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“Generalist” and “Specialist”: Plants Deploy Diverse Strategies to Thrive in West Texas

photograph by John Phelan. The pinyon-juniper woodland is the Southwest's dominant forest type – and in our region, these often open, park-like environments can be found in foothills and mountain slopes.

Twelve thousand years ago, the great ice sheets that had long covered most of Canada and the northern U.S. receded. The jet stream – the continent's storm track – abruptly shifted course. And the Southwest – which had been a lush land of lakes, woodlands and savannas – began a swift transformation into the arid land we know today.

Every living thing that endured here adapted to that reality of increasing aridity and heat – from kangaroo rats, which can live without drinking water, to the region's people, who developed techniques – like slow-roasting agaves – to turn otherwise inedible desert plants into food.  

It's true of our region's plant life as well. In ways that often elude our detection, every plant species here deploys strategies to survive, if not flourish, in an unforgiving land. 

Dr. Helen Poulos, of Wesleyan University, has studied forest ecology in West Texas for two decades, with special attention to environments known as “pinyon-juniper woodlands.”

“Pinyon pine – it's an underappreciated tree,” Poulos said. “I remember when I was doing my dissertation – 'Oh, you and your little pines, Helen!,' my adviser used to say. But they're the dry-site pine across the entire western United States.”

Big trees and dense forests have their charms, and their champions. And pinyons – which top out at 20 feet – can't compete with Ponderosa or Arizona pines for height. But the pinyon-juniper woodland is in fact the Southwest's dominant forest type – covering 100 million acres in 10 states, and extending deep into Mexico. These open, park-like woodlands are found in mid-elevations and foothills. 

And for some, they strike a deep aesthetic chord. Research suggests humans are instinctively attracted to savannas – grasslands with scattered areas of trees. The explanation could be evolutionary: unlike dense forests, savannas offered our forebears the perfect mix of “prospect” and “refuge” – the ability to see potential threats, while remaining hidden oneself. Pinyon-juniper woodlands are the savannas of the Southwest. 

Ponderosas need cooler, wetter conditions – in West Texas, they're confined to the high Guadalupes, and the shaded canyons of the Davis and Chisos. That's not the case for pinyons.

“Pinyon pine is a generalist because it can handle a variety of different kinds of conditions,” Poulos said, “and it also is more drought tolerant.” 

In one study, Poulos explored the physical mechanisms that allow pinyons to thrive across a range of elevations and environments. The secret to the tree's success as a generalist, she found, lies in its “plasticity.”

Poulos examined pinyon needles at three elevations in the Davis Mountains – at 6,000, 7,000 and 8,200 feet. As elevation increases, she found, so does the size and water content of the pine's needles. Those findings indicate lower-elevation pinyons are conserving their resources. Higher-elevation trees, by contrast, are capitalizing on wetter, cooler conditions to speed up photosynthesis.

It's striking evidence of how pinyons adapt across short distances, to accommodate specific niches. 

But such “generalizing” is just one survival strategy. 

In Mexico's Maderas del Carmen, opposite Big Bend National Park, Poulos looked at two “specialist” trees. Again, she found a physiological basis for the plants' distributions.

Lacey oak thrives in the lower elevations of the Carmen range. Santa Rosa oak dominates the high country. Poulos cultivated seedlings of the two oaks, and subjected them to a “cruel experiment”: she denied them water for seven weeks. 

Both species proved stunningly drought-tolerant – after five weeks, neither was worse for wear. But six weeks in, the high-elevation oaks' health declined. The high-elevation species can outcompete its kin in wet conditions – but the Lacey oaks are better equipped for drought. These different adaptations explain why the two species live where they do.

Such findings are fascinating in themselves. But as climate change makes the Southwest even hotter and drier, they also have potent implications for conservation. 

“Generalists” are apt to do better in a warming climate than “specialists.” Understanding the specific conditions specialists need, and the ways in which they're adapted to those niches, can help us preserve them.

“So this physiological approach,” Poulos said, “of thinking about what are the mechanisms explaining these patterns we're seeing – no one is really thinking in that way. But I think it's a powerful perspective, because it really helps you start to understand – why are we not seeing certain species regenerate abundantly?”

Beyond the postcard images of mountain vistas, our region's plant diversity is one of its hidden treasurers. There's always more to learn about this green and growing world.

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