Pulling a Fast One: Invasive Grass Can Tweak Genetic Timing for New Ground

To beat an enemy you have to know it — but cheatgrass makes that tough.

“Cheatgrass is a formidable enemy because it is remarkably adaptable,” said Peter Adler, ecologist from the Quinney College of Natural Resources and director of the USU Ecology Center.

New research from Adler, former USU post-doc Megan Vahsen, and a team of collaborators documents how nimble these adaptations can be.

Cheatgrass (Bromus tectorum) is aggressive, fast-growing and spreading across the American West. The invasive grass now dominates millions of acres from Arizona to Idaho, threatening ranching and agriculture and disrupting habitat for wildlife. Land managers and researchers are grappling for ways to better battle the costly and persistent invasion.

“Cheatgrass is now dominant across much of the Intermountain West,” Vahsen said. “Understanding why is not only important for current management but also for predicting where it might spread in the next few decades.”

People tend to think that plants grow in genetically programmed ways — that their genes tell them when to grow, how big to get and when to flower. But really successful species have more flexibility, adapting to new conditions in ways that let them flourish far from their native ground in relatively short timeframes.

Ecologists call this “plasticity” — the ability of a plant to adjust its growth and behavior in response to its environment within a single generation.

Cheatgrass competes with native plants by growing earlier in the season, depleting the soil of nutrients, taking up space and leaving behind dry, highly flammable fuels to feed summer fires.

Fire further knocks out the native plants, fire-disturbed soils become receptive to new cheatgrass seeds, and the cycle continues. But researchers have wondered: Is this pattern just the luck of the genetic draw, or is this species changing its behavior to match new environments?

The team looked at how cheatgrass alters timing of its reproductive flowering when introduced to new environments, paving the way for invasive success. They gathered cheatgrass seeds from genetically distinct populations around the country and grew them in experimental plots under different temperatures and crowding conditions at sites in Idaho and Wyoming.

Flowering timing of the grass adjusted to current environmental conditions in just one generation, they found. The warmer a site or the more crowded it was, the earlier the flowering occurred.

“Genetics and plasticity both affected the time of flowering,” Adler said. “Plants sourced from hot, dry locations flowered earlier than plants from cool, wet places, even when grown together in a common garden. But these genetic effects were smaller than the effects of plasticity.”

Depending on current conditions, the plant could adjust its flowering schedule by up to 25 days. When the rapid adaptations happened to coincide with genetic predisposition for earlier flowering, cheatgrass’s advantages began to look formidable.

The battle to fight cheatgrass will be a long one and may only get harder.

“Unfortunately, cheatgrass appears well prepared to deal with climate change,” Adler said. “Plasticity will help it in the short term and dispersal of early-flowering genotypes that are adapted to warmer conditions will help it in the longer term.”

But information like this can give land managers something of a strategic edge, allowing them to predict spread and range shifts as the plant invades, or assist with the timing of control efforts like herbicide application or grazing to target vulnerable life stages, Adler said.

It also helps land managers to know what they don’t know. For instance, control methods effective in one region with a certain cheatgrass genotype may be less effective in another.

The authors emphasized that understanding both the plant’s immediate responses to an environment and the longer-term evolutionary adaptations of different genetic types is crucial. Successful strategies will need to consider both.

This research is part of a larger project, the BromeCast research network, which is working to predict how climate change is influencing population dynamics of cheatgrass across western North America. The research could improve the accuracy of population models to help land managers mitigate the spread of cheatgrass in the face of rapid environmental changes.?