By Published: Nov. 14, 2023

In Rabbit Valley near the Colorado-Utah border, some signs indicate that aster could stymie the dominance of the invasive species

Cheatgrass, Bromus tectorum, is an invading species that now dominates millions of acres in North America. Although it is found in all 50 states, it has been particularly troublesome in rangeland in the western states and in sagebrush environments of the Great Basin.

Its original native distribution included the Mediterranean region of Europe, northern Africa and southwestern Asia. It was first reported in New York and Pennsylvania in 1861 and made its way to the western states by the 1880s. Cheatgrass is probably the most successful invader in North America—in the Great Basin alone it is estimated to cover 81,000 square miles.

Between the 1830s and 1880s, biologists were not monitoring the appearance and movement of invading species as they are today. But given that cheatgrass seemed to spread with agriculture and with ranching, it seems likely that its seeds were one of the impurities of seed lots bought by farmers and ranchers. But in addition, it is known that cheatgrass was also purposely planted in search of a grass that would support more livestock.

western aster in Rabbit Valley

Western aster and cheatgrass grow in Rabbit Valley near the Colorado-Utah border.

The most distressing facet of cheatgrass is how it invades communities, displaces established species and dominates grasslands and sagebrush communities. It certainly followed cows, for early management of cows generally led to overgrazed landscapes, which were quickly colonized by cheatgrass. But cheatgrass has a variety of advantages that make it a successful weed, and once established, a dominant that is difficult to eradicate.

Cheatgrass is a winter annual. It sets seeds in late summer and in contrast to many annual grass species, its seeds germinate in the fall. By spring, it has a large root system, and its shoots develop and are photosynthesizing long before its competitors. The early, rhizomatous root system is able to absorb and store much water, leaving parched soil for native grasses. Cheatgrass grows quickly in spring, reaching heights of 3 to 4 feet, creating abundant seeds and more bulk than native grasses.

The plants dry as their seeds mature, dramatically changing the appearance of the landscape—spring's lush and abundant greenery loses its nutritional value as green turns gold and the seeds develop long awns—spikes from the maturing seeds that can injure cattle. Perhaps the common name "cheat" comes from the sudden transformation from excellent food for cattle in spring to very little of available nutritive value in mid-summer.

Finally, once cheatgrass becomes established, its greater accumulation of dry grass is fuel that changes the fire regime of the local environment. Instead of fires returning in 50 to 150 years, fires recur more frequently, providing this winter annual with another advantage over native grasses.

When I first started camping in the McInnis Canyons National Conservation Area west of Grand Junction, I was beguiled by the mesas and canyons but horrified by the condition of Rabbit Valley, which appeared to be a monoculture of cheatgrass. However, during my most recent trip to Rabbit Valley, I saw something that suggested either evolutionary or ecological conditions were changing.

two purple western asters

Western aster grows amid a near-monoculture of cheatgrass in western Colorado's Rabbit Valley.

Western aster, Symphyotrichum ascendens, was growing among the cheatgrass in the Knowles Overlook campground. I had not seen them before, or perhaps I did not notice them before. But the asters were in flower and they grew taller than the cheatgrass, so they were conspicuous. I looked around with binoculars and was delighted to find that western asters were mixed with cheatgrass throughout Rabbit Valley. In a few places, asters were abundant.

Western aster is a perennial, rhizomatous forb native to all states west of the Great Plains. Under good conditions, it can grow to be 4 feet tall, bearing numerous flowers. It is known as the most common, widespread and variable aster species. It is extremely variable because it arose from hybridization of S. spathulatum and S. falcatum, and it now hybridizes with both species.

Furthermore, its geographic range is large, and it is adapted to a wide elevational range and both mesic and arid environments. Species with large geographic ranges and adaptations to numerous habitats generally have high levels of genetic variation. which is needed for rapid evolutionary and ecological responses to novel challenges.

Apparently, I am not the first to have noticed western asters moving into areas dominated by cheatgrass. A literature survey turned up a U.S. Department of Agriculture plant guide, which stated that western aster could compete with and even suppress cheatgrass.

I am hoping that Rabbit Valley is a theater featuring increasing competitive ability of a widespread native species to the most successful invading species in the West.

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