Bromus tectorum, commonly known as cheatgrass or downy brome, is an invasive annual grass that poses a major ecological threat in North America. It has rapidly spread across the western United States. Its presence impacts ecosystems, agriculture, and wildfire regimes.
Identification and Life Cycle
This annual grass grows 20 to 90 centimeters tall, forming distinct bunches with multiple stems. Its leaves and sheaths are covered with fine, soft, dense hairs, giving the plant a downy appearance. The plant develops characteristic drooping seed heads, which are soft and fuzzy when young and contain spikelets with long awns. As the plant matures, its color changes from green to a purplish-red hue, eventually turning a straw-like color as it dries out.
Bromus tectorum follows a winter annual life cycle, contributing to its successful establishment. Seeds germinate in the fall when moisture is available, with seedlings overwintering. Growth resumes rapidly in the early spring during cool weather, allowing the plant to mature and produce seeds by late spring or early summer. This early maturation means the plant completes its life cycle and dies by early summer, leaving behind dry, dead plant material. Its fibrous root system efficiently absorbs moisture from light precipitation events.
Origin and Proliferation
Bromus tectorum is native to Europe, southwestern Asia, and northern Africa. It was introduced to North America in the late 19th century, appearing in states like New York and Pennsylvania. Its introduction was largely accidental, often as a contaminant in grain, packing materials, or ship ballast.
The grass spread rapidly across the Intermountain West. Its proliferation was accelerated by extensive livestock grazing and cultivation practices in the late 1800s and early 1900s, which created disturbed soil conditions favorable for its establishment. Seeds disperse through various means, including animal fur, wind, and human activities like vehicles and contaminated hay.
Ecological Disruption
The early and rapid growth of Bromus tectorum provides a competitive advantage over native perennial grasses and forbs. It germinates earlier than many native species, allowing it to monopolize available soil moisture and nutrients before native plants can establish themselves. This competitive pressure reduces native plant biodiversity, often resulting in monocultures where cheatgrass dominates.
Ecological consequences extend to wildlife, as native sagebrush steppe ecosystems, primary habitat for species like the greater sage-grouse, are degraded. Wildlife also experiences negative impacts due to the displacement of natural food sources and cover. Beyond direct competition, Bromus tectorum alters ecosystem processes by disrupting nutrient cycling and changing hydrologic regimes. Conversion to cheatgrass can significantly reduce carbon sequestration.
The Cheatgrass-Fire Cycle
A major impact of Bromus tectorum is its role in altering natural fire regimes, creating a feedback loop known as the cheatgrass-fire cycle. The plant’s annual life cycle culminates by early summer, leaving behind a continuous, highly flammable mat of fine, dry fuel. This contrasts with native ecosystems, which have patchier fuel loads and are less prone to widespread, frequent fires.
The presence of this dense, dry fuel leads to wildfires that are more frequent, larger, and faster-spreading than historical norms. In areas heavily infested with cheatgrass, the fire return interval has shortened from a historical average of 25-30 years to as little as 3-5 years. This increased fire frequency further promotes cheatgrass by eliminating fire-sensitive native plants, while cheatgrass seeds are adapted to survive and rapidly germinate in post-fire environments. A single fire can initiate this conversion, and repeated fires solidify the transformation of native shrublands into cheatgrass-dominated grasslands. This feedback loop ensures cheatgrass quickly re-establishes, creating an even more fire-prone landscape.
Management Strategies
Managing Bromus tectorum and restoring affected ecosystems requires a multifaceted approach. Mechanical methods, such as mowing or tilling, can be effective if implemented before the plant sets seed, reducing the seed bank. However, these methods can disturb the soil, creating new opportunities for cheatgrass to colonize.
Chemical control involves applying herbicides, including pre-emergent options that prevent seed germination and post-emergent treatments for established plants. Timing herbicide application is important for maximizing effectiveness and minimizing harm to native species. Cultural and biological strategies also play a part. Targeted grazing by livestock can reduce cheatgrass biomass and fuel loads if carefully managed. While some biological agents have shown promise in laboratory settings, their effectiveness in widespread field conditions remains limited. The objective of these management efforts is to facilitate the long-term restoration and establishment of resilient native plant communities.