Native Annual Species Research Articles

Impact of Recurrent Fire on Annual Plants: a Case Study from the Western Edge of the Colorado Desert

Limited information exists regarding the impact of fire on annual plant composition in creosote bush scrub vegetation. The impact of recurrent fires on annual plants is even less understood. To investigate this matter, annual vegetation was sampled in a stand of creosote bush scrub in western Coachella Valley, California that had recently experienced two wildfires. The wildfires fragmented the once contiguous shrubland into three sections: unburned, once-burned, and twice-burned stands, all of which were separated by fuel breaks that contained each fire. For all three stands, annual plant cover and species richness were determined in the field, soil seed bank samples were collected and assayed in a glasshouse, and soil chemistry and physical properties were measured. We found that invasive annual grass cover was highest in the twice-burned stand and native annual plant cover was greatest in the unburned stand. Native annual species richness significantly decreased each time a stand burned resulting in low native annual plant diversity. Seed bank assays revealed that invasive annual grass germinants were orders of magnitude greater in the twice-burned stand compared with the other two stands. Lastly, soil total N, C, and soil pH were elevated in both burned stands. Overall, we found that recurrent fire can result in strong impacts to annual vegetation; however, the twice-burned stand was sampled only three years after burning while the once-burned stand was sampled 20 years after burning. Thus, longer-term fire effect studies plus replication with additional study sites are still needed to improve our understanding of how recurrent fire impacts annual plants.

Chirostoma jordani is a native annual species inhabiting lacustrine waters of the Central Mexico Plateau. It is widely distributed and is currently facing high environmental pressures. Five experiments were performed to study the reproductive performance

Berberine, a constituent of some traditional Chinese medicinal plants, has been reported to have cytotoxicity effects on different human cancer cell lines. There is no available information about the effects and mechanism of action of berberine on human colon cancer cell line HCT-8. In this paper, the cytotoxicity ofberberine on HCT-8 cancer cells was investigated by MTT assay, fluorescence microscopy and flow cytometry analysis. Our data revealed that berberine could significantly inhibit the growth of HCT-8 cells in a dose- and time-dependent manner. Morphology of apoptotic cells was studied with acridine orange/ethidium bromide staining. The concentrations of lactate dehydrogenase and both acid and alkaline phosphatases were significantly increased in cell supernatants after berberine treatment, suggesting cell death. Furthermore, flow cytometry analysis showed that berberine could arrest HCT-8 cells at S phase in a time-dependent manner. To further investigate the apoptotic molecular mechanism, reverse transcription-polymerase chain reaction (RT-PCR) and western blotting methods were used. The up-regulated mRNA and/or protein expressions of Fas, FasL, TNF-alpha, caspase-3 and down-regulation of pro-caspase-3 suggest that the death receptor pathway may be involved in the apoptotic pathway induced by berberine. Decrease of Bcl-2 and increase of Bax in mRNA and/or protein expressions showed that the Bcl-2 family proteins were involved in berberine-induced apoptosis. We also found that berberine-induced apoptosis was associated with an upregulated expressions of p53 and prohibitin (PHB), and decreased vimentin expression. These results suggest that berberine can suppress cell growth and reduce cell survival by arresting the cell-cycle and by inducing apoptosis of HCT-8 cells.

Reproductive performance of the Mesa silverside (Chirostoma jordani Woolman, 1894) under natural and controlled photoperiods

Chirostoma jordani is a native annual species inhabiting lacustrine waters of the Central Mexico Plateau. It is widely distributed and is currently facing high environmental pressures. Five experiments were performed to study the reproductive performance of this species. Four of the experiments were conducted in 270-L indoor recirculation tanks. Two males and one female at the first stage of reproduction were included in each test. A photoperiod of 14 light hours and 10 dark hours was used. In a fifth experiment, 10 females and 15 males were kept in an outdoor 3,000-L recirculation tank under natural photoperiod. The number of spawns, fertilised eggs and 30-day-old juveniles were counted and the survival rate was calculated. The results indicated significant differences (P < 0.05) between treatments. Higher spawn numbers and greater egg production were observed under controlled photoperiod, and higher numbers of juveniles and a higher survival rate were observed under natural photoperiod. The trials exhibited different patterns of egg production during the experiment. The egg production in the natural-photoperiod trials followed a polynomial curve model. In contrast, the trials under the controlled photoperiod showed an irregular pattern of increases and decreases in egg production.

Disturbance, resource pulses and invasion: short-term shifts in competitive effects, not growth responses, favour exotic annuals

1. Increased resource availability and resource pulses often promote invasion by exotic invasive plants, but the relative importance of increased resource supply for invaders with different life histories is likely to vary. It is also unclear whether increased resources allow invaders to outgrow their native neighbours or alter the outcome of competition. Understanding mechanisms by which different invasive life histories respond to resource supply will help to target the most important species to control. 2. We found that aerially dropped wildfire retardant on intermountain grassland provided a very high pulse of N and P and caused dramatic increases in the abundance of annual exotic invaders already present in the system. Field experiments with retardant stimulated similar shifts to dominance by exotic annuals and decreases in native perennial grasses, but elicited no responses by native annual species. 3. In a greenhouse experiment, the growth response of both exotic and native species to retardant was not related to the intensity of mean competitive effects or responses. Concomitantly, gram per gram competitive effects differed substantially among species and treatments. N-caused shifts in the strength of competitive interactions were far better predictors than growth responses of the effects of retardant in the field. For example, Centaurea stoebe, an invasive perennial forb, grew disproportionally more than any other species in response to retardant when grown alone, but retardant significantly shifted competitive outcomes with this invader in favour of exotic annuals and the native bunch grass Pseudoroegneria spicata. 4. Synthesis and applications. Our results indicate that an extreme nutrient pulse did not create conditions whereby invaders outgrew native neighbours or other exotics, but instead altered the competitive playing field in ways that resulted in considerable changes in community composition. Given the minuscule scale of retardant drops on the landscape, our results do not suggest changing the way retardant is used to combat wildfire but do suggest that localized post-retardant weed control may be important so that retardant drops do not become point sources for exotic invasion. Furthermore, our results suggest that under high N conditions, invaders with annual life histories should be prioritized for control.

Effects of natural and anthropogenic gradients on native and exotic winter annuals in a southern California Desert

Native annual plant species constitute a large proportion of the plant diversity found in arid vegetation types within the southwestern United States; yet, little is known about controls on diversity patterns along natural and anthropogenic gradients. In this study we evaluated native species richness and exotic species cover across overlapping gradients of precipitation, wind, and N deposition in the Colorado Desert of southern California. Factors allowing native diversity to persist under high N deposition and high wind were also evaluated in a second, focused study at one end of the gradient. We found that gradients in precipitation, nitrogen deposition, and wind were the most important factors to native richness and exotic species cover across the landscape, while local heterogeneity in bare ground influenced richness and cover at the high deposition/windy, or high-disturbance, end of the gradient. Patterns of native diversity were evaluated across the gradients using non-metric multidimensional scaling, which showed diversity was split into two axes: one strongly correlated to precipitation and the other strongly correlated with disturbance factors. The disturbance factors were also positively associated with exotic grass and forb cover. In total, these results indicate that large-scale patterns in disturbance and exotic species cover negatively affect native annual plant species diversity but native species can also persist due to local heterogeneity.

Anthropogenic fires increase alien and native annual species in the Chilean coastal matorral

Aim We tested the hypothesis that anthropogenic fires favour the successful establishment of alien annual species to the detriment of natives in the Chilean coastal matorral. Location Valparao´so Region, central Chile. Methods We sampled seed rain, seedbank emergence and establishment of species in four paired burned and unburned areas and compared (using GLMM) fire resistance and propagule arrival of alien and native species. To assess the relative importance of seed dispersal and seedbank survival in explaining plant establishment after fire, we compared seed rain and seedbank structure with postfire vegetation using ordination analyses. Results Fire did not change the proportion of alien species in the coastal matorral. However, fire increased the number of annual species (natives and aliens) of which 87% were aliens. Fire reduced the alien seedbank and not the native seedbank, but alien species remained dominant in burned soil samples (66% of the total species richness). Seed rain was higher for alien annuals than for native annuals or perennials, thus contributing to their establishment after fire. Nevertheless, seed rain was less important than seedbank survival in explaining plant establishment in burned areas. Main conclusions Anthropogenic fires favoured alien and native annuals. Thus, fire did not increase the alien/native ratio but increased the richness of alien species. The successful establishment of alien annuals was attributable to their ability to maintain rich seedbanks in burned areas and to the greater propagule arrival compared to native species. The native seedbank also survived fire, indicating that the herbaceous community has become highly resilient after centuries of human disturbances. Our results demonstrate that fire is a relevant factor for the maintenance of alien-dominated grasslands in the matorral and highlight the importance of considering the interactive effect of seed rain and seedbank survival to understand plant invasion patterns in fire-prone ecosystems.

Germination phenology of some Great Basin native annual forb species

AbstractGreat Basin native plant communities are being replaced by the annual invasive cheatgrass Bromus tectorum. Cheatgrass exhibits a germination syndrome that is characteristic of facultative winter annuals. Although perennials dominate these communities, native annuals are present at many sites. Germination timing is often an important predictor of competitive interactions, and might determine whether the use of annual species in restoration efforts will be successful. I used a laboratory experiment to determine whether a suite of native annuals exhibit winter or spring annual germination syndromes. Seeds of Amsinckia menziesii var. intermedia, Amsinckia tesselata, Blepharipappus scaber, Descurainia pinnata, Eriastrum sparsiflorum, Lappula occidentalis, Mentzelia veatchiana and Plagiobothrys tenellus were tested for dormancy, and for responsiveness to light, cold stratification and dry after‐ripening. Species that would be expected to be most similar to cheatgrass are those that have no requirement for cold stratification and are therefore likely to germinate under autumn or winter conditions. The species that clearly met this criterion in this laboratory study were A. menziesii var. intermedia, A. tesselata, D. pinnata and L. occidentalis. In contrast, B. scaber, E. sparsiflorum, M. veatchiana and P. tenellus had their highest germination after cold stratification and would be expected to be spring germinators. Blepharipappus scaber was not coaxed out of dormancy to a great degree by any of the treatments I applied and may exhibit cue‐non‐responsive dormancy. Field seed burial experiments, as well as experiments examining the competitive ability of these annuals versus cheatgrass will further inform us about their potential for success in restoration seedings.

Factors affecting exotic annual plant cover and richness along roadsides in the eastern Mojave Desert, USA

Roads are often considered a critical point of entry for invasive exotic plant species. We evaluated the cover and richness of exotic plants relative to road type (gravel or paved) and distance from roads in the eastern Mojave Desert where exotic annual species are increasing the flammability of the desert. Exotic and native annual plant cover and richness were sampled from five to 45m from roads in three prevalent microsite types for this system: Larrea tridentata microsites, Ambrosia dumosa microsites, and interspaces between shrubs. Six exotic and 58 native annual species were detected during sampling. There was no distinction between the behavior of exotics and natives relative to road type or distance from the roads. A. dumosa and L. tridentata microsites had more exotic cover than interspaces. Likewise, exotic richness was significantly higher in the shrub microsites. A taxonomically controlled analysis involving three plant families revealed no significant differences in cover of native or exotic species relative to their distance from the road. Our results suggest that exotic plant species in the Mojave Desert are not necessarily more prevalent near roadsides than in adjacent undisturbed desert. Therefore, roadside surveys alone may not be adequate to detect exotic species presence.

Variation in the establishment of a non-native annual grass influences competitive interactions with Mojave Desert perennials

Competition between native and non- native species can change the composition and structure of plant communities, but in deserts, the highly variable timing of resource availability also influences non-native plant establishment, thus modulating their impacts on native species. In a field experiment, we varied densities of the non- native annual grass Bromus madritensis ssp. rubens around individuals of three native Mojave Desert perennials—Larrea tridentata, Achnathe- rum hymenoides, and Pleuraphis rigida—in either winter or spring. For comparison, additional plots were prepared for the same perennial species and seasons, but with a mixture of native annual species as neighbors. Growth of perennials declined when Bromus was established in winter because Bromus stands had 2-3 months of growth and high water use before perennial growth began. However, water potentials for the peren- nials were not significantly reduced, suggesting that direct competition for water may not be the major mechanism driving reduced perennial growth. The impact of Bromus on Larrea was lower than for the two perennial grasses, likely because Larrea maintains low growth rates throughout the year, even after Bromus has completed its life cycle. This result contrasts with the perennial grasses, whose phenology com- pletely overlaps with (Achnatherum) or closely follows (Pleuraphis) that of Bromus. In compari- son, Bromus plants established in spring were smaller than those established in winter and thus did not effectively reduce growth of the perenni- als. Growth of perennials with mixed annuals as neighbors also did not differ from those with Bromus neighbors of equivalent biomass, but stands of these native annuals did not achieve the high biomass of Bromus stands that were neces- sary to reduce perennial growth. Seed dormancy and narrow requirements for seedling survivor- ship of native annuals produce densities and biomass lower than those achieved by Bromus; thus, impacts of native Mojave Desert annuals on perennials are expected to be lower than those of Bromus.

Life-history Habitat Matching in Invading Non-native Plant Species

We briefly reviewed the literature on habitat matching in invading non-native plant species. Then we hypothesized that the richness and cover of native annual and perennial plant species integrate complex local information of vegetation and soils that would help to predict invasion success by similarly adapted non-native plant species. We tested these ‘life-history habitat matching’ relationships in 603 0.1-ha plots, including 294 plots in Colorado, which were relatively high for the cover of native perennial plant species, and for 309 0.1-ha plots in southern Utah, which were relatively high in the cover of native annual plant species. We found strong positive relationships between the richness and foliar cover for both native and non-native species, whether they were annual or perennial species (0.34 > r 2 < 0.53; P < 0.0001). We also found significant positive relationships between the cover of native annual species at a site and the richness (r 2 = 0.13; P < 0.0001) and the foliar cover (r 2 = 0.06; P < 0.0001) of non-native annual species. The proportion of non-native annual species in the flora of a plot also increased significantly with the foliar cover of native annual species. Conversely, the richness and cover of non-native annual species were significantly negatively associated with the foliar cover of native perennial species (r 2 = 0.05 and 0.06, respectively; P < 0.0001). The cover of non-native annual or perennial species was not significantly correlated with soil texture variables, %N, or %C. We conclude that there may be a high degree of life-history habitat matching by non-native annual species in these study sites. Information on native annual and perennial species richness and cover may help characterize the complex soils, climate, and disturbance environment in which similarly adapted non-native plant species establish and gain foliar cover.

Cattle Grazing Impacts on Annual Forbs and Vegetation Composition of Mesic Grasslands in California

Abstract: Livestock grazing represents a major human alteration of natural disturbance regimes in grasslands throughout the world, and its impacts on plant communities have been highly debated. We investigated the impact of cattle grazing on the California coastal prairie plant community with a focus on native annual forbs, a number of which are of conservation concern. In spring 2000 and 2001, we surveyed the vegetation community composition, vegetation structure, and soil chemical parameters at 25 paired grazed and ungrazed sites over a 670‐km range of the ecosystem. Native annual forb species richness and cover were higher in grazed sites, and this effect was concomitant with decreased vegetation height and litter depth. Soil properties explained less of the variation. Exotic annual grass and forb cover were higher in grazed sites. Native grass cover and species richness did not differ in grazed and ungrazed sites, but cover and species richness of native perennial forbs were higher in ungrazed sites. Our results suggest that cattle grazing may be a valuable management tool with which to conserve native annual forbs in the ecosystem we studied but that grazing differentially affects the various life‐history guilds. Therefore, land managers must focus on creating a matrix of disturbance regimes to maintain the suite of species native to these mesic grasslands. The results of this and other studies highlight the importance of considering the adaptation of vegetation communities to disturbance in making recommendations for grazing management.

COMPETITION, SEED LIMITATION, DISTURBANCE, AND REESTABLISHMENT OF CALIFORNIA NATIVE ANNUAL FORBS

Invasion by exotic species is a major threat to global diversity. The invasion of native perennial grasslands in California by annual species from the southern Mediterranean region is one of the most dramatic invasions worldwide. As a result of this invasion, native species are often restricted to low-fertility, marginal habitat. An understanding of the mechanisms that prevent the recolonization of the more fertile sites by native species is critical to determining the prospects for conservation and restoration of the native flora. We present the results of a five-year experiment in which we used seeding, burning, and mowing treatments to investigate the mechanisms that constrain native annuals to the marginal habitat of a Californian serpentine grassland. The abundance and richness of native species declined with increasing soil fertility, and there was no effect of burning or mowing on native abundance or richness in the absence of seeding. We found that native annual forbs were strongly seed limited; a single seeding increased abundance of native forbs even in the presence of high densities of exotic species, and this effect was generally discernable after four years. These results suggest that current levels of dominance by exotic species are not simply the result of direct competitive interactions, and that seeding of native species is necessary and may be sufficient to create viable populations of native annual species in areas that are currently dominated by exotic species. Corresponding Editor: M. Rees

Physiological variation among native and exotic winter annual plants associated with microbiotic crusts in the Mojave Desert

Microbiotic crusts are important components of many aridland soils. Research on crusts typically focuses on the increase in soil fertility due to N-fixing micro-organisms, the stabilization of soils against water and wind erosion and the impact of disturbance on N-cycling. The effect of microbiotic crusts on the associated plant community has received little attention. We quantified the influence of crusts on the production, species diversity, nutrient content and water relations of winter annual plant species associated with microbiotic soil crusts in the northeast Mojave Desert. Shoot biomass of winter annuals was 37% greater and plant density was 77% greater on crusts than were biomass and density on soils lacking crust cover (=bare soils). This greater production of annuals on crusts was likely due to enhanced soil conditions including an almost two-fold increase in soil organic matter and inorganic N compared to bare soils. Crusted soils also had 53% greater volumetric water content than bare soils during November and December, the time when winter annuals become established. As plant development progressed into spring, however, soil water availability decreased: More negative plant xylem water potentials were associated with greater plant biomass on crusted soils. Plants associated with microbiotic soil crusts had lower concentrations of N in shoots (mg N g−1 dry mass). However, total shoot N (mg N m−2) was the same in plants growing on the different soil types when biomass production peaked in April. Shoots had similar patterns in their concentration and content of P. Species diversity of annuals was not statistically different between the two soil types. Yet, while native annuals comprised the greatest proportion of shoot biomass on bare soils, exotic forbs and grasses produced more biomass on crusts. Total shoot nutrient content (biomass×concentration) of the two exotic annual species examined was dramatically greater on crusts than bare soils; only one native species had greater shoot content of N and P when growing on crusts than bare soils. Microbiotic crusts appear to increase site fertility in the northeast Mojave Desert, but nutrients and water distributed within a greater biomass of annual plants growing on microbiotic crusts likely resulted in lower concentrations of nutrients in plant tissue and lower xylem pressure potentials than plants growing on bare soils. Exotic annuals growing on crusts appear to respond to the higher N availability by growing faster, potentially outcompeting native annual species.

Comparative ecology of Monochoria korsakowii and M. vaginalis

Two native annual species, Monochoria korsakowiiRegel et Maack and M. vaginalis (Burm. fil.) Kunth, occur in Japan. Although they show similar habitat preferences, these plants are now in contrasting situations in Japan; while M. vaginalis is a noxious weed, M. korsakowii, a once common weed of rice fields, has diminished its distributional range and is now listed as a vulnerable species. For the purpose of conservation and management of M. korsakowii, the life history traits of these species were investigated with particular emphasis on the seasonal growth cycles, patterns of dry matter distribution and photosynthetic and transpiration rates. We found significant differences in phenology and seasonal growth between M. korsakowii and M. vaginalis. M. korsakowii germinated about a month earlier and flowered one and a half months later than M. vaginalis. While shoot to root dry weight ratio in M. korsakowii was larger than 1.0, that in M. vaginalis was smaller than 1.0 during growth. Differences were also found in leaf life span, numbers of leaves and flowers per individual plant and transpiration rate. These findings support the belief that the decline of M. korsakowii relates, to some extent, to the recent changes in agricultural practices in Japan.

Recruitment and survival of native annual Trifolium species in the highlands of Ethiopia

Germination and survival of indigenous annual Trifolium species were studied. Seedlings naturally emerging in the field and from sown seeds in pots were regularly counted and uprooted, and survival was studied by monitoring colour‐coded seedlings. Differences in recruitment of Trifolium species were strongly related to the rainfall pattern. In the fallow (crop) lands, no seedling survived the dry season between the short and main rainy periods during the year, while in the natural pasture, 8% of the seedlings survived into the main growing (rainy) season. Occasional rains occurring in the dry period (between the two rainy periods) also induced successions of germination and seedlings death, and therefore depleted the soft seed reserve in the soil by the beginning of the main growing season. As temperature fluctuations were minimal during the main rainy season, the rate of seed softening was low, affecting new germinations. This had a significant impact on the quality of the natural pastures on which livestock in the highlands are dependent.

Solar tracking and photosynthesis in cotton leaves

Leaf solar tracking movements (ability of leaf to orient perpendicularly to the sun's direct rays) and photosynthetic responses to photon flux level were examined in three cultivars of Upland cotton ( Gossypium hirsutum) and one cultivar of Pima cotton ( Gossypium barbadense). Exposed canopy leaves of upland cotton exhibited strong solar tracking abilities diurnally, whereas leaves of Pima cotton remained in a fixed position throughout the day. Diurnal photosynthetic rates measured on an individual exposed leaf of Upland cotton (Delta Pine 62) that was allowed to solar track or fixed in a horizontal position did not differ except at the low irradiances occurring in the early morning and again in late afternoon. Upon further examination, it appeared that the photosynthetic dependence on photon flux was saturated by approximately 60% of midday irradiance levels and that these photosynthetic-irradiance relationships did not differ among the cotton cultivars examined. These data suggest that, in contrast to the responses observed in several native annual plant species, leaf solar tracking in cotton does not enhance photosynthetic rates of fully exposed sun-lit leaves.

Moisture Stress Effects on Biomass Partitioning in Two Sonoran Desert Annuals

The relative biomass accumulation rate and allocation to reproduction were quantified for Plantago insularis and Schismus barbatus, growing under four different levels of moisture stress. Analysis of growth by an allometric method indicated that seasonal growth rates were modified by water input, and for all water input treatments Schismus grew more rapidly than Plantago. As the growing season progressed and ambient temperatures rose, incremental water input had an inverse effect upon relative root growth of both species. Relative foliar growth rate of either species was not markedly altered by water input. When reproductive structures were present, their growth rates were higher than those of vegetative structures in both species. Schismus responded to water input by significantly increasing the biomass allocated to reproduction, which increased relative reproductive allocation. The computed reproductive allocation of Plantago is not correlated with water input, although the ultimate reproductive biomasses were correlated with the summation of plant moisture stress during reproduction. Thus, the bar-day formulation is useful in estimating the relative sensitivity of reproduction to drought for native annual species growing in similar habitats with varying levels of moisture stress. INTRODUCTION Several recent studies of biomass accumulation and allocation by desert annuals have been reported since the pioneer investigation by Klikoff (1966). His studies indicated the magnitude of biomass accumulation for plants of Plantago insularis was dependent upon the degree of moisture stress and was modified in some moisture conditions by plant density. Subsequently, an extensive survey of biomass accumulation and allocation was conducted with 32 species of Mojave Desert annuals (Turner, 1973, 1974). In the relatively dry year 1972, 22 of these species allocated a higher percentage of biomass to reproductive structures, as compared to the relatively wet year 1973, five species allocated a lower percentage of biomass to reproductive structures. The remaining five species allocated an equivalent percentage of biomass to reproductive structures during both dry and wet years. This investigation, along with the recent studies of Bell et al. (1979), Clark and Burk (1980) and Monson and Szarek (1981), established that biomass allocation in different moisture conditions is species-specific. However, a common deficiency of these three field studies is that no single species was investigated in similar habitats with different levels of moisture stress. Klikoff's investigation suggests that habitat-specific variations in moisture stress modify the rate of biomass accumulation, but he did not address the question of moisture stress-induced variations in allocation patterns. To evaluate the extent of habitat-specific modifications of biomass accumulation and allocation of desert annual species, we have expanded the original study of Klikoff (1966). Our purpose in this investigation was to quantify the biomass accumulation rate and allocation patterns of two abundant Sonoran Desert annuals, Plantago insularis and Schismus barbatus, treated with four different levels of moisture stress in adjacent experimental plots. One method of quantifying the biomass accumulation rate employs an allometric concept of growth analysis (Batschelet, 1979). The allometric concept assumes that the biomass accumulation rate of a plant part or organ may be different from, but dependent upon, the growth rate of the whole plant. Hence, relative growth rates are deterI Present address: Laboratory of Biomedical and Environmental Sciences, University of California, Los Angeles 90024.

Downy Brome: Intruder in the Plant Succession of Big Sagebrush Communities in the Great Basin

Highlight: We investigated succession on six big sagebrush sites of different potential ranging from the edge of the salt desert shrub to seral communities in the pinonljuniper woodland where the brush overstory had been removed by hand cutting. Depending upon the site, herbaceous communities that developed after elimination of overstory cover were composed of either a diversity or a relatively limited number of species of native annual forbs. Alien annual forbs were initial dominants on sites where seed source of these species was available. Dominance by downy brome caused a marked reduction of native annual species. Downy brome was seeded for 3 consecutive years in each of six different big sagebrush communities. On the majority of the sites, providing a seed source resulted in establishment and near total dominance by downy brome. The established populations have persisted and continued to dominate the communities. Some sites were outside the seedbed potential of downy brome, and seeding resulted in establishment only once in 3 years. Presence of plant litter and a rough microtopography were key seedbed characteristics permitting downy brome establishment on these sites.