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Texas’ Aspens

Broadcast on November 4, 2010

Quaking aspen in Big Bend National Park. (Photo by James Zech)

Aspen trunks. (Photo by James Zech)

By Megan Wilde

With their towering white trunks and fluttering leaves, quaking aspens are icons of the Rocky Mountains. Yet every fall, aspen stands paint our highest peaks with brilliant gold. What are these remarkable trees doing in the Chihuahuan Desert?

Sul Ross State University professor James Zech is studying Texas’ aspens, which grow only in the high Chisos, Davis and Guadalupe mountains.

Zech: Texas aspens are tough. They’re really tough. They are in extreme environments. Many of the sites are precarious in the sense that there’s rockslides, talus slopes. I give them a lot of credit for surviving as long as they have under these conditions.

Aspens are tough, period. Said to be North America’s most widely distributed tree species, aspens are found from arctic latitudes, down into Mexico’s Sierra Madre. Across their range, temperatures vary from 70 degrees below zero to 100 degrees above.

These tenacious trees are holdouts from the last Ice Age, when their distribution was even broader. But as the climate began warming about 10,000 years ago, aspens retreated with the glaciers, and some of these Pleistocene relics took refuge on the relatively cool, moist slopes of our region’s mountains.

They’ve adapted remarkably well to montane environments. The white powder on their trunks shields them from intense high-altitude sun, and their delicate leaves are adapted to mighty mountain winds.

Zech: The stalk of the leaves, the petiole of the leaves, are flat. So when the wind moves through, it makes them quake, it makes them tremble. That’s advantageous in that it reduces the wind pull on the entire tree that sometimes reaches the degree that it could snap off an entire trunk.

Also key to their success is their ability to make the most of disaster. Aspens excel at pioneering the sun-drenched barren landscapes left after a fire or landslide.

Zech: And because they’re clonal—they reproduce by extensive root systems—they have a tendency to be able to occupy these new scarred habitats pretty quick.

These clonal sprouts form dense stands that outcompete other trees. A grove of such clonal aspens is actually one giant organism. Some say a 106-acre aspen stand in Utah is the world’s most massive living organism.

But having an identical genetic make-up has its downsides, particularly when disease strikes. So aspens capitalize on fire in another way. Forest infernos knock aspens back to their roots, from which new, healthier stands later regenerate. In fact, without periodic fires, aspens actually suffer as conifers shade them out.

At the same time, they can survive an occasional blaze. Like ponderosa pines, aspens drop lower branches, which are liable to ignite. This self-pruning leaves the dark calligraphic scars that decorate aspen trunks.

As stalwart as they are, aspens may not be here forever, especially in a changing climate. Zech says these Pleistocene relics are a reminder of the long-term natural cycles that shape our world.

Zech: Just outside of Alpine there used to be ponderosa pine forest, and now it’s not there anymore because of changes in climate, etc. So things change. Landscapes change. Environments change. And while it would be great to always have aspens, I think that’s probably not going to be the case.

Such changes certainly won’t happen overnight. Still, don’t miss a chance to see these mountain treasures. One of the best places is Emory Peak in Big Bend National Park.

Have a question or comment about this episode? Contact Nature Notes Coordinator Megan Wilde at mwilde [at] cdri [dot] org. Or discuss this episode on Nature Notes’ Facebook page. This episode originally aired on November 4, 2010.

References & Resources for Educators

• Quaking Aspen: Bryce Canyon National Park
• “Trees born of fire and ice” by Chris Madson (National Wildlife, 1996)
• “Structure of Isolated Populations of Populus tremuloides (Quaking Aspen) in the Davis Mountains of Far-West Texas” by O. W. Van Auken, J. K. Bush, F. A. Richteb, and J. Karges (Natural Areas Journal, 2007)

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