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Thorny Questions: Club Cholla in West Texas

phyton-opuntia
Corynopuntia emoryi is one of five species of club cholla found in the Big Bend region.

Prickly pear and cholla cactus are among the most recognizable of West Texas plants. They're part of the "opuntioid" subfamily of cactus.

In a recent paper, an Alpine researcher shed new light on one member of this group. Her research has improved our understanding of the evolution and spread of the cacti.

Most Texans can identify a prickly pear cactus. And the shrub-like “cane” or “tree” chollas are also distinctive. But another kind of cacti belongs to this group, and it often escapes attention..

“Club” or “dog” chollas grow low to the ground. With club-shaped joints, they can form dense mats on the desert floor. Joselyn Fenstermacher, a research associate at Sul Ross State University, studies the plants.

“It's the face that only a mother could love,” Fenstermacher said. “They're really not all that attractive – until they're in flower, and then they're really neat. But that only happens for a very short time, and because most of these things are in the desert, away from where most people are, you don't really see it.”

Fenstermacher's interest in chollas began while she was at work on another project in Big Bend National Park. Her curiosity soon developed into an “obsession,” she said. But she was surprised that most existing descriptions were incomplete and even inaccurate.

Botanists had identified two “foundation” species in West Texas – Corynopuntia grahamii and Corynopuntia schottii. An isolated population of a third cholla species – more common to Arizona – was found in Presidio County. Club cholla that didn't fit these categories were often thought to be hybrids.

In botanical research, as in much else, the Big Bend remains a frontier. In the United States, cactus research is centered at universities in Arizona. And many of the botanists who described Big Bend chollas had never seen them in the field.

Fenstermacher thought the classifications were wrong, and she set out to fix them.

Over a 10-year period, she counted chromosomes for almost a hundred plants. She tested pollen, reviewed old specimens and collected hundreds of new specimens. She documented not only physical traits, but growth habits and soil conditions.

Questions had lingered about two forms of club cholla – aggeria and densispina. Fenstermacher's research confirmed that these plants were not hybrids, but independent species. Densispina is found in only in a handful of locations in Big Bend National Park.

In making species distinctions, Fenstermacher said, a botanist with field experience has an advantage.

“And that's one of the biggest things about the paper and why it happened – because I had a lot of field experience with these things – and I could see when stuff was different,” she said. “It's really important to record all these characteristics, to do genetics, to do the field stuff, to do chromosomes, because it all lends something to the picture of what this particular species is.”

Her research offers fascinating hints at the historic development and “migration” of club cholla in West Texas.

During the most recent Ice Age, which lasted 2 million years and ended about 10,000 years ago, the southern Big Bend was a sanctuary of warmth in a much colder world. The diversity of club cholla today suggests the plants may have evolved in the southern Big Bend, and then spread elsewhere.

How did that happen? Parts of some chollas detach easily, and they can be carried away in the fur or flesh of a passing animal. From present-day Brewster County, Corynopuntia schottii is found as far east as the Gulf of Mexico, while grahamii's range extends west into Arizona.

In fact, ancient mammals – like the mastodon – may have helped spread these chollas from their primordial home in the Big Bend. Perhaps even early humans from the Ice Age played a role.

“Maybe that's what happened here,” Fenstermacher said, “and that's why we have these five species essentially occurring on top of each other, where that hasn't happened in other places. These things kind of start here and sort of creep away, and become more like themselves the further they get from this area of diversity. Which is kind of cool, to think of these non-mobile things creeping along.”

As Fenstermacher's research shows, every West Texas life form is part of complex and multi-layered story.