Yellow starthistle already causes millions of dollars in damage to pastures in Western states each year, and as climate changes, land managers can expect the problem with that weed and others to escalate, says a Purdue University researcher.

When exposed to increased carbon dioxide, precipitation, nitrogen and temperature – all expected results of climate change – yellow starthistle in a Purdue study grew to up to six times its normal size while the other grassland species remained relatively unchanged. The plants were compared with those grown under ambient conditions.

"The rest of the grassland didn't respond much to changes in conditions except nitrogen," says Jeff Dukes, an associate professor of forestry and natural resources and the study's lead author. "We're likely to see these carbon dioxide concentrations in the second half of this century. Our results suggest that yellow starthistle will be a very happy camper in the coming decades."

The study is one of the first comparing the growth of invasive species vs. their local competitors under future climate scenarios. Dukes believes the results indicate problems land managers and crop growers could see in the coming decades, and not just with yellow starthistle.

"Plants are going to respond in a number of ways to climate change,” he says. “Sometimes the species we depend on will benefit, but other times it will be the weedy, problematic species that benefit most, and there can be economic and ecological damages associated that people should be aware of. These problems with yellow starthistle aren't going to go away on their own. If anything it's going to become more of a problem than it is now."

Yellow starthistle is a significant weed in the West, especially in California, where it has a longer growing season than native plants and depletes ground moisture, affecting water supplies.

"It reduces the quality of the area for animal forage, is toxic to horses and when it forms spines, cattle don't want to eat it," Dukes says. "Many consider yellow starthistle to be the worst grassland weed in the West."

The decreased pasture production, lost water and control costs associated with the weed cause economic impacts in many Western states. Experts suggest that, in Idaho alone, it may cause more than $12 million a year in economic damage and that yellow starthistle reduces pasture values by 6-7% across California.

Dukes says all plants increased in size as expected when exposed to more nitrogen. But yellow starthistle was especially responsive to increased carbon dioxide.

That might be in part because the weed can gain access to more soil resources, he says. Grassland plants' stomata don't have to be open as wide to take in carbon dioxide when there is a larger concentration in the air. Those smaller stomata allow less water to escape, and the extra water in the soil could favor yellow starthistle. The added carbon dioxide also changed the mix of species competing with the weed and may have allowed it to grow a more effective root system.

"It was an impressive increase in growth," says Dukes. "It was one of the largest responses to elevated carbon dioxide ever observed."

Biological control species introduced to control yellow starthistle have not been effective enough, and he says it’s becoming urgent that better controls be developed to address invasive species that could cause significant damage to pasture, cropland and wild lands.