Can Invasive Species Enhance Competitive Ability and Restoration Potential in Native Grass Populations?

Native plant individuals often persist within communities dominated by exotics but the influence of this exposure on native populations is poorly understood. Selection for traits contributing to competitive ability may lead to native plant populations that are more tolerant of the presence of exotic invaders. In this way, long‐term coexistence with an exotic may confer competitive advantages to remnant (experienced) native populations and be potentially beneficial to restoration. In past studies we have documented genetic differentiation within native grass populations exposed to the exotic invader Russian knapweed (Acroptilon repens). Here, we examine populations of a cool‐season grass, needle‐and‐thread (Hesperostipa comata [Trin. & Rupr.]) and a warm season, alkali sacaton (Sporobolus airoides [Torr.]) collected from Russian knapweed‐invaded sites and adjacent noninvaded sites to assess their relative competitive ability against a novel exotic neighbor, Canada thistle (Cirsium arvense). Experienced S. airoides (from within A. repens invasions) appear to better tolerate (accumulate biomass, leaf nitrogen content, and to initiate new tillers) the presence of a novel competitor (C. arvense). Experienced and inexperienced H. comata genets differ in their response to the presence of C. arvense. Relative neighbor effects of native grasses on C. arvense were generally greater from experienced grasses. The ability to compete with novel neighbors may be driven by general competitive traits rather than species‐specific coevolutionary trajectories. Irrespective of competitive mechanisms, the conservation of native species populations within weed invasions may provide an important restoration tool by retaining unique components of native gene pools selected by competitive interactions with exotics.     

Soil pathogen communities associated with native and non‐native Phragmites australis populations in freshwater wetlands

Soil pathogens are believed to be major contributors to negative plant–soil feedbacks that regulate plant community dynamics and plant invasions. While the theoretical basis for pathogen regulation of plant communities is well established within the plant–soil feedback framework, direct experimental evidence for pathogen community responses to plants has been limited, often relying largely on indirect evidence based on above‐ground plant responses. As a result, specific soil pathogen responses accompanying above‐ground plant community dynamics are largely unknown. Here, we examine the oomycete pathogens in soils conditioned by established populations of native noninvasive and non‐native invasive haplotypes of Phragmites australis (European common reed). Our aim was to assess whether populations of invasive plants harbor unique communities of pathogens that differ from those associated with noninvasive populations and whether the distribution of taxa within these communities may help to explain invasive success. We compared the composition and abundance of pathogenic and saprobic oomycete species over a 2‐year period. Despite a diversity of oomycete taxa detected in soils from both native and non‐native populations, pathogen communities from both invaded and noninvaded soils were dominated by species of Pythium. Pathogen species that contributed the most to the differences observed between invaded and noninvaded soils were distributed between invaded and noninvaded soils. However, the specific taxa in invaded soils responsible for community differences were distinct from those in noninvaded soils that contributed to community differences. Our results indicate that, despite the phylogenetic relatedness of native and non‐native P. australis haplotypes, pathogen communities associated with the dominant non‐native haplotype are distinct from those of the rare native haplotype. Pathogen taxa that dominate either noninvaded or invaded soils suggest different potential mechanisms of invasion facilitation. These findings are consistent with the hypothesis that non‐native plant species that dominate landscapes may “cultivate” a different soil pathogen community to their rhizosphere than those of rarer native species.     

Evolutionary responses of native plants to novel community members

Both ecological and evolutionary processes can influence community assembly and stability, and native community members may respond both ecologically and evolutionarily as additional species enter established communities. Biological invasions provide a unique opportunity to examine these responses of native community members to novel species additions. Here, I use reciprocal transplant experiments among naturally invaded and uninvaded environments, along with experimental removals of exotic species, to determine whether exotic plant competitors and exotic insect herbivores evoke evolutionary changes in native plants. Specifically, I address whether the common native plant species Lotus wrangelianus has responded evolutionarily to a series of biological invasions by adapting to the presence of the exotic plant Medicago polymorpha and the exotic insect herbivore Hypera brunneipennis. Despite differences in selection regimes between invaded and uninvaded environments and the presence of genetic variation for traits relevant to the novel competitive and plant-herbivore interactions, these experiments failed to reveal evidence that Lotus has responded evolutionarily to the double invasion of Medicago followed by H. brunneipennis. However, when herbivory from H. brunneipennis was experimentally reduced, Lotus plants from source populations invaded by Medicago outperformed plants from uninvaded source populations when transplanted into heavily invaded destination environments. Therefore, Lotus showed evidence of adaptation to Medicago invasion but not to the newer invasion of an exotic shared herbivore. The presence of this exotic insect herbivore alters the outcome of evolutionary responses in this system and counteracts adaptation by the native Lotus to invasion by the exotic plant Medicago. This result has broad implications for the conservation of native communities. While native species may be able to adapt to the presence of one or a few exotics, a multitude of invasions may limit the ability of natives to respond evolutionarily to the novel and frequently changing selection pressures that arise with subsequent invasions.     

Giant invasive Heracleum persicum: Friend or foe of plant diversity?

The impact of invasion on diversity varies widely and remains elusive. Despite the considerable attempts to understand mechanisms of biological invasion, it is largely unknown whether some communities’ characteristics promote biological invasion, or whether some inherent characteristics of invaders enable them to invade other communities. Our aims were to assess the impact of one of the massive plant invaders of Scandinavia on vascular plant species diversity, disentangle attributes of invasible and noninvasible communities, and evaluate the relationship between invasibility and genetic diversity of a dominant invader. We studied 56 pairs of Heracleum persicum Desf. ex Fisch.‐invaded and noninvaded plots from 12 locations in northern Norway. There was lower native cover, evenness, taxonomic diversity, native biomass, and species richness in the invaded plots than in the noninvaded plots. The invaded plots had nearly two native species fewer than the noninvaded plots on average. Within the invaded plots, cover of H. persicum had a strong negative effect on the native cover, evenness, and native biomass, and a positive association with the height of the native plants. Plant communities containing only native species appeared more invasible than those that included exotic species, particularly H. persicum. Genetic diversity of H. persicum was positively correlated with invasibility but not with community diversity. The invasion of a plant community by H. persicum exerts consistent negative pressure on vascular plant diversity. The lack of positive correlation between impacts and genetic diversity of H. persicum indicates that even a small founder population may cause high impact. We highlight community stability or saturation as an important determinant of invasibility. While the invasion by H. persicum may decrease susceptibility of a plant community to further invasion, it severely reduces the abundance of native species and makes them more vulnerable to competitive exclusion.     

Potential selection in native grass populations by exotic invasion

Ecological impacts of invasive plant species are well documented, but the genetic response of native species to invasive dominance has been often overlooked. Invasive plants can drastically alter site conditions where they reach dominance, potentially exerting novel selective pressures on persistent native plant populations. Do native plant populations in old exotic invasions show evidence of selection when compared to conspecific populations in adjacent, noninvaded areas? We employ amplified fragment length polymorphism (AFLP) analysis to screen a large number of loci from two native grass species (Hesperostipa comata (Trin. & Rupr.) Barkworth and Sporobolus airoides Torr.) that occur in old infestations of the invasive forb Acroptilon repens. We then compare observed locus by locus F_ST values with distributions of F_ST estimated from simulation models under expectation of neutrality. We also compare the proportion of loci possibly linked to selection and those not linked to selection which exhibit parallel trends in divergence between two community types (invaded, noninvaded). Few loci (H. comata, 2.6%; S. airoides, 8.7%) in the two native grasses may be linked to genes under the influence of selection. Also, loci linked to selection showed a greater portion of parallel trends in divergence than neutral loci. Genetic similarities between community types were less than genetic similarity within community types suggesting differentiation in response to community alteration. These results indicate that a small portion of scored AFLP loci may be linked to genes undergoing selection tied to community dominance by an invasive species. We propose that native plants in communities dominated by exotic invasives may be undergoing natural selection.     

Phenotypes of Two Generations of Sporobolus airoides Seedlings Derived from Acroptilon repens‐invaded and Non‐invaded Grass Populations

Although the ecological impacts of invasive species are well known, the evolutionary impacts on recipient native grass communities are not. We suggest that remnant native plants may provide desirable seed sources for restoration and native plant production. Native populations exposed to the selective pressures associated with exotic invasion may retain traits that increase their ability to coexist with invasive species. Two generations of Sporobolus airoides Torr. (Alkali sacaton) plants derived from lineages collected from within long‐term invaded areas of Acroptilon repens (L.) DC (Russian knapweed) and from adjacent non‐invaded areas were propagated in a greenhouse to evaluate generational changes in phenotypic traits from the production environment. Given the difference in invasion history of the two populations, we hypothesized that invaded and non‐invaded subpopulations would differ phenotypically. Phenotypic measurements revealed that invaded subpopulations had greater vegetative growth, whereas non‐invaded subpopulations had increased sexual reproduction. Phenotypic expression changed from the first to the second generation, predominantly in the invaded subpopulation. Generational phenotypic shifts are disadvantageous for native seed production which requires a standard product to sell commercially. However, phenotypic variation may improve field seed survival. This research demonstrates the potential value of targeting post‐invasion remnant grass populations for restoration.     

Native perennial grasses show evolutionary response to Bromus tectorum (cheatgrass) invasion

Invasive species can change selective pressures on native plants by altering biotic and abiotic conditions in invaded habitats. Although invasions can lead to native species extirpation, they may also induce rapid evolutionary changes in remnant native plants. We investigated whether adult plants of five native perennial grasses exhibited trait shifts consistent with evolution in response to invasion by the introduced annual grass Bromus tectorum L. (cheatgrass), and asked how much variation there was among species and populations in the ability to grow successfully with the invader. Three hundred and twenty adult plants were collected from invaded and uninvaded communities from four locations near Reno, Nevada, USA. Each plant was divided in two and transplanted into the greenhouse. One clone was grown with B. tectorum while the other was grown alone, and we measured tolerance (ability to maintain size) and the ability to reduce size of B. tectorum for each plant. Plants from invaded populations consistently had earlier phenology than those from uninvaded populations, and in two out of four sites, invaded populations were more tolerant of B. tectorum competition than uninvaded populations. Poa secunda and one population of E. multisetus had the strongest suppressive effect on B. tectorum, and these two species were the only ones that flowered in competition with B. tectorum. Our study indicates that response to B. tectorum is a function of both location and species identity, with some, but not all, populations of native grasses showing trait shifts consistent with evolution in response to B. tectorum invasion within the Great Basin.     

Ecological filtering of exotic plants in an Australian sub‐alpine environment

We investigated some of the factors influencing exotic invasion of native sub‐alpine plant communities at a site in southeast Australia. Structure, floristic composition and invasibility of the plant communities and attributes of the invasive species were studied. To determine the plant characteristics correlated with invasiveness, we distinguished between roadside invaders, native community invaders and non‐invasive exotic species, and compared these groups across a range of traits including functional group, taxonomic affinity, life history, mating system and morphology. Poa grasslands and Eucalyptus‐Poa woodlands contained the largest number of exotic species, although all communities studied appeared resilient to invasion by most species. Most community invaders were broad‐leaved herbs while roadside invaders contained both herbs and a range of grass species. Over the entire study area the richness and cover of native and exotic herbaceous species were positively related, but exotic herbs were more negatively related to cover of specific functional groups (e.g. trees) than native herbs. Compared with the overall pool of exotic species, those capable of invading native plant communities were disproportionately polycarpic, Asteracean and cross‐pollinating. Our data support the hypothesis that strong ecological filtering of exotic species generates an exotic assemblage containing few dominant species and which functionally converges on the native assemblage. These findings contrast with those observed in the majority of invaded natural systems. We conclude that the invasion of closed sub‐alpine communities must be viewed in terms of the unique attributes of the invading species, the structure and composition of the invaded communities and the strong extrinsic physical and climatic factors typical of the sub‐alpine environment.     

Identifying Native Vegetation for Reducing Exotic Species during the Restoration of Desert Ecosystems

There is currently much interest in restoration ecology in identifying native vegetation that can decrease the invasibility by exotic species of environments undergoing restoration. However, uncertainty remains about restoration's ability to limit exotic species, particularly in deserts where facilitative interactions between plants are prevalent. Using candidate native species for restoration in the Mojave Desert of the southwestern U.S.A., we experimentally assembled a range of plant communities from early successional forbs to late‐successional shrubs and assessed which vegetation types reduced the establishment of the priority invasive annuals Bromus rubens (red brome) and Schismus spp. (Mediterranean grass) in control and N‐enriched soils. Compared to early successional grass and shrub and late‐successional shrub communities, an early forb community best resisted invasion, reducing exotic species biomass by 88% (N added) and 97% (no N added) relative to controls (no native plants). In native species monocultures, Sphaeralcea ambigua (desert globemallow), an early successional forb, was the least invasible, reducing exotic biomass by 91%. However, the least‐invaded vegetation types did not reduce soil N or P relative to other vegetation types nor was native plant cover linked to invasibility, suggesting that other traits influenced native‐exotic species interactions. This study provides experimental field evidence that native vegetation types exist that may reduce exotic grass establishment in the Mojave Desert, and that these candidates for restoration are not necessarily late‐successional communities. More generally, results indicate the importance of careful native species selection when exotic species invasions must be constrained for restoration to be successful.     

Climate change and the invasion of California by grasses

Over the next century, changes in the global climate are expected to have major consequences for plant communities, possibly including the exacerbation of species invasions. We evaluated this possibility in the grass flora of California, which is economically and ecologically important and heavily invaded. We used a novel, trait‐based approach involving two components: identifying differences in trait composition between native and exotic components of the grass flora and evaluating contemporary trait–climate relationships across the state. The combination of trait–climate relationships and trait differences between groups allows us to predict changes in the exotic‐native balance under climate change scenarios. Exotic species are more likely to be annual, taller, with larger leaves, larger seeds, higher specific leaf area, and higher leaf N percentage than native species. Across the state, all these traits are associated with regions with higher temperature. Therefore, we predict that increasing temperatures will favor trait states that tend to be possessed by exotic species, increasing the dominance of exotic species. This prediction is corroborated by the current distribution of exotic species richness relative to native richness in California; warmer areas contain higher proportions of exotic species. This pattern was very well captured by a simple model that predicts invasion severity given only the trait–climate relationship for native species and trait differences between native and exotic species. This study provides some of the first evidence for an important interaction between climate change and species invasions across very broad geographic and taxonomic scales.     

Negative relationship between diversity and productivity under plant invasion

The relationship between diversity and productivity of plant community under plant invasion has been not well known up to now. Here, we investigated the relationship between diversity and productivity under plant invasion and studied the response of species level plant mass to species richness in native and invaded communities. A field experiment from 2008 to 2013 and a pot experiment in 2014 were conducted to study the effects of plant invasion on the relationship between diversity and productivity and the response of species level plant mass to species richness in native and invaded communities. The community level biomass was negatively correlated to plant species richness in invaded communities while the same relationship was positive in native communities. The species level plant mass of individual species responded differently to overall plant species richness in the native and invaded communities, namely, most of the species’ plant mass increased in native communities, but decreased in invaded communities with increasing species richness. The complementarity or selection effects might dominate in native communities while competition effects might dominate in invaded communities. Accordingly, the negative relationship between diversity and productivity under plant invasion is highlighted in our experiments.     

Current plant diversity but not its soil legacy influences exotic plant invasion

植物多样性而非其土壤遗留效应影响外来植物入侵 植物多样性可以影响外来植物入侵,然而植物多样性的土壤遗留效应是否能够影响外来植物入侵目前仍不清楚。植物多样性能够改变土壤微生物群落和土壤理化性质,这种遗留效应可能会对该土壤中外来植物的生长产生影响。因此,我们假设植物多样性的土壤遗留效应会影响外来植物的入侵。为了检验该假说,我们开展了一个两阶段的植物-土壤反馈实验。在土壤驯化阶段,我们将12个植物物种(4种禾草植物、3种豆科植物和5种杂类草植物)分别单独种植,或者随机选择8个物种(包含3个功能型)混合种植在土壤中。在反馈阶段,我们将入侵植物三叶鬼针草(Bidens pilosa)分别与本地禾草荩草(Arthraxon hispidus)、本地杂类草翅果菊(Pterocypsela indica)或者同时与荩草和翅果菊种植在被驯化过的土壤中。研 究结果显示,三叶鬼针草相对于其本地竞争植物的生长取决于驯化植物和竞争植物物种的功能型。驯化植物的多样性对三叶鬼针草与其本地竞争植物之间的生长差异没有显著影响。然而,随着本地竞争植物物种多样性的增加,三叶鬼针草相对于其本地竞争植物的生长显著降低。这些结果表明,当前的植物多样性可以通过增加入侵植物和本地植物之间的生长不平衡性来减少外来植物的入侵。但是,植物多样性的土壤遗留效应对外来植物入侵的影响可能很小。     

Biogeographic differences in plant–soil biota relationships contribute to the exotic range expansion of Verbascum thapsus

1. Exotic plant species can evolve adaptations to environmental conditions in the exotic range. Furthermore, soil biota can foster exotic spread in the absence of negative soil pathogen–plant interactions or because of increased positive soil biota–plant feedbacks in the exotic range. Little is known, however, about the evolutionary dimension of plant–soil biota interactions when comparing native and introduced ranges.
2. To assess the role of soil microbes for rapid evolution in plant invasion, we subjected Verbascum thapsus, a species native to Europe, to a reciprocal transplant experiment with soil and seed material originating from Germany (native) and New Zealand (exotic). Soil samples were treated with biocides to distinguish between effects of soil fungi and bacteria. Seedlings from each of five native and exotic populations were transplanted into soil biota communities originating from all populations and subjected to treatments of soil biota reduction: application of (a) fungicide, (b) biocide, (c) a combination of the two, and (d) control.
3. For most of the investigated traits, native populations showed higher performance than exotic populations; there was no effect of soil biota origin. However, plants developed longer leaves and larger rosettes when treated with their respective home soil communities, indicating that native and exotic plant populations differed in their interaction with soil biota origin. The absence of fungi and bacteria resulted in a higher specific root length, suggesting that V. thapsus may compensate the absence of mutualistic microbes by increasing its root–soil surface contact.
4. Synthesis. Introduced plants can evolve adaptations to soil biota in their new distribution range. This demonstrates the importance of biogeographic differences in plant–soil biota relationships and suggests that future studies addressing evolutionary divergence should account for differential effects of soil biota from the home and exotic range on native and exotic populations of successful plant invaders.

     

Plant litter effects on soil nutrient availability and vegetation dynamics: changes that occur when annual grasses invade shrub-steppe communities

Changes in the quantity and quality of plant litter occur in many ecosystems as they are invaded by exotic species, which impact soil nutrient cycling and plant community composition. Such changes in sagebrush-steppe communities are occurring with invasion of annual grasses (AG) into a perennial grass (PG) dominated system. We conducted a 5-year litter manipulation study located in the northern Great Basin, USA. Springtime litter was partially or completely removed in three communities with differing levels of invasion (invaded, mixed, and native) to determine how litter removal and litter biomass affected plant-available soil N and plant community composition. Litter biomass (prior to the removal treatment) was negatively correlated with plant-available N in the invaded community, but was positively correlated in the native community. Plant-available N had greater intra- and inter-annual fluctuations in the invaded compared to the mixed or native communities, but was not generally affected by removal treatments. Litter removal had negative effects on AG cover during a warm/dry year and negative effects on PG cover during a cool/wet year in the mixed community. Overall, the effectiveness of springtime litter manipulations on plant-available N were limited and weather dependent, and only removal treatments >75 % had effects on the plant community. Our study demonstrates how communities invaded by AGs have significantly increased temporal variability in nutrient cycling, which may decrease ecosystem stability. Further, we found that the ecological impacts from litter manipulation on sagebrush communities were dependent on the extent of AG invasion, the timing of removal, and seasonal precipitation.     

Composition of soil seed banks in southern California coastal sage scrub and adjacent exotic grassland

Soil seed banks are important to many plant communities and are recognized as an important component of management plans. Understanding seed bank composition and density is especially important when communities have been invaded by exotic species and must be managed to promote desirable species. We examined germinable soil seed banks in southern California coastal sage scrub (CSS) that is heavily invaded by exotic grasses and in adjacent exotic grassland. Soils from both communities had similar seed banks, dominated by high densities of exotic grass and forb species. Up to 4,000 exotic grass seeds and at least 400 exotic forb seeds/m² were found in most soils, regardless of aboveground vegetation type. Native forbs averaged 400 seeds/m² in grass-dominated areas and about 800 in shrub-dominated soils. Shrub seed density was <1 and <10 seeds/m² in grass- and shrub-dominated areas, respectively, indicating that the shrub seed bank is not persistent compared to annuals. We also compared pre- and post-burn soil seed banks from one location that burned in October 2003. Late-season burning in both grass- and CSS-dominated areas disproportionately reduced exotic grass seed densities relative to native seed densities. The similarity of the seed banks in adjacent grass and shrub communities suggests that without intervention, areas currently dominated by CSS may become more similar to grass-dominated areas in terms of aboveground vegetation. In such areas, the first growing season following a wildfire is a window of opportunity for increasing native diversity at a time when density of exotic grass seeds is low. At time of research, Robert D. Cox was graduate student.     

Effects of exotic grass invasion on spatial heterogeneity in the ground-layer of grassy woodlands

We quantified the effects of exotic annual grass invasion on the ground-layer structure of grassy eucalypt woodlands, with the aim of determining if weed invasion decreased gap size and plant basal area leading to reduced spatial heterogeneity. We measured plant density, distance between plants and basal plant area in woodland sites which ranged from zero to 100% exotic plant cover in the ground-layer. The ground-layer in uninvaded woodlands was heterogeneous, with a large variation in basal plant area and distance between plants. Exotic annual grass density was positively correlated with total plant density, whereas native plant density was negatively correlated. Total plant basal area decreased as total plant density increased, with a lower total plant area in exotic dominated transects compared to native dominated. Variation in basal plant area decreased with increasing plant density. Exotic annual grasses were more closely spaced together (smaller gap size) and had a smaller basal area than the native grasses and rushes. There was also less variation in basal area and gap size with individual exotic annual grasses compared to the native grasses. Inter-plant distance was greater for both the native and exotic grasses when they had native grasses neighbouring them instead of exotic grasses. These findings show that woodlands invaded by exotic annual grasses have relatively less spatial heterogeneity in the ground-layer. These results have implications for other aspects of perennial grassy ecosystems invaded by annual grasses, including plant recruitment and restoration strategies.     

Plant community diversity and native plant abundance decline with increasing abundance of an exotic annual grass

Exotic plants are generally considered a serious problem in wildlands around the globe. However, some argue that the impacts of exotic plants have been exaggerated and that biodiversity and other important plant community characteristics are commonly improved with invasion. Thus, disagreement exists among ecologists as to the relationship of exotic plants with biodiversity and native plant communities. A better understanding of the relationships between exotic plants and native plant communities is needed to improve funding allocation and legislation regarding exotic plants, and justify and prioritize invasion management. To evaluate these relationships, 65 shrub–bunchgrass plant communities with varying densities of an exotic annual grass, Taeniatherum caput-medusae (L.) Nevski (medusahead), were sampled across 160,000 ha in southeastern Oregon, United States. Environmental factors were generally not correlated with plant community characteristics when exotic annual grass density was included in models. Plant diversity and species richness were negatively correlated with exotic annual grass density. Exotic annual grass density explained 62% of the variation in plant diversity. All native plant functional groups, except annual forbs, exhibited a negative relationship with T. caput-medusae. The results of this study suggest that T. caput-medusae invasions probably have substantial negative impacts on biodiversity and native plant communities. The strength of the relationships between plant community characteristics and T. caput-medusae density suggests that some exotic plants are a major force of change in plant communities and subsequently threaten ecosystem functions and processes. However, experimental studies are needed to fully substantiate that annual grass invasion is the cause of these observed correlations.     

Effects of exotic annual grass litter and local environmental gradients on annual plant community structure

Exotic annual grasses have been introduced into many semi-arid ecosystems worldwide, often to the detriment of native plant communities. The accumulation of litter from these grasses (i.e. residual dry biomass) has been demonstrated to negatively impact native plant communities and promote positive feedbacks to exotic grass persistence. More targeted experiments are needed, however, to determine the relative impact of exotic grass litter on plant community structure across local environmental gradients. We experimentally added exotic grass litter to annual forb-dominated open woodland communities positioned along natural canopy cover gradients in southwest Western Australia. These communities are an important component of this region’s plant biodiversity hotspot and are documented to be under threat from exotic annual grasses. After a one-year treatment period, we measured the effects of exotic grass litter, soil properties, and canopy cover on native and exotic species richness and abundance, as well as common species’ biomass and abundances. Plant community structure was more strongly influenced by soil properties and canopy cover than by grass litter. Total plant abundances per plot, however, were significantly lower in litter addition plots than control plots, a trend driven by native species. Exotic grass litter was also associated with lower abundances of one very common native species: Waitzia acuminata. Our results suggest that exotic grass litter limits the establishment of some native species in this system. Over multiple years, these subtle impacts may contribute substantially to the successful advancement of exotic species into this system, particularly in certain microenvironments.     

Nitrogen limitation, 15N tracer retention, and growth response in intact and Bromus tectorum-invaded Artemisia tridentata ssp. wyomingensis communities

Annual grass invasion into shrub-dominated ecosystems is associated with changes in nutrient cycling that may alter nitrogen (N) limitation and retention. Carbon (C) applications that reduce plant-available N have been suggested to give native perennial vegetation a competitive advantage over exotic annual grasses, but plant community and N retention responses to C addition remain poorly understood in these ecosystems. The main objectives of this study were to (1) evaluate the degree of N limitation of plant biomass in intact versus B. tectorum-invaded sagebrush communities, (2) determine if plant N limitation patterns are reflected in the strength of tracer ¹⁵N retention over two growing seasons, and (3) assess if the strength of plant N limitation predicts the efficacy of carbon additions intended to reduce soil N availability and plant growth. Labile C additions reduced biomass of exotic annual species; however, growth of native A. tridentata shrubs also declined. Exotic annual and native perennial plant communities had divergent responses to added N, with B. tectorum displaying greater ability to use added N to rapidly increase aboveground biomass, and native perennials increasing their tissue N concentration but showing little growth response. Few differences in N pools between the annual and native communities were detected. In contrast to expectations, however, more ¹⁵N was retained over two growing seasons in the invaded annual grass than in the native shrub community. Our data suggest that N cycling in converted exotic annual grasslands of the northern Intermountain West, USA, may retain N more strongly than previously thought.     

The ecological barriers to the recovery of bridal creeper (Asparagus asparagoides (L.) Druce) infested sites: Impacts on vegetation and the potential increase in other exotic species

Abstract To protect native biodiversity from environmental weeds, the impacts that these weeds cause need to be known before weed control commences. Asparagus asparagoides (L.) Druce (bridal creeper) (Asparagaceae) is a serious environmental weed and has been selected for biological control in Australia. To predict the responses of plant communities to the control of bridal creeper, a prerelease baseline of the impacts of bridal creeper on native plant communities was undertaken. Plant assemblages in areas invaded by bridal creeper were compared with reference areas that contained little or no bridal creeper. Areas invaded by bridal creeper contained 52% fewer native plant species when compared with nearby reference areas. However, there was no difference in the number of other exotic plant species between areas. Similar trends were found for the germinable seed bank. Although a greater number of exotic species were present in the seed bank compared with the vegetation surveys, there was still no difference between areas with and without bridal creeper. In a glasshouse trial, exotic species germinated more frequently compared with native species. This could indicate that as bridal creeper density decreases following control, exotic species have an advantage over native species when colonizing areas left vacant by bridal creeper. Second, as bridal creeper areas contained reduced native species richness and cover, they may be susceptible to further weed invasion after bridal creeper is removed. Therefore, simply reducing the presence of bridal creeper may not guarantee successful restoration of invaded areas and additional restoration efforts will be needed to ensure the ultimate goal of protecting native biodiversity is reached.