GENERAL LIFE CYCLE: Like all annual grasses, the development of Bromus rubens is comprised of six stages: germination, vegetative growth, floral bud development, maturation of flowers, fruiting, and senescence (Hufstader 1978). The prevailing environmental conditions influence the various stages of development in different ways. Germination of Bromus rubens seeds is particularly dependent on the moisture level of the soil. The ability to germinate throughout the fall, winter and spring, provide the seeds an opportunity to maximize the utilization of available moisture in order for a vigorous growth phase early in the development of the plant. In southern California, the majority of red brome seeds germinate during the end of November; the seedlings grow slowly throughout the winter with the maximum growth rate of 0.04 g/m2/day starting at the end of March and continuing through the beginning of May, at which time senescence commences (Hufstader 1978). Flowering starts in late winter and continues throughout the spring.
VEGETATIVE GROWTH: Vegetative growth commences with germination and terminates in the spring when floral development begins (Hulbert 1955). The growth rate and total standing crop appears to be relatively independent of the amount of precipitation once germination has occurred (Hufstader 1976). Plant development subsequent to germination is more dependent on the genetics of the species than it is on the environmental conditions. Growth proceeds slowly through the winter and reaches its maximum growth rate shortly before flowering (Beatley 1966, Hufstader 1978). Spring germination followed by a rapid growth period results in floral development at approximately the same time as flowering of plants that germinated in the fall (Beatley 1966). Plants that germinate in the fall are susceptible to winter freezes. Bromus rubens is not frost hardy and thus temperatures below 32 F will kill the plants (Hulbert 1955).
Crowding, especially in pure stands of red brome, decreases the survivability of individual plants (Wu and Jain 1979). A lack of reduction in number of seeds produced and a high mortality rate accompany higher density plots (Wu and Jain 1979). The section on Population Dynamics addresses these nonadaptive characteristics of Bromus rubens.
Several reasons, particularly the shallow root system and the lack of shade tolerance, account for the inability of this species to compete with established plants. In certain areas of central and southern California, red brome is an understory plant and unable to adequately compete with the larger plants for sunlight because of its delayed initial development relative to the taller species (Hufstader 1978). Competition for nutrients along with competition for light appears to be a determining factor in the size and distribution of Bromus rubens; the shallow root system limits the ability of the plant to search for nutrients deep in the soil (Humphrey 1977). Nitrogen fertilizers, but not mulch, increase the growth rate of red brome (Hulbert 1955, Bartolome et al. 1980). Hulbert (1955) speculates that the readily available nitrogen from fertilizers aids in the production of a more extensive root system; the roots are then able to compete with larger plants for water and nutrient supplies, this, in turn, allows for greater above-ground growth.
COMPETITIVE RELATIONSHIPS AND POPULATION DYNAMICS: Annual plants have an intrinsic competitive advantage: a short life-cycle, including a rapid growth phase, and the ability to produce an abundant seed source under adverse conditions (Burcham 1957, Naveh and Whittaker 1979, Cox pers. comm.). Bromus rubens follows this pattern, and grows during the cool season when adequate moisture is available. The seeds remain dormant when the environmental conditions are severe. Whereas species with low seed dormancy exhibit higher initial germination and subsequent higher mortality than species with high dormancy (Jain 1982). Bromus rubens growing in soils disturbed by testing of nuclear and other explosive devices in southern Nevada out-competes native plants which fill the same niche (Beatley 1966). It does this by requiring half as much moisture and having less exact temperature demands than the natives: 36% of native species versus 73% of Bromus rubens survive. More information on the competitive ability of red brome is available in the next section on Effects of Disturbances.
Bromus rubens is adapted to and competitive in disturbed areas, however it is not considered a problem in undisturbed sites; although it grows on open hillsides, woodlands and chaparral, most research pertains to competition with grasses and forbs in grasslands and chaparral scrub vegetation (Crampton 1974, Daubenmire 1978, Wu and Jain 1979). Compared to other annual bromes, Bromus rubens has a patchy, limited distribution (Hulbert 1955, Wu and Jain 1979). This patchiness is due, in part, to the relatively low plasticity of red brome in response to conditions of crowding (Wu and Jain 1979). In crowding experiments, Bromus rubens displays greater self-thinning characteristics (death of individual plants), and no reduction in seed production in high plant density plots, as compared to Bromus mollis which maintains a higher plant survival rate possibly by partitioning less energy to seed production. The decrease in survival of individual plants, both in test plots and natural sites, is possibly due to the shade intolerance of Bromus rubens (Hufstader 1976). The limited intrapopulation genetic variability, resulting partially from a low outcrossing rate, may be a factor contributing to the narrow niche of Bromus rubens in California (Wu and Jain 1979).
Bromus rubens grows where sunlight and nutrients are available. The possibility of it growing in undisturbed sites exists only if bare soil and available light is present. The removal of understory herbaceous vegetation will provide a site for this winter annual to grow. Bromus rubens grows readily in open woodlands, below cottonwoods, willows and mesquite trees, where light penetrates through the canopy or through deciduous trees (Richter pers. comm., Naveh and Whittaker 1979).
High seed production aids in the survival of this species. However, the limited dispersion of the seeds results in added intra- and inter-specific competition (Wu and Jain 1979). The seeds must find a location with sufficient moisture and with limited competition from other species (Hufstader 1976). In natural populations, Bromus rubens produces an average of 76 seeds per plant, of which 18% find a safe site and result in established seedlings; however only 10% of the seedlings reach maturity (Wu and Jain 1979). With a less than 2% seed carry-over rate from the year produced to the following year, in conjunction with the low seedling establishment and low survivability quality, the invasive potential of this species is limited.
GRAZING AND FIRE: The major types of disturbances that influence the invasive potential of this species are livestock grazing and rangeland fires. The forage value of Bromus rubens is relatively low. Only during a short period of the life-cycle is it palatable to livestock; livestock, primarily sheep, graze on the plant during the winter months when it is young and green (Gould 1951, Sampson et al. 1951, Crampton 1974, Humphrey 1977). The poor quality of forage is due to the sparse foliage, the early maturity, and the stiff awns and sharp pointed florets which irritate livestock (Crampton 1974). In addition, the shallow root system is inadequate at anchoring the plant when tugged by grazers and the resulting soil-covered roots of Bromus rubens are disfavored by livestock (Humphrey 1977).
Bromus rubens has moderate erosion control abilities (Crampton 1974). Red brome is fourth to last of ten annual exotic range plants at improving range quality (Sampson et al. 1951). The poor range improvement and forage quality of Bromus rubens may explain why this species was not intentionally disseminated in western United States (Burcham 1957).
Before horses and livestock were introduced into western California the vegetation was primarily perennial species characteristic of steppe vegetation (Burcham 1957, Daubenmire 1978). By the mid 1800s the vegetation and soil had been disturbed to such an extent that plants which were adapted to disturbed environments and that were relatively unpalatable to livestock began to flourish (Burcham 1957).
Grazing and burning may increase the amount of Bromus rubens by clearing vegetation and providing adequate sites for the seeds to germinate (Hulbert 1955, Naveh and Whittaker 1979). Because seeds of annual species have a short dormancy period, they can utilize optimum conditions to germinate and complete their rapid life-cycle during the same period that disturbed perennials are slowly recovering (Naveh and Whittaker 1979).
Clipping of BROMUS spp. seedlings only slightly reduces the yield (Hulbert 1955). Mowing the plants prior to seed development results in the development of new culms; however, plants are usually killed when cut at soil level once seeds have developed (Hulbert 1955). Mowing at this stage is pointless since the seeds will be dispersed and the plant left alone would have senesced. Increasing the frequency of mowing throughout the entire growing season decreases the quantity of the yield (Hulbert 1955).
Burning (in June, October and unknown months) increases the abundance of Bromus rubens, especially in areas where the land had previously undergone disturbances (Beatley 1966, O'Leary and Westman 1988). Experiments conducted with coastal sage vegetation burned one time in June or October resulted in drastic increases in the amount of red brome in the sites where there were few vigorous native perennial plants plus a supply of Bromus rubens seeds present prior to the fire (O'Leary and Westman 1988). Possibly the effects of pollution or other stresses on summer growing perennial plants weaken the plants' ability to recover after a fire and thus the annual weeds are able to increase their percent land cover (O'Leary and Westman 1988).
Fires in sagebrush vegetation in Utah resulted in density changes of vegetation, with the burned plots having a 32% reduction in perennial grass cover and a considerable increase in the amount of annual bromes as compared to the unburned plots (Pickford 1932). A reduction in the amount of available nitrogen in burned plots in the Sonoran Desert may have a greater detrimental effect on the native perennial plants than on the introduced annuals; no deleterious effects of these fires were observed on Bromus rubens (Whysong and Heisler 1978). Over a three year period the number of red brome plants in both unburned and burned (month of burn unknown) plots in a COLEOGYNE plant community at the Nevada test site increased from 376 and 429 plants per plot, respectively, to 615 and 1,626 plants per plot, respectively (Beatley 1966).
Johnson and Smathers (1974) feel that overgrazing followed by fire suppression has resulted in an increase in the abundance of the annual brome, Bromus tectorum, in Lava Beds National Monument. Overgrazing resulted in a reduction in native perennials and an increase in the amount of annual weeds. Possibly the suppression of fires has prevented the occurrence of natural succession.
RELATIONSHIP WITH FAUNA: The eyesight of red-shouldered hawks in California is affected by Bromus rubens (McCrary and Bloom 1984). The sharp florets become lodged in the corners of the eyes, causing eye infections which lead to a reduction in vision. When the problem afflicts both eyes the hunting ability is reduced often leading to death by starvation.
Red brome is second to Tridens pulchellus in frequency of occurrence in the stomach of desert cottontails (Turkowski 1975). The animals ate the plants throughout the year, including during the flowering season. The article does not indicate whether flowers and seeds were found in the rabbits' stomachs. Rabbits ate 98% of the Bromus rubens plants which were grown in containers in the Mojave Desert (Slayback et al. 1981). Only 2% of the unprotected container plants survived.
Bromus rubens appears to be a food source of the Great Basin kangaroo rat (DIPODOMYS MICROPS). This rodent nests in areas separate from where it harvests its food (Rowland and Turner 1964).
A change in composition from a perennial shortgrass prairie to a field of introduced annual species results in changes in grasshopper distribution (Pfadt 1982). Annual grasses alone cannot support the density and diversity of grasshopper species that the native grasslands supported in this study.
