Bromus tectorum

Bromus tectorum grows in many different climates. It is found primarily in the 150–560 millimetres (5.9–22.0 in) precipitation zone.[10] It will grow in almost any type of soil, including B and C horizons of eroded areas and areas low in nitrogen.[10] B. tectorum is quick to colonize disturbed areas.[10] It is most often found on coarse-textured soils and does not grow well on heavy, dry, and/or saline soils. It grows in a relatively narrow range of soil temperatures; growth starts at 2.0–3.5 °C (35.6–38.3 °F) and slows when temperatures exceed 15 °C (59 °F).[14]

The seeds ripen and disperse in the late spring and early summer.[10] They are dispersed by wind, small rodents, or attachment to animal fur, within a week of maturity.[10] They are also moved as a contaminant in hay, grain, straw, and machinery.[10] Bromus tectorum is an abundant seed producer, with a potential in excess of 300 seeds per plant; seed production per plant is dependent on plant density. Under optimal conditions, B. tectorum may produce 450 kg of seed per hectare (400 pounds per acre) with about 330,000 seeds/kg (150,000 seeds/pound).[10] As the seed of B. tectorum ripen the plant goes from green to purple to straw-colored.[10]

B. tectorum seeds demonstrate rapid germination as soon as the seed lands in appropriate conditions.[10] If winter rainfall is limiting and germination is inhibited, but spring moisture is adequate, then seeds will germinate in the spring, and the plants will flower that summer.[10] The seeds maintain high viability (ability to germinate under optimal conditions) in dry storage, lasting over 11 years. In the field, under buried conditions, seeds will lose their viability in 2–5 years. Seeds can withstand high soil temperatures, and the primary limit to germination is inadequate moisture. Germination is best in the dark or in diffuse light. They germinate most quickly when covered with soil, but do not need to be in contact with bare soil. Some leaf litter cover will generally improve germination and establishment of seedlings. Seedlings emerge rapidly from the top 2.5 cm (1") of soil, and a few plants emerge from depths of 8 cm (3"), but not from seeds 10 cm (4") below the surface.[15]

In the US, it grows on rangelands, pastures, prairies, fields, waste areas, eroded sites, and roadsides. It is much reviled by ranchers and land managers. B. tectorum seeds are also a critical portion of the diet of the chukar and grey partridge which have been introduced to the US. Intensive sheep browsing of B. tectorum in early spring has been used as a fire fuels reduction strategy in the hills adjacent to Carson City, Nevada.[19] Because of rangeland fires and the invasion of Bromus tectorum, in 2010 the United States Fish and Wildlife Service (USFWS) considered the possibility of extending the protections of the Endangered Species Act to the greater sage-grouse.[20] The primary focus of Secretarial Order 3336, signed in 2015 in response to the USFWS status review, was to reduce threats greater sage-grouse habitat by reducing the frequency and severity of rangeland fire.[20] Specifically, Secretarial Order 3336 focused on how reducing B. tectorum could reduce the frequency and extent of rangeland fires. Since the review of the status of the greater sage-grouse by the USFWS in 2010 and the implementation Secretarial Order 3336 in 2015 the bulk of the research focusing on B. tectorum ecology and control has been completed. 

Bromus tectorum has demonstrated a quantitative and qualitative response to recent and near-term changes in the concentration of atmospheric carbon dioxide. Laboratory experiments have shown that above-ground biomass increased 1.5–2.7 gram per plant for every 10 part per million (ppm) increase above the 270 ppm pre-industrial baseline.[21] On the qualitative side, rising carbon dioxide decreased the digestibility and potential decomposition of B. tectorum. In addition to stimulation of biomass, rising carbon dioxide may also increase the above ground retention of B. tectorum biomass by decreasing removal by animals or bacteria.[21] Ongoing increases in atmospheric carbon dioxide may contribute significantly to B. tectorum productivity and fuel load with subsequent effects on wildfire frequency and intensity.[21][22]

Bromus tectorum has been shown to benefit from endophytic colonization by morels (Morchella sextelata, M. snyderi) in western North America.[23]    

The availability of native seed will always be a limiting factor in restoration of sagebrush ecosystems after a rangeland fire. Little is understood about the germination requirements of native species. This lack of understanding is complicated by the episodic nature of establishment in arid grasslands.[8][24] In response to the limited availability of native seed, land managers have been seeding Agropyron cristatum, a perennial bunchgrass native to Russia and Asia. The use of seeding another non-native to control an exotic, problem species is called assisted succession.[25] A. cristatum is much easier to establish than the native perennials and has been shown to be a strong competitor of Bromus tectorum.[4][26]

However, A. cristatum can exhibit invasive behavior and is a strong competitor of native perennials.[4][27] The reason it is used, regardless of its invasive behavior, is because it restores some function a perennial grassland.  A. cristatum is resistant to wildfire and it is suitable forage for cattle and wildlife.[25] But the intensive control that would be needed to establish a native plant community in a  A. cristatum monoculture would cause disturbance that would also increase the invasive species it was planted to out compete.[26] An alternative to using A. cristatum as a placeholder species in assisted succession is to establish it alongside foundation species like sagebrush. Adding sagebrush would diversity the ecosystem and provide habitat for sagebrush obligates.[26] However, this would mean accepting the possibility that the native plant community may never establish.

A majority of research in 2011-2017 has focused on the use of herbicides to control B. tectorum and their effect on native plant communities. When using herbicides to suppress winter annual grasses the two most important factors that influence success are application timing and residual soil activity. Application timing is split into three main categories: pre-emergence in the fall before Bromus tectorum germinates, early post-emergence in early spring when B. tectorum is a seedling, and late post-emergence in late spring after B. tectorum is mature.

To be most effective post-emergence application needs to be done as late in the spring as possible to ensure that the herbicide treatment hits the majority of the B. tectorum population.[28] However, the late application puts the native perennial vegetation at risk as they may be coming out of dormancy.[28] Herbicides with no residual soil activity are not generally used because they are only effective in the year of application. If the herbicide has no residual soil activity the herbicide must be applied post-emergence in the early spring, but pre-emergence application is preferred because pre-emergence application is less likely to harm the native vegetation.

Studies have suggested that herbicide usage may select for warm season grasses and decrease the abundance of cool season grasses.[28] There are five main herbicides used to control B. tectorum: imazapic, rimsulfuron, tebuthiuron, glyphosate, and indaziflam. However, the bulk of the recent research has been done on glyphosate, indaziflam, and imazapic.

Glyphosate has no residual soil activity and must be used post-emergence, which limits its control of B. tectorum to one year. For effective control it must be applied to the same area for more than five years to get ahead of seed production to prevent recolonization.[28] Imazapic is the herbicide most widely used by land managers for B. tectorum control. Of the herbicides listed it is also the most commonly studied. Imazapic is preferred because it can be applied both pre and post-emergence, is approved for rangeland use, and has residual soil activity that allows for one to two year control.[29] Indaziflam is one of the newest herbicides, licensed in 2010. It has a residual soil activity of 2–3 years and its also useful against many other invasive grasses. Not only does it reduce the abundance and biomass of B. tectorum, but it also reduces the highly flammable litter that B. tectorum produces.[7] In early trials it has consistently out competed imazapic.[29] As of 2017, indaziflam has not been approved for use outside of residential and commercial properties.

Prescribed burning alone reduces Bromus tectorum biomass for approximately two years.[30] The goal of a prescribed burn in a B. tectorum invaded area is to remove the highly flammable plant litter in a controlled manner. The timing of prescribed burns can affect the variety and amount of returning vegetation. A study shows spring burns may result in a significant reduction of native vegetation, but fall burns have been shown to increase species richness.[31] Fall burns may also promote select grasses and fire resilient plants.[31] Another control for B. tectorum burns is consideration for the densities and fire adaptations of nearby foliage.[32] In some cases, the existence of adjacent morchella can trigger mutual relationships like increased fiber and, by extension, fuels that nurture the return of cheatgrass.[32] Similarly, when densely packed conifers begin infilling sagebrush communities the understory perennial vegetation is reduced; when these areas are prescribed burned the succession is dominated by B. tectorum in favor of taller grasses, making burns situationally inferior.[33]

Water availably has a large impact on the success of Bromus tectorum treatments. In years of high precipitation, B. tectorum recruitment and biomass will increase and may render the treatment ineffective.[30] In most long-term B. tectorum studies precipitation differences between years are speculated to be the cause of variation in effectiveness.[30][34] However, well-timed precipitation after herbicide application can increase the amount of herbicide that makes it into the soil. When herbicide is applied to an area and there is B. tectorum litter on the ground, much of the herbicide will be absorbed into the litter and some will adhere to the litter. The litter creates a blanket that B. tectorum can germinate under even after herbicide application.[30] But if it rains shortly after herbicide application, some of the herbicide that is trapped in the litter can be released and work into the soil.[30] The rain may also allow native species to over come herbicide impacts.[34] Increased precipitation in early spring may increase the success of seeding by increasing the germination rate of native grasses and remove B. tectorum’s competitive advantage.[25]

Invasive Bromus tectorum on Spruce Mountain, Nevada

There is a positive correlation between native vegetation communities and biological soil crust (BSC).[35] BSC is composed of cyanobacteria, algae, lichens, and mosses living on the soil. In arid regions BSCs colonize the spaces in-between plants, increase the biodiversity of the area, are often the dominant cover, and are vital in ecosystem function.[5] In addition to providing erosion control, BSC is vital for nutrient cycling and carbon fixing.[5] Fire and trampling by cattle are the major threats to the BSC communities, and once disturbed it can take decades to centuries for BSC to reform.[35][5]  A decline in the health of the BSC community serves as an early warning indicator for Bromus tectorum invasion. If the BSC community is healthy then it will impede B. tectorum germination and reduce the likelihood of invasion.[5][6] However, if there is a disturbance in the biological soil crust and downy brome is able to establish, then B. tectorum will impede the recovery of the BSC community.[35]

Native perennial grasses have roots that often reach four feet into the soil. These roots provide organic matter, which feeds the soil organisms, who assist in water and nutrient cycling in arid ecosystems and improve soil quality[36]. Bromus tectorum has a shallow spreading root system, which makes it much more efficient at absorbing moisture from light precipitation episodes and disrupts nutrient cycling.[12][36] Several studies have shown that native plant biomass, especially that of bunchgrasses, negatively effects B. tectorum cover and biomass,[4][37][38] suggesting that a diverse native perennial community will be more resistant to B. tectorum invasion.

Studies have identified Poa secunda, Pseudoroegneria spicata, and Achnatherum thurberianum as key grasses for B. tectorum resistance.[39][40] The life strategies of these three grasses differ in such a way that they provide constant interaction and competition with B. tectorum.[39] P. spicata, and A. thurberianum are deep rooted and complete most of their growth in the late spring, and P. secunda is shallow rooted and completes most of its growth in the late winter and early spring.[39] 

Perennial grass ecosystems are less prone to burning. B. tectorum has been historically thought to create a positive feedback loop. However, Taylor et al. (2014) suggests that fire alone does not promote B. tectorum.[12] If an area burns, the B. tectorum cover and biomass does not increase as was once thought, but recovers to previous levels.[12] Increased fires, because of B. tectorum, may serve to maintain, not increase, the B. tectorum population by preventing the natives from establishing.