Established native perennial grasses out-compete an invasive annual grass regardless of soil water and nutrient availability

Competition and resource availability influence invasions into native perennial grasslands by non-native annual grasses such as Bromus tectorum. In two greenhouse experiments we examined the influence of competition, water availability, and elevated nitrogen (N) and phosphorus (P) availability on growth and reproduction of the invasive annual grass B. tectorum and two native perennial grasses (Elymus elymoides, Pascopyrum smithii). Bromus tectorum aboveground biomass and seed production were significantly reduced when grown with one or more established native perennial grasses. Conversely, average seed weight and germination were significantly lower in the B. tectorum monoculture than in competition native perennial grasses. Intraspecific competition reduced per-plant production of both established native grasses, whereas interspecific competition from B. tectorum increased production. Established native perennial grasses were highly competitive against B. tectorum, regardless of water, N, or P availability. Bromus tectorum reproductive potential (viable seed production) was not significantly influenced by any experimental manipulation, except for competition with P. smithii. In all cases, B. tectorum per-plant production of viable seeds exceeded parental replacement. Our results show that established plants of Elymus elymoides and Pascopyrum smithii compete successfully against B. tectorum over a wide range of soil resource availability.     

Direct Effects of Soil Amendments on Field Emergence and Growth of the Invasive Annual Grass Bromus tectorum L. and the Native Perennial Grass Hilaria jamesii (Torr.) Benth

Bromus tectorum L. is a non-native, annual grass that has invaded western North America. In SE Utah, B. tectorum generally occurs in grasslands dominated by the native perennial grass, Hilaria jamesii (Torr.) Benth. and rarely where the natives Stipa hymenoides Roem. and Schult. and S. comata Trin. & Rupr. are dominant. This patchy invasion is likely due to differences in soil chemistry. Previous laboratory experiments investigated using soil amendments that would allow B. tectorum to germinate but would reduce B. tectorum emergence without affecting H. jamesii. For this study we selected the most successful treatments (CaCl₂, MgCl₂, NaCl and zeolite) from a previous laboratory study and applied them in the field in two different years at B. tectorum-dominated field sites. All amendments except the lowest level of CaCl₂ and zeolite negatively affected B. tectorum emergence and/or biomass. No amendments negatively affected the biomass of H. jamesii but NaCl reduced emergence. Amendment effectiveness depended on year of application and the length of time since application. The medium concentration of zeolite had the strongest negative effect on B. tectorum with little effect on H. jamesii. We conducted a laboratory experiment to determine why zeolite was effective and found it released large amounts of Na⁺, adsorbed Ca²⁺, and increased Zn²⁺, Fe²⁺, Mn²⁺, Cu²⁺, exchangeable Mg²⁺, exchangeable K, and NH₄⁺ in the soil. Our results suggest several possible amendments to control B. tectorum. However, variability in effectiveness due to abiotic factors such as precipitation and soil type must be accounted for when establishing management plans.     

Arbuscular Mycorrhizal Assemblages in Native Plant Roots Change in the Presence of Invasive Exotic Grasses

Plant invasions have the potential to significantly alter soil microbial communities, given their often considerable aboveground effects. We examined how plant invasions altered the arbuscular mycorrhizal fungi of native plant roots in a grassland site in California and one in Utah. In the California site, we used experimentally created plant communities composed of exotic (Avena barbata, Bromus hordeaceus) and native (Nassella pulchra, Lupinus bicolor) monocultures and mixtures. In the Utah semi-arid grassland, we took advantage of invasion by Bromus tectorum into long-term plots dominated by either of two native grasses, Hilaria jamesii or Stipa hymenoides. Arbuscular mycorrhizal fungi colonizing roots were characterized with PCR amplification of the ITS region, cloning, and sequencing. We saw a significant effect of the presence of exotic grasses on the diversity of mycorrhizal fungi colonizing native plant roots. In the three native grasses, richness of mycorrhizal fungi decreased; in the native forb at the California site, the number of fungal RFLP patterns increased in the presence of exotics. The exotic grasses also caused the composition of the mycorrhizal community in native roots to shift dramatically both in California, with turnover of Glomus spp., and Utah, with replacement of Glomus spp. by apparently non-mycorrhizal fungi. Invading plants may be able to influence the network of mycorrhizal fungi in soil that is available to natives through either earlier root activity or differential carbon provision compared to natives. Alteration of the soil microbial community by plant invasion can provide a mechanism for both successful invasion and the resulting effects of invaders on the ecosystem.     

Mowing Reduces Exotic Annual Grasses but Increases Exotic Forbs in a Semiarid Grassland

Cheatgrass (Bromus tectorum) and other exotic winter‐active plants can be persistent invaders in native grasslands, growing earlier in the spring than native plants and pre‐empting soil resources. Effective management strategies are needed to reduce their abundance while encouraging the reestablishment of desirable native plants. In this 4‐year study, we investigated whether mowing and seeding with native perennial grasses could limit growth of exotic winter‐actives, and benefit growth of native plants in an invaded grassland in Colorado, United States. We established a split‐plot experiment in October 2008 with 3 mowing treatments: control, spring‐mowed, and spring/summer‐mowed (late spring, mid‐summer, and late summer), and 3 within‐plot seeding treatments: control, added B. tectorum seeds, and added native grass seeds. Cover of plant species and aboveground biomass were measured for 3 years. In March and June of 2010, 2011, and March of 2012, B. tectorum and other winter‐annual grasses were half as abundant in both mowing treatments as in control plots; however, cover of non‐native winter‐active forbs increased 2‐fold in spring‐mowed plots and almost 3‐fold in spring/summer‐mowed plots relative to controls. These patterns remained consistent 1 year after termination of treatments. Native cool‐season grasses were most abundant in spring‐mowed plots, and least abundant in control plots. There was higher cover of native warm‐season grasses in spring/summer‐mowed plots than in control plots in July 2011 and 2012. The timing of management can have strong effects on plant community dynamics in grasslands, and this experiment indicates that adaptive management can target the temporal niche of undesirable invasive species.     

Effects of resource availability and propagule supply on native species recruitment in sagebrush ecosystems invaded by Bromus tectorum

Resource availability and propagule supply are major factors influencing establishment and persistence of both native and invasive species. Increased soil nitrogen (N) availability and high propagule inputs contribute to the ability of annual invasive grasses to dominate disturbed ecosystems. Nitrogen reduction through carbon (C) additions can potentially immobilize soil N and reduce the competitiveness of annual invasive grasses. Native perennial species are more tolerant of resource limiting conditions and may benefit if N reduction decreases the competitive advantage of annual invaders and if sufficient propagules are available for their establishment. Bromus tectorum, an exotic annual grass in the sagebrush steppe of western North America, is rapidly displacing native plant species and causing widespread changes in ecosystem processes. We tested whether nitrogen reduction would negatively affect B. tectorum while creating an opportunity for establishment of native perennial species. A C source, sucrose, was added to the soil, and then plots were seeded with different densities of both B. tectorum (0, 150, 300, 600, and 1,200 viable seeds m⁻²) and native species (0, 150, 300, and 600 viable seeds m⁻²). Adding sucrose had short-term (1 year) negative effects on available nitrogen and B. tectorum density, biomass and seed numbers, but did not increase establishment of native species. Increasing propagule availability increased both B. tectorum and native species establishment. Effects of B. tectorum on native species were density dependent and native establishment increased as B. tectorum propagule availability decreased. Survival of native seedlings was low indicating that recruitment is governed by the seedling stage.     

Soil water exploitation after fire: competition between Bromus tectorum (cheatgrass) and two native species

Summary Causes for the widespread abundance of the alien grass Bromus tectorum (cheatgrass) after fire in semiarid areas of western North America may include: (1) utilization of resources freed by the removal of fireintolerant plants; and (2) successful competition between B. tectorum and individual plants that survive fire. On a site in northwestern Nevada (USA), measurements of soil water content, plant water potential, aboveground biomass production, water use efficiency, and B. tectorum tiller density were used to determine if B. tectorum competes with either of two native species (Stipa comata and Chrysothamnus viscidiflorus) or simply uses unclaimed resources. Soil water content around native species occurring with B. tectorum was significantly lower (P<0.05) than around individuals without B. tectorum nearby. Native species had significantly more negative plant water potential when they occurred with B. tectorum. Aboveground biomass was significantly higher for native species without B. tectorum. However, the carbon isotope ratio of leaves for native species with B. tectorum was not significantly different from individuals without B. tectorum. Thus, B. tectorum competes with native species for soil water and negatively affects their wate status and productivity, but the competition for water does not affect water use efficiency of the native species. These adverse effects of B. tectorum competition on the productivity and water status of native species are also evident at 12 years after a fire. This competitive ability of B. tectorum greatly enhances its capability to exploit soil resources after fire and to enhance its status in the community.     

Soil amendment interacts with invasive grass and drought to uniquely influence aboveground versus belowground biomass in aridland restoration

Water‐holding soil amendments such as super‐absorbent polymer (SAP) may improve native species establishment in restoration but may also interact with precipitation or invasive species such as Bromus tectorum L. (cheatgrass or downy brome) to influence revegetation outcomes. We implemented an experiment at two sites in Colorado, U.S.A., in which we investigated the interactions of drought (66% reduction of ambient rainfall), B. tectorum seed addition (BRTE, 465 seeds/m²), and SAP soil amendment (25 g/m²) on initial plant establishment and 3‐year aboveground and belowground biomass and allocation. At one site, SAP resulted in higher native seeded species establishment but only with ambient precipitation. However, by the third year, we detected no SAP effects on native seeded species biomass. Treatments interacted to influence aboveground and belowground biomass and allocation differently. At one site, a SAP × precipitation interaction resulted in lower belowground biomass in plots with SAP and drought (61.7 ± 7.3 g/m²) than plots with drought alone (91.6 ± 18.1 g/m²). At the other site, a SAP × BRTE interaction resulted in higher belowground biomass in plots with SAP and BRTE (56.6 ± 11.2 g/m²) than BRTE alone (35.0 ± 3.7 g/m²). These patterns were not reflected in aboveground biomass. SAP should be used with caution in aridland restoration because initial positive effects may not translate to long‐term benefits, SAP may uniquely influence aboveground versus belowground biomass, and SAP can interact with environmental variables to impact developing plant communities in positive and negative ways.     

Nitrogen acquisition by annual and perennial grass seedlings: testing the roles of performance and plasticity to explain plant invasion

Differences in resource acquisition between native and exotic plants is one hypothesis to explain invasive plant success. Mechanisms include greater resource acquisition rates and greater plasticity in resource acquisition by invasive exotic species compared to non-invasive natives. We assess the support for these mechanisms by comparing nitrate acquisition and growth of invasive annual and perennial grass seedlings in western North America. Two invasive exotic grasses (Bromus tectorum and Taeniatherum caput-medusae) and three perennial native and exotic grasses (Pseudoroegneria spicata, Elymus elymoides, and Agropyron cristatum) were grown at various temperatures typical of autumn and springtime when resource are abundant and dominance is determined by rapid growth and acquisition of resources. Bromus tectorum and perennial grasses had similar rates of nitrate acquisition at low temperature, but acquisition by B. tectorum significantly exceeded perennial grasses at higher temperature. Consequently, B. tectorum had the highest acquisition plasticity, showcasing its ability to take advantage of transient warm periods in autumn and spring. Nitrate acquisition by perennial grasses was limited either by root production or rate of acquisition per unit root mass, suggesting a trade-off between nutrient acquisition and allocation of growth to structural tissues. Our results indicate the importance of plasticity in resource acquisition when temperatures are warm such as following autumn emergence by B. tectorum. Highly flexible and opportunistic nitrate acquisition appears to be a mechanism whereby invasive annual grasses exploit soil nitrogen that perennials cannot use.     

Abiotic constraints on the competitive ability of exotic and native grasses in a Pacific Northwest prairie

In prairie ecosystems, abiotic constraints on competition can structure plant communities; however, the extent to which competition between native and exotic plant species is constrained by environmental factors is still debated. The objective of our study was to use paired field and greenhouse experiments to evaluate the competitive dynamics between two native (Danthonia californica and Deschampsia cespitosa) and two exotic (Schedonorus arundinaceus and Lolium multiflorum) grass species under varying nutrient and moisture conditions in an upland prairie in the Willamette Valley, Oregon. We hypothesized the two invasive, exotic grasses would be more competitive under high-nutrient, moderate-moisture conditions, resulting in the displacement of native grasses from these environments. In the field, the experimental reduction of competition resulted in shorter, wider plants, but only the annual grass, Lolium multiflorum, produced more aboveground biomass when competition was reduced. In the greenhouse, the two exotic grasses produced more total biomass than the two native grasses. Competitive hierarchies were influenced by nutrient and/or moisture treatments for the two exotic grasses, but not for the two native grasses. L. multiflorum dominated competitive interactions with all other grasses across treatments. In general, S. arundinaceus dominated when in competition with native grasses, and D. cespitosa produced the most biomass in monoculture or under interspecific competition with the other native grass, D. californica. D. californica, D. cespitosa, and S. arundinaceus all produced more biomass in high-moisture, high-nutrient environments, and D. cespitosa, L. multiflorum, and S. arundinaceus allocated more biomass belowground in the low nutrient treatment. Taken together, these experiments suggest the competitive superiority of the exotic grasses, especially L. multiflorum, but, contrary to our hypothesis, the native grasses were not preferentially excluded from nutrient-rich, moderately wet environments. Laurel Pfeifer-Meister and Esther M. Cole contributed equally to this work.     

Plant Recruitment and Soil Microbial Characteristics of Rehabilitation Seedings Following Wildfire in Northern Utah

One goal of post‐fire native species seeding is to increase plant community resistance to exotic weed invasions, yet few studies address the impacts of seeding on exotic annual establishment and persistence. In 2010 and 2011, we investigated the influence of seedings on exotic annuals and the underlying microbial communities. The wildfire site in northern Utah was formerly dominated by Artemisia tridentata ssp. wyomingensis, but burned in September 2008. Experimental seeding treatments were installed in November 2008 to examine strategies for establishing native species using two drills, hand broadcasts and different timing of seed applications (resulting in 13 seeding treatments). We collected aboveground biomass of invasive annuals (Halogeton glomeratus, Salsola kali, and Bromus tectorum), other volunteer plants from the extant seed bank, and seeded species from all treatments in the second and third years after fire. We sampled soils within microsites beneath native perennial bunchgrass and exotic annuals to characterize underlying soil microbial communities. High precipitation following seeding led to strong seedling establishment and we found few differences between seeding treatments established with either drill. All seeded treatments reduced exotic biomass by at least 90% relative to unseeded controls. Soil microbial communities (phospholipid fatty acid analysis), beneath B. tectorum, Poa secunda, and Pseudoroegneria spicata microsites differed little 3 years after fire. However, microbial abundance beneath P. spicata increased from June to July, suggesting that microbial communities beneath successful seedings can vary greatly within a single growing season.     

Fire, native species, and soil resource interactions influence the spatio-temporal invasion pattern of Bromus tectorum

Bromus tectorum (cheatgrass) is an invasive annual that occupies perennial grass and shrub communities throughout the western United States. Bronus tectorum exhibits an intriguing spatio‐temporal pattern of invasion in low elevation ponderosa pine Pinus ponderosa/bunchgrass communities in western Montana where it forms dense rings beneath solitary pines following fire. This pattern provides a unique opportunity to investigate several indirect effects of native vegetation that influence the invasion pattern of B. tectorum, and specifically how native species, disturbance, and soil resources interact to influence the spatio‐temporal pattern of invasion. We established four replicate field sites, each containing burned‐tree, burned‐grass, unburned‐tree, and unburned‐grass sampling locations, and initiated a series of field sampling and greenhouse experiments utilizing these locations. The objective of our first greenhouse experiment was to identify whether belowground factors contributed to the pattern of B. tectorum biomass observed in these field locations. This experiment generated a B. tectorum biomass response that was nearly identical to the invasion pattern observed in the field, suggesting further investigation of belowground factors was necessary. We measured resin‐sorbed NH₄⁺ and NO₃⁻ during one generation of B. tectorum, and measured a suite of P fractions through a sequential extraction procedure from these soils. These data revealed that a resource island of high N and P exists beneath pine trees. Through a second greenhouse experiment, we determined that N limited B. tectorum biomass in tree soil, whereas P limited biomass in bunchgrass soil. Finally, through a germination experiment we determined that pine litter strongly inhibited B. tectorum germination. These data suggest B. tectorum is regulated by P in bunchgrass soil, and by N and inhibition by pine litter beneath trees, effects that are likely alleviated by fire. These data demonstrate the combined role of direct and indirect interactions between native and invasive species in regulating biological invasions.     

Unexpected hybridization reveals the utility of genetics in native plant restoration

Native plant materials (NPMs) are increasingly utilized during the restoration of disturbed plant communities. Here, we analyze next‐generation genetic sequencing data for Hilaria jamesii, a dominant graminoid across drylands of the southwestern United States, and document that the species' only commercially‐available NPM, “Viva,” is a hybrid between H. jamesii and its sister species, Hilaria mutica. In fact, hybrids between these species are common where they geographically overlap. Furthermore, we show that the “Viva” hybrid has successfully been moved beyond the hybrid zone and into the core range of H. jamesii. The potential ramifications of introducing novel genetic material into H. jamesii are discussed, as well as the utility of genetic analyses to protect species' natural patterns of genetic diversity and help managers make informed decisions regarding the development and deployment of NPMs.     

Nitrogen deposition enhances Bromus tectorum invasion: biogeographic differences in growth and competitive ability between China and North America

Increased resource supply commonly facilitates invasion by exotic plants, raising concerns over atmospheric nitrogen (N) deposition; fast‐growing annual invaders may have exceptional abilities to outperform native perennials in response to N pulses. However, it remains unclear whether this advantage is due to growth differences or to shifts in competitive outcomes, and whether annual invaders are favored by N deposition in their introduced range over native range. We conducted an experiment to compare the growth and competitive ability of Bromus tectorum and its native perennial grasses either at three different N regimes or between China and North America. The soil used in this experiment was from mountain grasslands as a neutral growth medium. The total biomass of three natives from China and North America did not increase along the N deposition gradient. Nitrogen addition enhanced the growth of North American B. tectorum instead of Chinese B. tectorum. Nitrogen addition increased the competitive ability of B. tectorum, but had no effect on that of natives. North American B. tectorum was bigger and had greater competitive ability and root weight ratio than Chinese B. tectorum. In contrast, North American natives were less competitive than Chinese natives. There was a significantly positive correlation between the growth of B. tectorum grown alone and its competitive ability. These findings suggest that N deposition may enhance the B. tectorum invasion through disproportionally increasing the growth and maintaining inherent competitive advantages of North American B. tectorum, further increasing threats to introduced ranges. There were differences in the growth and competitive ability of B. tectorum and natives between China and North America, which explains why B. tectorum is a minor component at home and becomes a successful invader abroad.     

Experimental removal of intraspecific competitors — effects on water relations and productivity of a desert bunchgrass,Hilaria rigida

Summary Intraspecific competition in the C₄ bunchgrass Hilaria rigida was examined on a Sonoran Desert site in southeastern California. Potential competition within monospecific stands was experimentally altered by removal of the aboveground portions of all plants within a 1.5 m radius of a monitored plant. Compared with unaltered plots, altered plots had less negative soil water potentials during periods of soil drying. Leaf blades on monitored plants of altered plots remained green longer and had greater stomatal conductances than those on monitored plants on unaltered plots. Production of new culms was twice as great on altered plots. Greater root biomass and root length were observed in altered plots, and root extension into soil areas formerly occupied by roots of neighboring plants occurred within one year after treatment. The results indicate that removal of the aboveground biomass of neighboring plants reduces the competition for limited available soil water in this desert environment.     

Cheatgrass is favored by warming but not CO2 enrichment in a semi‐arid grassland

Elevated CO₂ and warming may alter terrestrial ecosystems by promoting invasive plants with strong community and ecosystem impacts. Invasive plant responses to elevated CO₂ and warming are difficult to predict, however, because of the many mechanisms involved, including modification of phenology, physiology, and cycling of nitrogen and water. Understanding the relative and interactive importance of these processes requires multifactor experiments under realistic field conditions. Here, we test how free‐air CO₂ enrichment (to 600 ppmv) and infrared warming (+1.5 °C day/3 °C night) influence a functionally and phenologically distinct invasive plant in semi‐arid mixed‐grass prairie. Bromus tectorum (cheatgrass), a fast‐growing Eurasian winter annual grass, increases fire frequency and reduces biological diversity across millions of hectares in western North America. Across 2 years, we found that warming more than tripled B. tectorum biomass and seed production, due to a combination of increased recruitment and increased growth. These results were observed with and without competition from native species, under wet and dry conditions (corresponding with tenfold differences in B. tectorum biomass), and despite the fact that warming reduced soil water. In contrast, elevated CO₂ had little effect on B. tectorum invasion or soil water, while reducing soil and plant nitrogen (N). We conclude that (1) warming may expand B. tectorum's phenological niche, allowing it to more successfully colonize the extensive, invasion‐resistant northern mixed‐grass prairie, and (2) in ecosystems where elevated CO₂ decreases N availability, CO₂ may have limited effects on B. tectorum and other nitrophilic invasive species.     

The type of competition modulates the ecophysiological response of grassland species to elevated CO2 and drought

The effects of elevated CO₂ and drought on ecophysiological parameters in grassland species have been examined, but few studies have investigated the effect of competition on those parameters under climate change conditions. The objective of this study was to determine the effect of elevated CO₂ and drought on the response of plant water relations, gas exchange, chlorophyll a fluorescence and aboveground biomass in four grassland species, as well as to assess whether the type of competition modulates that response. Elevated CO₂ in well‐watered conditions increased aboveground biomass by augmenting CO₂ assimilation. Drought reduced biomass by reducing CO₂ assimilation rate via stomatal limitation and, when drought was more severe, also non‐stomatal limitation. When plants were grown under the combined conditions of elevated CO₂ and drought, drought limitation observed under ambient CO₂ was reduced, permitting higher CO₂ assimilation and consequently reducing the observed decrease in aboveground biomass. The response to climate change was species‐specific and dependent on the type of competition. Thus, the response to elevated CO₂ in well‐watered grasses was higher in monoculture than in mixture, while it was higher in mixture compared to monoculture for forbs. On the other hand, forbs were more affected than grasses by drought in monoculture, while in mixture the negative effect of drought was higher in grasses than in forbs, due to a lower capacity to acquire water and mineral nutrients. These differences in species‐level growth responses to CO₂ and drought may lead to changes in the composition and biodiversity of the grassland plant community in future climate conditions.     

Clipping defoliation and nitrogen addition shift competition between a C3 grass (Leymus chinensis) and a C4 grass (Hemarthria altissima)

• Human‐induced disturbances, including grazing and clipping, that cause defoliation are common in natural grasslands. Plant functional type differences in the ability to compensate for this tissue loss may influence interspecific competition.
• To explore the effects of different intensities of clipping and nitrogen (N) addition on compensatory growth and interspecific competition, we measured accumulated aboveground biomass (AGB), belowground biomass (BGB), tiller number, non‐structural carbohydrates concentrations and leaf gas exchange parameters in two locally co‐occurring species (the C₃ grass Leymus chinensis and the C₄ grass Hemarthria altissima) growing in monoculture and in mixture.
• For both grasses, the clipping treatment had significant impacts on the accumulated AGB, and the 40% clipping treatment had the largest effect. BGB gradually decreased with increasing defoliation intensity. Severe defoliation caused a significant increase in tiller number. Stored carbohydrates in the belowground biomass were mobilised and transported aboveground for the growth of new leaves to compensate for clipping‐induced injury. The net CO₂ assimilation rate (A) of the remaining leaves increased with clipping intensity and peaked under clipping intensities of 20% or 40%. Nitrogen addition, at a rate of 10 g·N·m^?2·year^?1, enhanced A of the remaining leaves and non‐structural carbohydrate concentrations, which benefited plant compensatory growth, especially for the C₃ grass. Under the mixed planting conditions, the clipping and N addition treatments lowered the competitive advantage of the C₄ grass.
• The results suggest that a combination of defoliation and N deposition have the potential to benefit the coexistence of C₃ and C₄ grasses.

     

Local soil,but not commercial AMF inoculum,increases native and non‐native grass growth at a mine restoration site

Soil communities are often degraded in mined sites, and facilitating the recovery of soil mutualists such as arbuscular mycorrhizal fungi (AMF) may assist with the restoration of native plants. At a grassland mine restoration site, I compared a commercial AMF inoculum with soil collected from beneath native grasses as a source of inoculum, as well as a control treatment. Field plots were broadcast‐inoculated and seeded with native grasses, and biomass of native and non‐native species was measured in three consecutive years. In addition, greenhouse‐grown seedlings of a native bunchgrass (Stipa pulchra) were inoculated with similar treatments, transplanted into the field, and assessed after 18 months. When broadcast inoculation was used, the local soil inoculum tended to increase non‐native grass biomass, and marginally decreased non‐native forb biomass in the second year of study, but did not significantly affect native grass biomass. Broadcast commercial inoculum had no detectable effects on biomass of any plant group. Stipa pulchra transplants had greater N content and mycorrhizal colonization, and marginally higher shoot mass and K content, when pre‐inoculated with local soil (relative to controls). Pre‐inoculation with commercial AMF increased AMF colonization of the S. pulchra transplants, but did not significantly affect biomass or nutrient content. The findings indicate that at this site, the use of local soil as an inoculum had greater effects on native and non‐native plants than the commercial product used. In order to substantially increase native grass performance, inoculation of transplanted plugs may be one potential strategy.     

3种本地植物与入侵植物紫茎泽兰的竞争

为筛选替代控制紫茎泽兰的本地植物及探讨提高替代控制效率的方法,通过盆栽实验并利用相对产量(RY)和竞争攻击力系数(A)衡量了3种本地植物与紫茎泽兰的竞争关系,同时评估了活性炭(AC)、杀真菌剂(FC)和二者联合(AC+FC)对它们竞争的影响。结果表明：(1)不添加任何物质条件下：紫茎泽兰与南酸枣混种时,紫茎泽兰的生物量明显高于单种(P<0.05),其RY和A分别显著大于1和0(P<0.05);紫茎泽兰分别与假地豆和狗尾草混种时,紫茎泽兰的株高和生物量均明显低于单种(P<0.05),其RY和A均分别显著小于1和0(P<0.05)。说明紫茎泽兰的竞争力强于南酸枣而弱于假地豆和狗尾草,假地豆和狗尾草可以在一定区域作为替代控制紫茎泽兰的潜在目标植物。(2)与不添加任何物质相比,紫茎泽兰与南酸枣混种时,AC、FC及AC+FC处理增加了紫茎泽兰的根冠比,降低了其地上生物量比,也降低了南酸枣的生物量(P<0.05);紫茎泽兰与假地豆混种时,AC和AC+FC处理增加了假地豆的株高和生物量,FC处理增加了紫茎泽兰的株高(P<0.05);紫茎泽兰与狗尾草混种时,AC和A...     

California native and exotic perennial grasses differ in their response to soil nitrogen,exotic annual grass density,and order of emergence

Early emergence of plant seedlings can offer strong competitive advantages over later-germinating neighbors through the preemption of limiting resources. This phenomenon may have contributed to the persistent dominance of European annual grasses over native perennial grasses in California grasslands, since the former species typically germinate earlier in the growing season than the latter and grow rapidly after establishing. Recently, European perennial grasses have been spreading into both non-native annual and native perennial coastal grass stands in California. These exotic perennials appear to be less affected by the priority effects arising from earlier germination by European annual grasses. In addition, these species interactions in California grasslands may be mediated by increasing anthropogenic or natural soil nitrogen inputs. We conducted a greenhouse experiment to test the effects of order of emergence and annual grass seedling density on native and exotic perennial grass seedling performance across different levels of nitrogen availability. We manipulated the order of emergence and density of an exotic annual grass (Bromus diandrus) grown with either Nassella pulchra (native perennial grass), Festuca rubra (native perennial grass), or Holcus lanatus (exotic perennial grass), with and without added nitrogen. Earlier B. diandrus emergence and higher B. diandrus density resulted in greater reduction in the aboveground productivity of the perennial grasses. However, B. diandrus suppressed both native perennials to a greater extent than it did H. lanatus. Nitrogen addition had no effect on the productivity of native perennials, but greatly increased the growth of the exotic perennial H. lanatus, grown with B. diandrus. These results suggest that the order of emergence of exotic annual versus native perennial grass seedlings could play an important role in the continued dominance of exotic annual grasses in California. The expansion of the exotic perennial grass H. lanatus in coastal California may be linked to its higher tolerance of earlier-emerging annual grasses and its ability to access soil resources amidst high densities of annual grasses.