Opuntia sp.).

The spines themselves often are referred to as highly-modified leaves, but we still aren’t sure.  There are two interpretations as to how spines developed evolutionarily.  They may be highly-modified leaves on specialized condensed shoots (side branches), or they may be modified bud scales (also modified leaves, which cover an embryonic shoot).

At maturity, spines and leaves are very different.  Spines have none of the usual leaf features, like blades, veins, stomates and photosynthetic tissue.  Instead, they contain fibers and thickened, almost woody epidermal tissue, neither of which is found in leaves. Thus different sets of genes are utilized during development.  Finally, spines are basically dead, while leaves are composed of living tissue.  So why call spines modified leaves? ... because both develop from the same type of embryonic structures -- leaf primordia.  Bud scales come from these primordia too, hence the confusion.  See Mauseth 1976 and 2006 for more details and discussion.

Prickly pear spine clusters:  large spines radiate outwards,  pale-colored glochids form clusters at base (click photo for closer view).

Members of the prickly pear subfamily (Opuntioideae) have two types of spines -- the normal type we’re used to seeing, and insidious hair-like little buggers called glochids.  The larger ones generally stay put when bumped, but glochids come off easily.  They can burrow into the skin, are hard to find, and sometimes seem impossible to remove.  Chart courtesy REI (click to view).

Spines develop on structures called areoles.  Wikipedia explains that these are “highly specialized branches on cacti ... apparently they evolved as abortive branch buds while their spines evolved as vestigial leaves.”  “apparently” is the proper adverb in this case; as with spines, more work needs to be done to understand the evolutionary path to areoles. Above and below:  a pincushion cactus (Coryphantha vivipara) post-burn on Sheep Mountain, southeast Wyoming.  In the recovering plant, green tubercules are visible, with clusters of spines growing from the areoles on top.

Left:  Mammillaria bombycina, by Brian Johnson (Micscape); used with permission.  Spines are clustered on areoles at the top of tubercules.  For more stunning photos and photomicrographs of cacti, see Brian’s A Close-up View of Several Members of the Cactus Family.  Even if technical details of cactus anatomy are not your thing, this site is still well worth visiting -- for the incredible beauty of the portraits.

Areoles also are where cactus flowers develop -- those beautiful, colorful blooms that contrast so strikingly with the dull green plants.

What's that bright red patch in the piñon - juniper woodland? (San Rafael Swell, Colorado)

It's that show-off prima donna -- claret cup cactus (Echinocereus triglochidiatus).

Areoles commonly occur on tubercules, but they also may be arranged along ribs, as in Echinocereus viridiflorus, the hedgehog cactus below.

Along with spines, this "old man cactus" has "whiskers" -- hairs said to have evolved to shade the stem and reduce water loss.  But who knows?  Maybe they really evolved to appeal to people -- who take them in and care for them  ;-)

Sources (in addition to links in post)

Websites:

CactiGide.com is a handy reference site for cacti.

James D. Mauseth’s research site (cacti):  http://www.sbs.utexas.edu/mauseth/researchoncacti/index.htm

Dalhousie Collection of Cacti & Other Succulents at Dalhousie University  http://cactus.biology.dal.ca/index.html

Literature:

Mauseth, J.D.  1976.  Cytokinin- and gibberellic acid-induced effects on the structure and metabolism of shoot apical meristems in Opuntia polyacantha (Cactaceae).  American Journal of Botany 63: 1295-1301.

Mauseth, J.D.  2006.  Structure-function relationships in highly modified shoots of Cactaceae.  Invited review.  Annals of Botany 98: 901–926.