Invasive Annual Grass Research Articles

Invasive annual grasses indirectly increase virus incidence in California native perennial bunchgrasses

In California valley grasslands, Avena fatua L. and other exotic annual grasses have largely displaced native perennial bunchgrasses such as Elymus glaucus Buckley and Nassella pulchra (A. Hitchc.) Barkworth. The invasion success and continued dominance of the exotics has been generally attributed to changes in disturbance regimes and the outcome of direct competition between species. Here, we report that exotic grasses can also indirectly increase disease incidence in nearby native grasses. We found that the presence of A. fatua more than doubled incidence of infection by barley and cereal yellow dwarf viruses (B/CYDVs) in E. glaucus. Because B/CYDV infection can stunt E. glaucus and other native bunchgrasses, the indirect effects of A. fatua on virus incidence in natives suggests that apparent competition may be an additional mechanism influencing interactions among exotic and native grasses in California. A. fatua's influence on virus incidence is likely mediated by its effects on populations of aphids that vector B/CYDVs. In our study, aphids consistently preferred exotic annuals as hosts and experienced higher fecundity on them, suggesting that the exotics can attract and amplify vector populations. To the best of our knowledge, these findings are the first demonstration that exotic plants can indirectly influence virus incidence in natives. We suggest that invasion success may be influenced by the capacity of exotic plant species to increase the pathogen loads of native species with which they compete.

Light limitation creates patchy distribution of an invasive grass in eastern deciduous forests

Species interactions and their indirect effects on the availability and distribution of resources have been considered strong determinants of community structure in many different ecological systems. In deciduous forests, the presence of overstory trees and shrubs creates a shifting mosaic of resources for understory plants, with implications for their distribution and abundance. Determination of the ultimate resource constraints on understory vegetation may aid management of these systems that have become increasingly susceptible to invasions by non-native plants. Microstegium vimineum (Japanese grass) is an invasive annual grass that has spread rapidly throughout the understory of forests across the eastern United States since it was first observed in Tennessee in 1919. M. vimineum occurs as extensive, dense patches in the understory of eastern deciduous forests, yet these patches often exhibit sharp boundaries and distinct gaps in cover. One example of this distributional pattern was observed relative to the native midstory tree Asimina triloba (pawpaw), whereby dense M. vimineum cover stopped abruptly at the drip line of the A. triloba patch and was absent beneath the A. triloba canopy. We conducted field and greenhouse experiments to test several hypotheses regarding the causes of this observed pattern of M. vimineum distribution, including allelopathy, seed dispersal, light limitations, and soil moisture, texture, and nutrient content. We concluded that light reduction by the A. triloba canopy was the environmental constraint that prevented establishment of M. vimineum beneath this tree. Whereas overstory tree canopy apparently facilitates the establishment of this shade-tolerant grass, the interaction of overstory canopy with midstory canopy interferes with M. vimineum by reducing the availability of sunflecks at the ground layer. It is likely that other midstory species influence the distribution and abundance of other herb-layer species, with implications for management of understory invasive plant species.

Native harvester ants threatened with widespread displacement exert localized effects on serpentine grassland plant community composition

Seed‐harvesting ants can influence the abundance and distribution of plant species through both the selective harvesting of seeds and the construction of nutrient‐rich nest mounds, but the relative contributions of these two mechanisms have not been addressed by previous studies. Furthermore, the impact of ant seed harvesting in California serpentine grasslands remains unresolved because of divergent results from several previous experiments. This study investigates the influence of harvester ants on serpentine grassland plant species composition by examining two potential signatures of seed harvesting ants on plant community composition: species composition on versus off ant nest mounds, and species abundance as a function of distance from nest mounds. Of the 28 plant species identified in this study, 22 exhibited spatial patterns consistent with effects of seed harvesting, nest construction, or both. Although most species showed significant gradients in abundance with distance from a nest, there were no clear relationships between plant species distributions and previously reported harvester ant seed foraging patterns. Harvester ant nest mounds supported plant communities that were distinct from the surrounding serpentine grassland, with notably higher densities of legumes and invasive annual grasses. Comparison of our results with those of previous studies indicates that the patterns we observed are generally consistent over time, but affect more species and a larger fraction of the grassland than previously reported. Unaffected areas of the grassland seem likely to serve as important refuges for some plant species.

Plant invaders, global change and landscape restoration

Modifications in land uses, technology, transportation and biogeochemical cycles currently influence the spread of organisms by reducing the barriers that once restricted their movements. We provide an overview of the spatial and temporal extent for agents of environmental change (land and disturbance transformations, biogeochemical modifications, biotic additions and losses) and highlight those that strongly influence rangeland ecosystems. Restoration may provide a mechanism for ameliorating the impacts of invasive species, but applications of restoration practices over large scales, e.g. ecoregions, will yield benefits earlier when the landscape is prioritised by criteria that identify locations where critical restoration species can grow and where success will be high. We used the Great Basin, USA as our region of interest where the invasive annual grass, cheatgrass (Bromus tectorum), dominates millions of hectares. A landscape-level restoration model for sagebrush (Artemisia tridentata ssp. tridentata and ssp. wyomingensis) was developed to meet the goal of establishing priority habitat for wildlife. This approach could be used in long-range planning of rangeland ecosystems where funds and labour for restoration projects may vary annually.

Sulfosulfuron effects on growth and photosynthesis of 15 range grasses

We conducted greenhouse experiments to compare photosynthetic and growth responses of 2 invasive annual grasses (downy brome = Bromus tectorum L. and medusahead = Taeniatherum caput-medusae (L.) [Nevski]), 6 caespitose grasses, and 7 rhizomatous grasses to the herbicide sulfosulfuron (1-(2-ethylsulfonylimidazo[1,2-a]pyridin-3-ylsulfonyl)-3-(4,6-dimethoxypyrimidin-2-yl)urea). Our objectives were to identify general patterns of species responsiveness and test the hypothesis that sulfosulfuron induced reduction in photosynthetic activity and shoot growth would be more pronounced in small relative to larger plants. Small plants in a spring experiment and large plants in a summer experiment were treated with sulfosulfuron (70 g ai ha−1). Wildryes and bromes were consistently injured; whereas, 5 of the 7 wheatgrasses were not susceptible to sulfosulfuron. Rhizomatous grasses generally experienced greater damage from sulfosulfuron than caespitose grasses. These results suggest that sulfosulfuron would provide a useful rangeland management tool to control unwanted invasive annual grasses without significantly hindering growth and physiology of desirable rangeland grasses.

Maternal and sibling factors induce dormancy in dimorphic seed pairs of Aegilops triuncialis

Germination and emergence are stimulated by environmental cues, but strongly influenced by maternal controls. However, traits related to seed dispersal may have important influences on germination as well. For example, the “sibling rivalry” hypothesis suggests that germination may be inhibited when sibling seeds remain within a single dispersal unit. These two influences on germination suggest different, and possibly conflicting, evolutionary strategies for optimizing individual fitness. Using an invasive annual grass that produces dispersal units with dimorphic seeds, I found significant reductions in seedling emergence that suggested the presence of both strong maternal and sibling influences on the germination of the smaller seed of dimorphic pairs. Both influences were capable of nearly complete germination suppression of the small seed, but there was no strong evidence for a hierarchy among the two factors. The maternal effect is consistent with a bet-hedging strategy for survival in variable environments where resource availability can be unpredictable, but the sibling effect likely represents a mechanism for reducing competition between closely related individuals, particularly under conditions of resource limitation.

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.

Sulfosulfuron effects on growth and photosynthesis of 15 range grasses

We conducted greenhouse experiments to compare photosynthetic and growth responses of 2 invasive annual grasses (downy brome = Bromus tectorum L. and medusahead = Taeniatherum caput-medusae (L.) [Nevski]), 6 caespitose grasses, and 7 rhizomatous grasses to the herbicide sulfosulfuron (1-(2-ethylsulfonylimidazo[1,2-a]pyridin-3-ylsulfonyl)-3-(4,6-dimethoxypyrimidin-2-yl)urea). Our objectives were to identify general patterns of species responsiveness and test the hypothesis that sulfosulfuron induced reduction in photosynthetic activity and shoot growth would be more pronounced in small relative to larger plants. Small plants in a spring experiment and large plants in a summer experiment were treated with sulfosulfuron (70 g ai ha-1). Wildryes and bromes were consistently injured; whereas, 5 of the 7 wheatgrasses were not susceptible to sulfosulfuron. Rhizomatous grasses generally experienced greater damage from sulfosulfuron than caespitose grasses. These results suggest that sulfosulfuron would provide a useful rangeland management tool to control unwanted invasive annual grasses without significantly hindering growth and physiology of desirable rangeland grasses.

Sulfosulfuron effects on growth and photosynthesis of 15 range grasses

We conducted greenhouse experiments to compare photosynthetic and growth responses of 2 invasive annual grasses (downy brome = Bromus tectorum L. and medusahead = Taeniatherum caput-medusae (L.) [Nevski]), 6 caespitose grasses, and 7 rhizomatous grasses to the herbicide sulfosulfuron (1-(2-ethylsulfonylimidazo[1,2-a]pyridin-3-ylsulfonyl)-3-(4,6-dimethoxypyrimidin-2-yl)urea). Our objectives were to identify general patterns of species responsiveness and test the hypothesis that sulfosulfuron induced reduction in photosynthetic activity and shoot growth would be more pronounced in small relative to larger plants. Small plants in a spring experiment and large plants in a summer experiment were treated with sulfosulfuron (70 g ai ha−1). Wildryes and bromes were consistently injured; whereas, 5 of the 7 wheatgrasses were not susceptible to sulfosulfuron. Rhizomatous grasses generally experienced greater damage from sulfosulfuron than caespitose grasses. These results suggest that sulfosulfuron would provide a useful rangeland management tool to control unwanted invasive annual grasses without significantly hindering growth and physiology of desirable rangeland grasses.

Nitrogen Effects on Seed Germination and Seedling Growth

Recent evidence associates the persistence of invasive plant species with disturbance and fluctuations in distinct forms of mineral N in soils. We conducted soil and hydroponic experiments to investigate the influence of N form and availability on germination and seedling development of 2 invasive annual grasses, cheatgrass (Bromus tectorum) and medusahead (Taeniatherum caput-medusae) and 6 perennial grasses, bluebunch wheatgrass (Pseudoroegneria spicata), crested wheatgrass (Agropyron cristatum x A. desertorum), Sand Hollow and Seaman's Gulch big squirreltail (Elymus multisetus), and Little Camas and Little Wood bottlebrush squirreltail (E. elymoides ssp. brevifolius and E. elymoides ssp. elymoides, respectively). Seeds were sown in soils with no soil additions, barley straw (1 mg kg-1), NH4+ = 10 mg N kg-1, NH4+ + I (nitrification inhibitor) = 10 mg N kg-1 + 37 ml nitrapyrin, or NO3- = 10 mg N kg-1 to evaluate cumulative germination percentage for 20 days in an incubator. For the hydroponic experiment, grass seedlings were exposed to distinct forms and uniform concentrations of mineral N to monitor root and shoot growth for 21 days. Treatments were no N added, NH4+ (1 mM), NO3- (1 mM), and NH4NO3 (0.5 mM). Treatments did not alter germination in the soil experiment. Lack of soil N effect on seed germination is attributed to the absence of seed dormancy in the populations of grasses we evaluated. Initial root length and overall shoot growth of grasses were greater in the NO3- than in the NH4+ treatment more frequently for perennial grasses. Root and shoot growth of medusahead and cheatgrass generally exceeded that of the other grasses except crested wheatgrass. However, relative decreases in root dry mass for the no N treatment were greater for the invasive annual grasses than the perennial grasses when compared to the N-addition treatments. DOI:10.2458/azu_jrm_v56i6_monaco

Are Mojave Desert annual species equal? Resource acquisition and allocation for the invasive grass Bromus madritensis subsp. rubens (Poaceae) and two native species

Abundance of invasive plants is often attributed to their ability ot outcompete native species. We compared resource acquisition and allocation of the invasive annual grass Bromus madritensis subsp. rubens with that of two native Mojave Desert annuals, Vulpia octoflora and Descurainia pinnata, in a glasshouse experiment. Each species was grown in monoculture at two densities and two levels of N availability to compare how these annuals capture resources and to understand their relative sensitivities to environmental change. During >4 mo of growth, Bromus used water more rapidly and had greater biomass and N content than the natives, partly because of its greater root-surface area and its exploitation of deep soils. Bromus also had greater N uptake, net assimilation and transpiration rates, and canopy area than Vulpia. Resource use by Bromus was less sensitive to changes in N availability or density than were the natives. The two native species in this study produced numerous small seeds that tended to remain dormant, thus ensuring escape of offspring from unfavorable germination conditions; Bromus produced fewer but larger seeds that readily germinated. Collectively, these traits give Bromus the potential to rapidly establish in diverse habitats of the Mojave Desert, thereby gaining an advantage over coexisting native species.

Contrasting Responses of Intermountain West Grasses to Soil Nitrogen

The mechanisms responsible for soil-N-mediated species replacement of native perennial grasses by the invasive annual grasses cheatgrass (Bromus tectorum L.) and medusahead (Taeniatherum caput-medusae [L.] Nevski) on rangelands are not completely understood. In addition, the contributions of distinct forms of inorganic N (i.e., NH 4 + and NO 3 -) to these shifts in species composition are currently unclear. Consequently, we conducted a greenhouse experiment to test 2 hypotheses: 1) that low N availability reduces growth (root and shoot) and N allocation of invasive annual seedlings more than native perennial species, and 2) that seedling growth and N allocation of invasive annual grasses is more responsive than native perennial grasses when supplied with NO 3 - relative to NH 4 +. We grew seedlings of 2 annual grasses and the native perennial grasses bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Love), and 4 populations of squirreltail (Elymus elymoides [Raf.] Swezey; E. multisetus [J.G. Smith] M.E. Jones) in separate pots and exposed them to treatments differing in N form and availability for 17 weeks. Unexpectedly, root and shoot growth of annual grasses were equal or greater than native perennial grasses under low N availability. Annual grasses took up more NO 3 - and allocated more growth and N to shoots than the perennial grasses (P < 0.05). Perennial grasses had significantly greater root:shoot dry mass ratios than the invasive annual grasses across treatments (P < 0.05). Invasive annual and native perennial grasses both had greater (P < 0.05) shoot and root mass and allocated more N to these structures when supplied with NO 3 - relative to NH 4 +. The ecological implications of these growth and N allocation patterns in response to N availability and form provide important clues regarding the specific traits responsible for differences in competitive ability between invasive annual and native perennial grasses on semiarid rangelands.

Overlapping resource use in three Great Basin species: implications for community invasibility and vegetation dynamics

Summary In the Great Basin of the western United States of America, the invasive annual grass Bromus tectorum has extensively replaced native shrub and bunchgrass communities, but the native bunchgrass Elymus elymoides has been reported to suppress Bromus. Curlew Valley, a site in Northern Utah, provides a model community to test the effects of particular species on invasion by examining competitive relationships among Elymus, Bromus and the native shrub Artemisia tridentata. The site contains Bromus/Elymus, Elymus/Artemisia and monodominant Elymus stands. Transect data indicate that Elymus suppresses Bromus disproportionately relative to its above‐ground cover. Artemisia seedlings recruit in Elymus stands but rarely in the presence of Bromus. This relationship might be explained by competition between the two grasses involving a different resource or occurring in a different season to that between each grass and Artemisia. Time reflectometry data collected in monodominant patches indicated that in spring, soil moisture use by Bromus is rapid, whereas depletion under Elymus and Artemisia is more moderate. Artemisia seedlings may therefore encounter a similar moisture environment in monodominant or mixed perennial stands. However, efficient autumn soil moisture use by Elymus may help suppress Bromus. In competition plots, target Artemisia grown with Bromus were stunted relative to those grown with Elymus, despite equivalent above‐ground biomass of the two grasses. Competition for nitrogen in spring and autumn, assessed with 15N tracer, appears to be secondary to moisture availability in determining competitive outcomes. Elymus physiology and function appear to play an important role in determining the composition of communities in Curlew Valley, by maintaining zones free of Bromus where Artemisia can recruit.

Contrasting responses of Intermountain West grasses to soil nitrogen

The mechanisms responsible for soil-N-mediated species replacement of native perennial grasses by the invasive annual grasses cheatgrass (Bromus tectorum L.) and medusahead (Taeniatherum caput-medusae [L.] Nevski) on rangelands are not completely understood. In addition, the contributions of distinct forms of inorganic N (i.e., NH4+ and NO3-) to these shifts in species composition are currently unclear. Consequently, we conducted a greenhouse experiment to test 2 hypotheses: 1) that low N availability reduces growth (root and shoot) and N allocation of invasive annual seedlings more than native perennial species, and 2) that seedling growth and N allocation of invasive annual grasses is more responsive than native perennial grasses when supplied with NO3- relative to NH4+. We grew seedlings of 2 annual grasses and the native perennial grasses bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Love), and 4 populations of squirreltail (Elymus elymoides [Raf.] Swezey; E. multisetus [J.G. Smith] M.E. Jones) in separate pots and exposed them to treatments differing in N form and availability for 17 weeks. Unexpectedly, root and shoot growth of annual grasses were equal or greater than native perennial grasses under low N availability. Annual grasses took up more NO3- and allocated more growth and N to shoots than the perennial grasses (P 0.05). Perennial grasses had significantly greater root:shoot dry mass ratios than the invasive annual grasses across treatments (P 0.05). Invasive annual and native perennial grasses both had greater (P 0.05) shoot and root mass and allocated more N to these structures when supplied with NO3- relative to NH4+. The ecological implications of these growth and N allocation patterns in response to N availability and form provide important clues regarding the specific traits responsible for differences in competitive ability between invasive annual and native perennial grasses on semiarid rangelands.

Nitrogen effects on seed germination and seedling growth

Recent evidence associates the persistence of invasive plant species with disturbance and fluctuations in distinct forms of mineral N in soils. We conducted soil and hydroponic experiments to investigate the influence of N form and availability on germination and seedling development of 2 invasive annual grasses, cheatgrass (Bromus tectorum) and medusahead (Taeniatherum caput-medusae) and 6 perennial grasses, bluebunch wheatgrass (Pseudoroegneria spicata), crested wheatgrass (Agropyron cristatum x A. desertorum), Sand Hollow and Seaman's Gulch big squirreltail (Elymus multisetus), and Little Camas and Little Wood bottlebrush squirreltail (E. elymoides ssp. brevifolius and E. elymoides ssp. elymoides, respectively). Seeds were sown in soils with no soil additions, barley straw (1 mg kg-1), NH4+ = 10 mg N kg-1, NH4+ + I (nitrification inhibitor) = 10 mg N kg-1 + 37 ml nitrapyrin, or NO3- = 10 mg N kg-1 to evaluate cumulative germination percentage for 20 days in an incubator. For the hydroponic experiment, grass seedlings were exposed to distinct forms and uniform concentrations of mineral N to monitor root and shoot growth for 21 days. Treatments were no N added, NH4+ (1 mM), NO3- (1 mM), and NH4NO3 (0.5 mM). Treatments did not alter germination in the soil experiment. Lack of soil N effect on seed germination is attributed to the absence of seed dormancy in the populations of grasses we evaluated. Initial root length and overall shoot growth of grasses were greater in the NO3- than in the NH4+ treatment more frequently for perennial grasses. Root and shoot growth of medusahead and cheatgrass generally exceeded that of the other grasses except crested wheatgrass. However, relative decreases in root dry mass for the no N treatment were greater for the invasive annual grasses than the perennial grasses when compared to the N-addition treatments.

PEAK FIRE TEMPERATURES AND EFFECTS ON ANNUAL PLANTS IN THE MOJAVE DESERT

Very little is known about the behavior and effects of fire in the Mojave Desert, because fire was historically uncommon. However, fire has become more frequent since the 1970s with increased dominance of the invasive annual grasses Bromus rubens and Schismus spp., and land managers are concerned about its ecological effect. In this paper, I describe patterns of peak fire temperature and their effect on annual plants in creosote bush scrub vegetation of the Mojave Desert. Temperatures were monitored among microhabitats and distances from the soil surface, and between spring and summer. Microhabitats ranged from high amounts of fuel beneath creosote bush (Larrea tridentata) canopies, to intermediate amounts at the canopy drip line, to low amounts in the interspaces between them. Distances from the soil surface were within the vertical range where most annual plant seeds occur (−2, 0, 5, and 10 cm). I also compare temperature patterns with postfire changes in soil properties and annual plant biomass and spe...

Photosynthesis in an invasive grass and native forb at elevated CO2 during an El Niño year in the Mojave Desert.

Annual and short-lived perennial plant performance during wet years is important for long-term persistence in the Mojave Desert. Additionally, the effects of elevated CO2 on desert plants may be relatively greater during years of high resource availability compared to dry years. Therefore, during an El Niño year at the Nevada Desert FACE Facility (a whole-ecosystem CO2 manipulation), we characterized photosynthetic investment (by assimilation rate-internal CO2 concentration relationships) and evaluated the seasonal pattern of net photosynthesis (A net) and stomatal conductance (g s) for an invasive annual grass, Bromus madritensis ssp. rubens and a native herbaceous perennial, Eriogonum inflatum. Prior to and following flowering, Bromus showed consistent increases in both the maximum rate of carboxylation by Rubisco (V Cmax) and the light-saturated rate of electron flow (J max) at elevated CO2. This resulted in greater A net at elevated CO2 throughout most of the life cycle and a decrease in the seasonal decline of maximum midday A net upon flowering as compared to ambient CO2. Eriogonum showed significant photosynthetic down-regulation to elevated CO2 late in the season, but the overall pattern of maximum midday A net was not altered with respect to phenology. For Eriogonum, this resulted in similar levels of A net on a leaf area basis as the season progressed between CO2 treatments, but greater photosynthetic activity over a typical diurnal period. While g s did not consistently vary with CO2 in Bromus, it did decrease in Eriogonum at elevated CO2 throughout much of the season. Since the biomass of both plants increased significantly at elevated CO2, these patterns of gas exchange highlight the differential mechanisms for increased plant growth. The species-specific interaction between CO2 and phenology in different growth forms suggests that important plant strategies may be altered by elevated CO2 in natural settings. These results indicate the importance of evaluating the effects of elevated CO2 at all life cycle stages to better understand the effects of elevated CO2 on whole-plant performance in natural ecosystems.

Rising CO2– future ecosystems

Rising CO₂– future ecosystems

Phosphorus acquisition by Bromus madritensis ssp. rubens from soil interspaces shared with Mojave Desert shrubs

Abstract Bromus madritensis ssp. rubens (L.) Husnot (Red Brome) is an invasive annual grass that is associated with increases in fire frequency and decreases in perennial plant diversity. The success of Red Brome in the Mojave Desert has been attributed to its competitive ability, but competition between Red Brome and native shrubs for below‐ground resources has not been investigated previously. In this study we present a modification of previous dual‐isotope methods that assesses competitive interactions and responses among plants under field conditions. We then use this method to (i) determine if direct competition for phosphorus (P) occurs between Red Brome and native shrubs, and (ii) evaluate the effectiveness of phosphate acquisition by Red Brome from soil interspaces shared with different Mojave Desert shrub species. Clipping Red Brome to remove ≈85% of its foliage on the day prior to labelling soil interspaces with P isotopes did not have the desired effect of inhibiting phosphorus uptake by Red Brome. Therefore we were unable to verify that direct competition for P occurred between Red Brome and native shrubs. Nonetheless, by sampling plant tissue from unclipped strips of Red Brome that were centred between two shrubs, we were able to evaluate the effectiveness of phosphate acquisition by Red Brome from interspaces shared with different shrubs. There were no differences in the amount of phosphorus acquired by Red Brome from soil interspaces shared with the evergreen shrub Larreatridentata (D.C.) Cov. versus soil interspaces shared with the drought deciduous shrub Lycium pallidum Miers. However, Red Brome acquired more than six times as much P from soil interspaces shared with Larrea than from soil interspaces shared with the drought deciduous shrub Ambrosia dumosa Payne. Less P uptake by Red Brome from soil interspaces shared with Ambrosia suggests greater effectiveness of Ambrosia compared with Larrea to deplete soil P.

Elevated CO2 increases productivity and invasive species success in an arid ecosystem.

Arid ecosystems, which occupy about 20% of the earth's terrestrial surface area, have been predicted to be one of the most responsive ecosystem types to elevated atmospheric CO2 and associated global climate change. Here we show, using free-air CO2 enrichment (FACE) technology in an intact Mojave Desert ecosystem, that new shoot production of a dominant perennial shrub is doubled by a 50% increase in atmospheric CO2 concentration in a high rainfall year. However, elevated CO2 does not enhance production in a drought year. We also found that above-ground production and seed rain of an invasive annual grass increases more at elevated CO2 than in several species of native annuals. Consequently, elevated CO2 might enhance the long-term success and dominance of exotic annual grasses in the region. This shift in species composition in favour of exotic annual grasses, driven by global change, has the potential to accelerate the fire cycle, reduce biodiversity and alter ecosystem function in the deserts of western North America.