Bromus Rubens Research Articles

Relationships of Native Desert Plants with Red Brome (Bromus rubens): Toward Identifying Invasion-Reducing Species

AbstractThe interactions between native and exotic species occur on a continuum from facilitative to competitive. A growing thrust in invasive species science is differentiating where particular native species occur along this continuum, with practical implications for identifying species that might reduce the invasibility of ecosystems. We used a greenhouse experiment to develop a competitive hierarchy of 27 native species with red brome, an invasive annual grass in the arid lands of the southwestern United States, and a field study to assess in situ responses of brome to native perennial species in the Mojave Desert. Native species most competitive with brome in the competition experiment included the annuals Esteve's pincushion and western fiddleneck and the perennials eastern Mojave buckwheat, sweetbush, and brittlebush, which reduced brome biomass to 49 to 70% of its grown-alone amount. There was no clear difference in competitive abilities with brome between annual and perennial natives, and competiveness was not strongly correlated (r= 0.15) with the biomass of the native species. In the field, sweetbush and brittlebush supported among the least cover of brome, suggesting congruence of the strong early competitive abilities of these species with in situ patterns of brome distribution. At the other extreme, brome attained its highest average cover (19%) below littleleaf ratany, significantly greater than all but 3 of the 16 species evaluated. Cover by brome was only weakly related (r= 0.19) to the area of the perennial canopy, suggesting that factors other than the sizes of perennial plants were linked to differences in brome cover among species. Results suggest that (1) interactions with brome differ substantially among native species, (2) these interactions are not as closely linked to biomass production as in more temperate regions, and (3) there is potential for identifying native species that can reduce invasion of desert ecosystems.

Species specific plant-soil interactions influence plant distribution on serpentine soils

Where serpentine soils exist, variation in soil properties affects plant species distribution at both coarse and fine spatial scales. The New Idria (California, USA) serpentine mass has barren areas, supporting only sparse shrub and tree islands, adjacent to areas of densely-vegetated serpentine chaparral. To identify factors limiting growth on barren relative to vegetated serpentine soils, we analyzed soils from barren, shrub-island within barren, and vegetated areas and foliage from shrub-island and vegetated areas. We also grew Ceanothus cuneatus (native evergreen shrub), Achillea millefolium (native perennial forb), and Bromus madritensis ssp. rubens (invasive annual grass) in soils from barren and vegetated areas amended factorially with N, K, and Ca in a pot study. In well-watered pots, biomass was greater by 5-, 14-, and 33-fold for Ceanothus, Achillea, and Bromus, respectively, on vegetated-area-collected soils than on barren-collected soils, indicating a strong soil chemistry effect. Although field soil data suggested nutrient deficiency and not heavy metal toxicity, pot study plant data indicated otherwise for two of the three species. On barren-collected soils, only Ceanothus responded positively to added N and Ca and did not show greater foliar Mg or heavy metal (Fe, Ni, Cr, Co, Zn) concentrations than on vegetated-area-collected soils. Ceanothus maintained lower root Mg and heavy metal (Fe, Ni, Cr, Co) concentrations on barren soils and translocated less heavy metal (Fe, Ni, Cr, Co, Mn, Cu) from roots to foliage than Achillea and Bromus. Achillea and Bromus showed significant log-log biomass relationships with foliar Ca:Mg (+), Mg (-), and heavy metals (Fe, Ni, Cr, Co, Mn, Cu, Zn) (-), while Ceanothus showed relationships only with Ca:Mg (+) and Mg (-). The New Idria barren-vegetated pattern appears to be maintained by different factors for different species or functional types— low Ca:Mg ratios on barrens for all species tested, high heavy metal concentrations for Achillea and Bromus, and low macronutrient (N) concentrations for Ceanothus. Combined data from this and other studies suggest high heavy metal concentrations more strongly affect herbaceous than woody species, contributing to variation in species distribution on serpentine soils.

Ecotoxicological diagnosis of a sealed municipal landfill

Assessing the environmental impact of a soil-topped landfill requires an accurate ecotoxicological diagnosis. This paper describes various diagnostic protocols for this purpose and their application to a real case: the urban solid waste (USW) municipal landfill of Getafe (Madrid, Spain). After their initial sealing with soil from the surroundings about 20 years ago, most USW landfills in the autonomous community of Madrid have continued to receive waste. This has hindered precise assessment of their impact on their environment and affected ecosystems. The procedure proposed here overcomes this problem by assessing the situation in edaphic, aquatic and ecological terms. The present study focused on the most influential soil variables (viz. salinity due largely to the presence of anions, and heavy metals and organic compounds). These variables were also determined in surface waters of the wetland most strongly affected by leachates running down landfill slopes. Determinations included the characterization of plant communities and microbial biodiversity. The study was supplemented with a bioassay under controlled conditions in pots containing soil contaminated with variable concentrations of Zn (as ZnCl(2)) intended to assess ecochemical actions in a population of Bromus rubens, which grows profusely in the landfill.

Short-term effects of experimental fires on a Mojave Desert seed bank

A Mojave Desert shrub community was experimentally burned to understand changes in seed bank of desert annual plant species in response to wildfire. Seed mortality ranged from 55 to 80%, and fire caused significant losses of native and alien annual seeds. Schismus arabicus, Schismus barbatus, Bromus madritensis, Bromus tectorum, Erodium cicutarium and Plantago spp. made up >95% of the seed bank. Bromus spp. and Plantago spp. had proportionately greater mortality of seeds than did Schismus spp. and E. cicutarium. Schismus spp. can be lodged into soil cracks thus avoiding lethal temperatures. E. cicutarium has a self-drilling mechanism that places the seeds at greater depth in the soil. Greater seed mortality occurred beneath shrub canopies than interspaces for most species ( Plantago, spp., Bromus spp., and E. cicutarium), but microsite had little effect on Schismus spp. Fire reduced the perennial Ambrosia dumosa densities under canopies. Fire reduced the mean number of species found in samples by about one species per plot and no species was extirpated on experimental plots. The relative abundances of common species did not change dramatically as a result of fire or microsite, however; seed densities varied by treatment and affected interpretations of species compositions.

Early Post-Fire Plant Establishment on a Mojave Desert Burn

Fire has become more extensive in recent decades in southwestern United States arid lands. Burned areas pose management challenges and opportunities, and increasing our understanding of post-fire plant colonization may assist management decision-making. We examined plant communities, soils, and soil seed banks two years after the 2005 Loop Fire, located in a creosote-blackbrush community in Red Rock Canyon National Conservation Area in southern Nevada’s Mojave Desert. Based on a spring sampling of 20, 0.01-ha plots, live + dead cover of the exotic annual Bromus rubens averaged nine times lower on the burn than on a paired unburned area. Perennial species composition shifted from dominance by late-successional native shrubs (e.g., Coleogyne ramosissima) on the unburned area, to dominance by native perennial forbs (e.g., Sphaeralcea ambigua, Baileya multiradiata )o n the burn. Species richness of live plants averaged 26% (100 m 2 scale) and 239% (1 m 2 scale) greater on the burn compared to the unburned area. Only 5% of Larrea tridentata individuals resprouted, compared to 64% of Yucca schidigera and baccata. Fire and microsite (interspace, below L. tridentata, or below Yucca) interacted to affect several 0–5 cm soil properties, with higher pH, conductivity, and total P and K on burned Yucca microsites. Bromus rubens density in 0–5 cm soil seed banks was four times lower on the burn, and its distribution among microsites reversed. Below-shrub microsites contained the most B. rubens seeds on the unburned area, but the least on the burned area. Intense fire below shrubs may have increased seed mortality, an idea supported by .3-fold decreases we found in emergence density after heating seed bank samples to 100uC. Our study occurred after a post-fire period of below-average precipitation, underscoring a need for longer term monitoring that characterizes moister years.

Interactive Effects of Simulated Herbivory and Interspecific Competition on Survival of Blackbrush (Coleogyne ramosissima) Seedlings

Abstract I investigated the individual and joint effects of simulated herbivory and interspecific competition on survival of Coleogyne ramosissima Torr. (blackbrush) seedlings. Seeds of C. ramosissima and Bromus rubens L. (red brome grass) were collected at mid-elevations (1220 to 1770 m) of the Spring Mountains in southern Nevada. A pot trial experiment was conducted for six months (27 wk) in a controlled environmental greenhouse. This trial experiment, consisting of a 2 × 2 factorial design with simulated rodent herbivory and interspecific competition with B. rubens as the main effects, resulted in four treatments. Herbivory on C. ramosissima by heteromyid and non-heteromyid species of rodents was simulated by clipping the top 3 cm of young shoots. Significant interaction was detected between herbivory and competition for C. ramosissima seedling survival. When herbivory and competition were examined independently, both factors had significant adverse effects on C. ramosissima seedling survival, with the...

Assessing Cheatgrass (Bromus tectorum) Genetic Diversity and Population Structure Using Rapd and Microsatellite Molecular Markers

Two molecular marker systems, random amplified polymorphic DNA (RAPD) and microsatellites, were used to evaluate population diversity and differentiation in 4 northern Nevada Bromus tectorum populations. From 80 RAPD primers, we found 16 (20%) that yielded 165 strong repeatable bands. Of those bands, 60 (35.8%) were polymor- phic. Of those, 21 met data-pruning guidelines for final analysis. RAPD variation was moderate (x = 0.363, s x -= 0.022), ranging from 0.312 to 0.404. Microsatellite variation was similar ( x - = 0.234, s x 0.051) but varied more widely, rang- ing from 0.009 to 0.551. All populations were out of Hardy-Weinberg equilibrium, as expected in a predominantly self- ing species. RAPDs revealed significant differentiation (P < 0.0001) across populations, whereas microsatellites only resolved 2 of the 4 populations. RAPDs revealed a considerable amount of variation in the 2 populations, Hot Springs and Truckee, which had nearly identical microsatellite profiles. Of 184 individuals, we found 182 unique RAPD pheno- types. We found 51 microsatellite genotypes across individuals, one of which was present in 95.3% of the individuals in the Hot Springs-Truckee group and 29% of the Stillwater individuals. The UPGMA phenograms were similar in group- ing Hot Springs with Truckee, and Stillwater with Peavine, and were highly correlated but not significantly. Both marker systems proved useful in assessing population genetic variation. One population-specific RAPD marker and 6 new microsatellite-length polymorphisms were identified. Both marker systems may also prove useful in studying other closely related species such as red brome (Bromus rubens).

Mercury in soils and plants in an abandoned cinnabar mining area (SW Spain)

An abandoned cinnabar mining area located in the South-West of Spain has been studied with the aim of assessing its mercury pollution level and enhancing the knowledge about the Hg soil/plant relationship. To do so, soils and plants were sampled near an inactive smelter and around two mining sites present in this area. Critical total Hg concentrations were found in the close environs of pollutant sources. These also show high levels of elemental Hg (up to 8 mg kg −1), but quite low exchangeable Hg contents (0.008–0.038 mg kg −1). Most plant specimens display in their aboveground tissues Hg concentrations comprised in the range 0.1–10 mg kg −1, with a great proportion (50%) showing critical levels. Greater Hg contents were found in plant specimens growing in soils with higher elemental Hg concentrations. The plant species displaying the greatest Hg levels are either perennial species of small-medium size and/or showing medium-highly corrugated leaves, or annual plants of small size. Marrubium vulgare L., Bromus madritensis L. and Trifolium angustifolium L. are the plant species with the highest Hg contents (37.6, 12.7 and 9.0 mg kg −1, respectively). Leaf specific surface seems an important feature in the atmospheric Hg uptake by plants.

Trace elements in wild grasses: a phytoavailability study on a remediated field

There have been significant efforts to establish a widely usable method for the prediction of trace element bioavailability in soil. In this work, we used extraction with 0.01 M CaCl2 and 0.05 M ethylenediaminetetraacetic acid (EDTA) to estimate bioavailable concentrations of As, Cd, Cu, Pb, and Zn in a soil moderately contaminated with trace elements 1 and 2 years after the application of three amendments. The experiment took place in a field plot of a soil affected by the toxic spill of the Aznalcóllar mine. Four treatments were established: three with amendments (biosolid compost, sugar beet lime, and a combination of leonardite plus sugar beet lime) and a control without amendment. Trace element concentrations of two representative species in each year (Lamarckia aurea and Poa annua in 2004 and Lamarckia aurea and Bromus rubens in 2005) were analyzed. The results showed a positive effect of the amendments both on soil and vegetation. Trace element concentrations in plants growing in the amended subplots were lower than those in plants from nonamended subplots. As a rule, concentrations of CaCl2-soluble Cd, Cu, and Zn in soil were positively correlated with trace elements in plants, whereas EDTA extraction was scarcely correlated with plant concentration. For species of grasses, especially L. aurea, CaCl2 seems to be a more suitable extractant to predict trace element bioavailability in this contaminated soil.

Root characteristics of representative Mediterranean plant species and their erosion-reducing potential during concentrated runoff

Gully erosion is an important soil degradation process in Mediterranean environments. Revegetation strategies for erosion control rely in most cases on the effects of the above-ground biomass on reducing water erosion rates, whereas the role of the below-ground biomass is often neglected. In a Mediterranean context, the above-ground biomass can temporally disappear because of fire or overgrazing and when concentrated flow erosion occurs, roots can play an important role in controlling soil erosion rates. Unfortunately, information on root characteristics of Mediterranean plants, growing on semi-natural lands, and their effects on the topsoil resistance to concentrated flow erosion is lacking. Therefore, typical Mediterranean grass, herb, reed, shrub and tree root systems of plants growing in habitats that are prone to concentrated flow erosion (i.e. in ephemeral channels, abandoned fields and steep badland slopes) are examined and their erosion-reducing potential was evaluated. Root density (RD), root length density (RLD) and root diameters are measured for 26 typical Mediterranean plant species. RD values and root diameter distribution within the upper 0.10–0.90 m of the soil profile are then transformed into relative soil detachment rates using an empirical relationship in order to predict the erosion-reducing effect of root systems during concentrated runoff. Comparing the erosion-reducing potential of different plant species allows ranking them according to their effectiveness in preventing or reducing soil erosion rates by concentrated flow. RD in the 0.10 m thick topsoil ranges between 0.13 kg m−3 for Bromus rubens (L.) and 19.77 kg m−3 for Lygeum spartum (L.), whereas RLD ranges between 0.01 km m−3 for Nerium oleander (L.) and 120.43 km m−3 for Avenula bromoides ((Gouan) H. Scholz.) Relative soil detachment rates, compared to bare soils, range between 0.3 × 10-12 and 0.7 for the 0.10 m thick topsoil. The results show that grasses such as Helictotrichon filifolium ((Lag.) Henrard), Piptatherum miliaceum ((L.) Coss.), Juncus acutus (L.), Avenula bromoides ((Gouan) H. Scholz), Lygeum spartum (L.) and Brachypodium retusum ((Pers.) Beauv.) have the highest potential to reduce soil erosion rates by concentrated flow in the 0–0.1 m topsoil. But also shrubs such as Anthyllis cytisoides (L.) and Tamarix canariensis (Willd.), having high root densities in the topsoil, can reduce erosion rates drastically. Among the species growing in channels, Juncus acutus (L.) has the highest erosion reducing potential, whereas Phragmites australis (Cav.) is the least effective. On abandoned fields, Avenula bromoides ((Gouan) H. Scholz) and Plantago albicans (L.) are the most effective species in reducing concentrated flow erosion rates, while Thymelaea hirsuta (L. (Endl.)) and Bromus rubens (L.) perform the worst. On steep badland slopes, Helictotrichon filifolium ((Lag.) Henrard) and Anthyllis cytisoides (L.) perform the best in the analysis of erosion reducing potential, while Ononis tridentata (L.) is the least effective species. These findings have implications for ecological restoration and management of erosion-prone slopes.

Feedbacks of Soil Inoculum of Mycorrhizal Fungi Altered by N Deposition on the Growth of a Native Shrub and an Invasive Annual Grass

Anthropogenic nitrogen (N) deposition causes shifts in vegetation types as well as species composition of arbuscular mycorrhizal (AM) fungi and other soil microorganisms. A greenhouse experiment was done to determine whether there are feedbacks between N-altered soil inoculum and growth of a dominant native shrub and an invasive grass species in southern California. The region is experiencing large-scale loss of Artemisia californica shrublands and replacement by invasive annual grasses under N deposition. Artemisia californica and Bromus madritensis ssp. rubens were grown with soil inoculum from experimental plots in a low N deposition site that had (1) N-fertilized and (2) unfertilized soil used for inoculum, as well as (3) high-N soil inoculum from a site exposed to atmospheric N deposition for four decades. All treatments plus a nonmycorrhizal control were given two levels of N fertilizer solution. A. californica biomass was reduced by each of the three inocula compared to uninoculated controls under at least one of the two N fertilizer solutions. The inoculum from the N-deposition site caused the greatest growth depressions. By contrast, B. madritensis biomass increased with each of the three inocula under at least one, or both, of the N solutions. The different growth responses of the two plant species may be related to the types of AM fungal colonization. B. madritensis was mainly colonized by a fine mycorrhizal endophyte, while A. californica had primarily coarse endophytes. Furthermore, A. californica had a high level of septate, nonmycorrhizal root endophytes, while B. madritensis overall had low levels of these endophytes. The negative biomass response of A. californica seedlings to high N-deposition inoculum may in part explain its decline; a microbially-mediated negative feedback may occur in this system that causes poor seedling growth and establishment of A. californica in sites subject to N deposition and B. madritensis invasion.

Interspecific Competition Between Coleogyne ramosissima Seedlings and Bromus rubens

Abstract Interactive effects of red brome grass (Bromus rubens) density and time of establishment on the early survival and growth of blackbrush (Coleogyne ramosissima) seedlings were quantitatively investigated. Seeds of Coleogyne and Bromus were collected from Cold Creek of the Spring Mountains in southern Nevada. A series of pot trial experiments were conducted in a controlled environmental glasshouse. In mixed culture pots when Coleogyne seedlings planted four weeks later than Bromus at medium and high density levels, survival of Coleogyne seedlings (experimental populations) was greatly reduced compared to single Coleogyne seedlings that grew alone (control population). Significant interactions were detected between neighboring Bromus density and time of planting for shoot height, root/shoot ratio, leaf length, and shoot water potential of experimental Coleogyne populations. When Bromus density was examined independently, all measured growth parameters of experimental Coleogyne populations were signi...

Dominance and environmental correlates of alien annual plants in the Mojave Desert, USA

Land managers are concerned about the negative effects of alien annual plants on native plants, threatened and endangered species such as the desert tortoise ( Gopherus agassizii), and ecosystem integrity in the Mojave Desert. Management of alien plants is hampered by a lack of information regarding the dominance and environmental correlates of these species. The results of this study indicate that alien plant species comprised a small fraction of the total annual plant flora, but most of the annual plant community biomass. When rainfall was high in 1995, aliens comprised 6% of the flora and 66% of the biomass. When rainfall was low in 1999, aliens comprised 27% of the flora and 91% of the biomass. Bromus rubens, Schismus spp. ( S. arabicus and S. barbatus), and Erodium cicutarium were the predominant alien species during both years, comprising 99% of the alien biomass. B. rubens was more abundant in relatively mesic microhabitats beneath shrub canopies and at higher elevations above 800–1000 m, whereas Schismus spp. and E. cicutarium were more abundant in the relatively arid interspaces between shrubs, and, for Schismus spp., at lower elevations as well. Disturbance variables were more reliable indicators of alien dominance than were productivity or native plant diversity variables, although relationships often varied between years of contrasting rainfall. The strongest environmental correlates occurred between dirt road density and alien species richness and biomass of E. cicutarium, and between frequency and size of fires and biomass of B. rubens.

Spatial and temporal patterns of wildfires in the Mojave Desert, 1980–2004

Fire has been historically infrequent in the Mojave Desert, and its increased prevalence caused by the invasion of non-native annual grasses is a major concern for land managers there. The most dramatic changes have occurred in middle elevation shrublands dominated by creosotebush ( Larrea tridentata), Joshua tree ( Yucca brevifolia), and/or blackbrush ( Coleogyne ramossissima), where most of the fires occurred between 1980 and 2004. This zone is more susceptible than other areas of the Mojave Desert to increased fire size following years of high rainfall. Increases in fire size are likely related to the flush of non-native annual grasses, Bromus rubens in particular, that produces continuous fuelbeds following years of high rainfall. This dynamic also has occurred to some degree at lower elevations, but the background cover of native perennial fuels there is already very low, muting the effects of the ephemeral fuels. At elevations above the middle elevation shrublands, fire size does not vary with rainfall, indicating that native woody fuels dictate fire regimes. These results suggest that an invasive plant/fire regime cycle is currently establishing in the middle and possibly the low elevation shrublands of the Mojave Desert, but not at higher elevations.

Effects of livestock watering sites on alien and native plants in the Mojave Desert, USA

Increased livestock densities near artificial watering sites create disturbance gradients called piospheres. We studied responses of alien and native annual plants and native perennial plants within 9 piospheres in the Mojave Desert of North America. Absolute and proportional cover of alien annual plants increased with proximity to watering sites, whereas cover and species richness of native annual plants decreased. Not all alien species responded the same, as the alien forb Erodium cicutarium and the alien grass Schismus spp. increased with proximity to watering sites, and the alien annual grass Bromus madritensis ssp. rubens decreased. Perennial plant cover and species richness also declined with proximity to watering sites, as did the structural diversity of perennial plant cover classes. Significant effects were focused within 200 m of the watering sites, suggesting that control efforts for alien annual plants and restoration efforts for native plants should optimally be focused within this central part of the piosphere gradient.

Soil microorganisms of a native shrub and exotic grasses along a nitrogen deposition gradient in southern California

Both anthropogenic nitrogen deposition and exotic annual grass invasion are occurring in the coastal sage scrub vegetation (CSS) of southern California. A study was carried out to determine the effects of these changes on soil microbial communities. Soils were sampled under exotic grasses and the native shrub Artemisia californica along an urban-to-rural N deposition gradient, and in N-fertilized plots in a rural site with low deposition. Arbuscular mycorrhizal (AM) colonization of A. californica was highest in low-N soils, but annual grasses (mainly Bromus madritensis) were colonized primarily by a fine endophyte that showed no pattern of response to soil N level. In addition, annual grasses generally had higher colonization by nonmycorrhizal, primarily septate, fungi in high N soils, while nonmycorrhizal colonization of A. californica was low in all soils. Spore density declined in the rhizosphere of A. californica with elevated N, but not of B. madritensis. Fatty acid methyl esters (FAME) were extracted from soil and their profiles were used to describe the soil microbial communities. Principal components analysis of FAME profiles showed a significant but weak relationship with levels of soil N. At the low N-deposition site that received N fertilizer, host plant was more important than soil N in determining FAME profiles. The most abundant fatty acids were the biomarkers for AM fungi. The different microbial communities of the two species, especially the predominance of fine endophyte and nonmycorrhizal fungi in roots of B. madritensis, warrant further research on functional responses to understand how these microorganisms may be involved in the invasion of native shrubland by exotic grasses.

Effects of litter and standing biomass on growth and reproduction of two annual species in a Mediterranean old‐field

Summary We tested the hypothesis that the competitive ability of a plant changes during its lifetime, by evaluating variations in the relative influence of standing biomass and litter on plant performance. Seedlings of two annual herbaceous species (‘targets’), of contrasted life‐forms (Crepis foetida and Bromus madritensis) were transplanted into an old‐field. The competitive ability of seedlings, vegetative and reproductive plants towards neighbours was recorded over one growing season. Fourteen traits related to plant morphology, growth and reproduction were measured to assess the competitive ability of targets. Relationships among traits were characterized to identify a set of traits as a surrogate of target competitive ability. The two target species responded similarly. Early growth was facilitated by litter and, to a greater degree, by vegetative biomass. Thereafter, the effect of neighbouring vegetation on target performance was negative, with a maximum depressing effect on reproduction (especially seed number and date of flowering) despite a weak facilitative effect of litter. Plant basal diameter, measured at peak growth period, and total number of inflorescences per plant, were found to predict growth and reproductive components of competitive ability, respectively. Assessing the effects of competition for population success from the response of vegetative plants will underestimate its importance because competition appears to exert its maximal impact on seed production.

Sonoran Desert Winter Annuals Affected by Density of Red Brome and Soil Nitrogen

Red brome [Bromus madritensis subsp. rubens (L.) Husn.] is a Mediterranean winter annual grass that has invaded Southwestern USA deserts. This study evaluated interactions among 13 Sonoran Desert annual species at four densities of red brome from 0 to the equivalent of 1200 plants m−2. We examined these interactions at low (3 μg) and high (537 μg NO3− g soil−1) nitrogen (N) to evaluate the relative effects of soil N level on survival and growth of native annuals and red brome. Red brome did not affect emergence or survival of native annuals, but significantly reduced growth of natives, raising concerns about effects of this exotic grass on the fecundity of these species. Differences in growth of red brome and of the three dominant non nitrogen-fixing native annuals at the two levels of soil N were similar. Total species biomass of red brome was reduced by 83% at low, compared to high, N levels, whereas that of the three native species was reduced by from 42 to 95%. Mean individual biomass of red brome was reduced by 87% at low, compared to high, N levels, whereas that of the three native species was reduced by from 72 to 89%.

Red brome (Bromus rubens subsp. madritensis) in North America: possible modes for early introductions, subsequent spread

Although invasions by exotic plants have increased dramatically as human travel and commerce have increased, few have been comprehensively described. Understanding the patterns of invasive species’ spread over space and time will help guide management activities and policy. Tracing the earliest appearances of an exotic plant reveals likely sites of introduction, paving the way for genetic studies to quantify founder events and identify potential source populations. Red brome (Bromus madritensis subsp. rubens) is a Mediterranean winter annual grass that has invaded even relatively undisturbed areas of western North America, where it threatens native plant communities. This study used herbarium records and contemporary published accounts to trace the early introductions and subsequent spread of red brome in western North America. The results challenge the most frequently cited sources describing the early history of this grass and suggest three possible modes for early introductions: the California Gold Rush and Central Valley wheat, southern California shipping, and northern California sheep. Subsequent periods of most rapid spread into new areas, from 1930 to 1942, and of greatest spread into new regions, during the past 50 years, coincide with ‘warm’ Pacific Decadal Oscillation regimes, which are linked to increased winter precipitation in the southwestern USA and northern Mexico. Global environmental change, including increased atmospheric CO2 levels and N deposition, may be contributing to the success of red brome, relative to native species.

15 N uptake by mycorrhizal native and invasive plants from a N-eutrophied shrubland: a greenhouse experiment

In the coastal sage scrub (CSS) community of southern California, Artemisia californica and other native shrubs are gradually being replaced by invasive annual grasses, especially Bromus madritensis ssp. rubens. This decline may be attributed, in part, to local atmospheric N deposition, which causes elevated soil NO3 −. Unaffected soils have low N with a relatively higher concentration of NH4 + than NO3 −. The objectives in this study were: (1) to compare the short-term uptake of 15NO3 − and 15NH4 + by B. madritensis and A. californica and (2) to discern whether 15NO3 − or 15NH4 + uptake is influenced by the mycorrhizal status of either plant species. Analysis of 15N concentrations indicated that both A. californica and B. madritensis took up more 15NO3 − than 15NH4 +, but overall, B. madritensis took up 6–15 times more of both forms of 15N than A. californica. Mycorrhizal A. californica had an increased 15NH4 + concentration in roots but not shoots. In B. madritensis, the only mycorrhizal response was a reallocation of 15NO3 −, with mycorrhizal plants retaining a higher proportion of 15NO3 − in roots. Overall, arbuscular mycorrhizae had a small effect on 15N uptake in this short-term study. However, the ability of B. madritensis to take up so much 15N may explain in part why it has been so successful in replacing A. californica in CSS.