Monster Invasive Tumbleweed Is Outgrowing Its Parent Species

Scientists once thought the hybrid Salsola ryanii would not be able to survive the hot, dry conditions of the West. They were wrong

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Shana Welles/UC Riverside

The image of a tumbleweed rolling across a dusty Western expanse is as American as apple pie—but, of course, less palatable. But tumbleweed are, in actual fact, invasive plants that can wreak havoc upon native ecosystems, agriculture and property—just ask residents of the town of Victorville, California, which was buried by an invasion of tumbleweeds last year.

Now, as Peter Dockrill reports for Science Alert, a new study has taken a closer look at a relatively new tumbleweed species already causing problems in the Golden State—and found that the plant’s genetic makeup may help it become even more invasive in the future.

Salsola ryanii is a hybrid of two other invasive tumbleweeds: Salsola tragus, which is native to Russia and China and invasive in 48 U.S. states, and Salsola australis, which is native to Australia and South Africa and invasive in California and Arizona. Salsola ryanii was first identified 15 years ago, and scientists initially predicted that it would soon go extinct because it did not seem well adapted to the hot, dry conditions of the West. But as it turns out, Salsola ryanii thrived in California; over the course of just 10 years, it spread from two to 15 areas. It can also grow taller than its parent species, reaching a towering height of six feet.

The Good, the Bad and the Tumbleweed

Salsola ryanii is a nasty species replacing other nasty species of tumbleweed in the U.S.,” says Norman Ellstrand, a professor of genetics at the University of California Riverside and co-author of the new study, which has been accepted to the journal AoB Plants. “It’s healthier than earlier versions, and now we know why.”

Ellstrand and Shana Welles, a graduate student in Ellstrand’s laboratory and lead author of the new report, suspected that Salsola ryanii might be getting a leg up because it is an allopolyploid, meaning that it has more than two complete sets of chromosomes. (Humans, by way of comparison, are diploids, inheriting one set of chromosomes from their mother and one from their father.) Allopolyploidy is commonly seen in plant species, so scientists have hypothesized that it confers an important advantage—namely, helping the new plant grow more vigorously than its parents. With their investigation into Salsola ryanii, Ellstrand and Welles have shown that this is in fact the case.

Over the course of two year-long experiments, the researchers planted Salsola ryanii, Salsola tragus and Salsola australis in a test garden and monitored the plants’ “fitness,” which is their volume, mass and estimated seed numbers. (Each species can produce more than 100,000 seeded fruits each year, so counting the actual number of seeds was deemed prohibitive.) In the first year of the study, the hybrid Salsola ryanii exhibited greater mass and volume than Salsola tragus, and “significantly greater” mass, volume and seed number than Salsola australis. In the second year, Salsola ryanii displayed “significantly greater mass than both of its progenitors, significantly larger volume than S. tragus and significantly higher estimated seed number than S. australis,” the researchers write.

Given just how robust Salsola ryanii seems to be, it is likely that the tumbleweed will continue to expand its range. Climate change may exacerbate the problem.

“[Salsola ryanii is] one of the only things that’s still green in late summer,” Welles says. “They may be well positioned to take advantage of summer rains if climate changes make those more prevalent.”

This could spell bad news in multiple respects. Though tumbleweeds start off rooted to the soil, they break away upon reaching maturity, rolling across landscapes and dispersing their seeds as they go. They compete with crops for resources, and are thus nightmares for farmers, reports Adam Allington of Bloomberg. Tumbleweeds can also disturb oil and gas pads, spread forest fires and cause traffic accidents.

Worryingly, Salsola tragus, one of the parent species of Salsola ryanii, has shown resistance to glyphosate, a common herbicide. Learning more about S. ryanii could help scientists figure out a way to suppress the plant before it too starts tumbling through dozens of states.

“An ounce of prevention,” Ellstrand says, “is a pound of cure.”

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