Weeds From Hell in a Greenhouse World

By Michael D. Lemonick

A yellow starthistle. Credit: Doug Greenberg/Flickr.

Beekeepers love yellow starthistle. “It makes fantastic honey,” says Jeffrey Dukes, an ecologist at Purdue University. “But for ranchers, it’s a real problem.” That’s an understatement. Since its accidental introduction to the American West in the mid-1800's, this aggressive plant, which originally came from Eurasia, has invaded grasslands in California, Idaho, Oregon and Washington. If horses eat too much of it, they can die; cows won’t perish, but they’ll avoid it when the plant develops its spiny flowers, so they need extra feed. A 2007 study pegged the plant’s economic damage in Idaho alone at more than $12 million annually. Here’s what the California Invasive Pest Council had to say in a 2006 report:

Yellow starthistle is a major consumer of groundwater, costing the state millions of dollars in lost water for wildlife, agriculture and municipal uses… It can also reduce land value and reduce access to recreational areas… On military bases such as Fort Hunter Liggett, yellow starthistle can severely impact training exercises and can impair the use of equipment (e.g., snagged parachutes, torn clothing) or clog air filters on vehicles. In addition, yellow starthistle can cause mechanical injury to humans (particularly to the face) when the spines are encountered…

All of that being the case, a new study published by Dukes and several colleagues in the journal Ecological Applications is not exactly cause for celebration. The short version: if you think it’s bad now, just wait. Increasing greenhouse gases — specifically, carbon dioxide, or CO2 — will aggravate problems related to yellow starthistle.

The problem in this case isn’t related to the global warming that CO2 is helping to cause, but rather has to do with how the gas affects the weed's growth. Carbon dioxide acts as sort of fertilizer, making yellow starthistle grow like… well, it’s already a weed, so perhaps a “super-weed” is the right term.

Researchers at the Jasper Ridge Global Change Experiment site test the response of a California grassland to climate change, elevated atmospheric carbon dioxide, and increased nitrogen pollution. Credit: JRGCE.

Dukes and his colleagues found this out on the grounds of Stanford University’s Jasper Ridge Biological Preserve, in the foothills of the Santa Cruz Mountains west of the Stanford campus. Since 1997, the Jasper Ridge Global Change Experiment has been artificially subjecting small plots of grassland to the climate conditions scientists are projecting for later this century — higher temperatures, changes in precipitation, increased soil deposits of nitrogen compounds from air pollution, and higher concentrations of CO2 due to continued burning of oil and coal for energy.

A couple of years ago, Dukes realized this ongoing experiment was ideal for answering a question he’d been interested in for a long time: how are invasive plants, in particular, going to respond to climate change? Since most plants use CO2 as a primary nutrient, it wouldn’t be surprising for them to thrive, but earlier experiments have shown that different species of plants respond in unique ways. “So we took what’s probably the worst grassland invasive species in the West,” says Dukes, “and introduced it.”

The results were stark. Most of the plants didn't respond to CO2 and some were actually supressed by it, but the yellow starthistle loved it.

“Some plants were monsters,” says Dukes. “We saw a range of responses, but the biggest plants benefited the most. On average, starthistle had one of the strongest responses to elevated CO2 we’ve ever seen.”

The mechanism, he believes, is that the extra CO2 allows the starthistle to grow a more extensive root system, thereby sucking up even more water and nutrients than normal. “In California, soil moisture is a precious resource, so when you have a species using more than its share, there are economic consequences,” he says. The same is true in semi-arid eastern Washington and Oregon, as well as southern Idaho, where the weed is also problematic.

It’s possible to control yellow starthistle, but it’s not easy. “Pulling it out is effective in the short term, but you need an army to do it,” says Dukes. Burning works if you do it for two years in a row, but setting huge grassland fires isn’t necessarily the best idea. Agricultural researchers have tried introducing insects that eat the plants, with some success, but, says Dukes, “there’s no evidence you can rely on that alone.”

And that’s the situation today. If yellow starthistle really does take off in a CO2-enriched world, controlling it could be much more difficult.

“The take home message,” he says, “is that there are a bunch of species that are going to benefit in the future, and there’s reason to believe that in many cases they're the species we don’t like. Poison ivy benefits immensely from increased CO2; it’s more competitive, and it produces worse toxins.” His research, along with other studies, “suggests that our costs for managing the landscape to be the way we like it are going to go up.”

“The question,” he says, “is how we’re going to prepare for that.”