High richness and dense seeding enhance grassland restoration establishment but have little effect on drought response

Restorations commonly utilize seed addition to formerly arable lands where the development of native plant communities is severely dispersal limited. However, variation in seed addition practices may profoundly affect restoration outcomes. Theory and observations predict that species-rich seed mixtures and seeding at high densities should enhance native plant community establishment, minimize exotic species cover, and may promote resistance and resilience to, and recovery from, environmental perturbations. We studied the post-seeding establishment of native plant communities in large grassland restoration plots, which were sown at two densities crossed with two levels of species richness on formerly arable land in Nebraska, USA, and their responses to drought. To evaluate drought resistance, recovery, and resilience of restored plant communities, we erected rainfall manipulation structures and tracked the response of seeded species cover and total plant biomass during experimental drought relative to controls and in the post-drought growing season. High seed richness and high-density seeding treatments resulted in greater richness and cover of native, seeded species per 0.5 m2 compared to low-richness and low-density treatments. Cover differences in response to seed mixture richness were driven by native forbs. Richness and cover of exotic species were lowest in high-richness and high-density treatments. We found little evidence of differential drought resistance, recovery, and resilience among seeding treatments. Increases in exotic species across years were restricted to drought subplots, and were not affected by seeding treatments. Grassland restoration was generally enhanced and exotic cover reduced both by the use of high-richness seed mixtures and high-density seeding. Given the lack of restoration treatment effects on the resistance, recovery, or resilience of seeded species exposed to drought, and the increases in exotic species following drought, other forms of active management may be needed to produce restored plant communities that are robust to climate change.     

Response of plant species to coal-mine soil materials

A two-year experiment was conducted on the Black Mesa Coal Mine near Kayenta, Arizona to investigate the growth and establishment of seven plant species in unmined soil (undisturbed soil) and coal-mine soil (spoils). Natural rainfall (20 cm/yr) and natural rainfull plus sprinkler irrigation (50 cm/yr) were the irrigation treatments applied to each soil material. Plant species grew better in unmined soil than in coal-mine soil. Supplemental irrigation water resulted in more plant growth than did natural rainfall alone in both soil materials; however, there were highly significant differences among species. Alfalfa (Medicago sativa L.) and all of the annual perennial grasses used, excluding Indian ricegrass (Oryzopsis hymenoides Ricker), produced effective ground cover on both soil materials when they received supplemental irrigation water. Fourwing saltbush (Atriplex canescens Pursh.) had low germination (emergence), seedling establishment, and stem production during the first year of growth; however, in the second year of growth, this species produced a dense ground cover on coal-mine soil with natural rainfall plus irrigation. In revegetating the Black Mesa Coal Mine, Harlan II barley (Hordeum vulgare L.) and Super X wheat (Triticum aestivum L.) provided initial protective cover until adapted perennial species could be established for permanent stabilisation. The Black Mesa Research Study indicated that irrigation water during seedling establishment was essential for the effective stabilisation of coal-mine soils in a semiarid environment.     

祁连山北坡自然恢复杨桦林地植物功能型组成及其影响因素

依据生活型(乔木、藤本、灌木、半灌木、多年生禾草、多年生杂类草和一二年生草本)和水分生态型(旱生、旱中生、中生和湿生)将祁连山北坡次生杨桦林28个调查样地中的81个物种划分为18种植物功能型(Plant functiontypes,PFTs),并通过典范对应分析(CCA)方法研究植物功能型与环境因子间的关系.结果表明:自然恢复杨桦林地植物功能型主要集中在中生多年生杂类草(PFTs16)、湿生多年生杂类草(PFTs23)、旱中生多年生杂类草(PFTs9)和中生灌木(PFTs18)4个植物功能型上;乔、灌、草三层冠层特征和中生植物的大量出现,反映了群落植物生活型趋于复杂化和结构化,生态型逐渐向中生化方向演替的特点;土壤容重、有机碳和全氮含量主要影响植物的生活型分异特征,土壤含水量和林冠郁闭度是植物的水分生态型分异的主要环境因子,而坡位、坡度控制着群落植物总体分布格局的形成.图1表3参31     

Postfire Seeding for Erosion Control: Effectiveness and Impacts on Native Plant Communities

Abstract:  Large, high-severity wildfires remove vegetation cover and expose mineral soil, ofen causing erosion and runoff during postfire rain events to increase dramatically. Land-management agencies in the United States are required to assess site conditions after wildfire and, where necessary, implement emergency watershed rehabilitation measures to help stabilize soil; control movement of water, sediment, and debris; prevent permanent impairment of ecosystem structure and function; and mitigate significant threats to human health, safety, life, property, or downstream values. One of the most common postfire treatments is broadcast seeding of grasses, usually from aircraft. Non-native annual or perennial grasses typically are used to provide quick, temporary ground cover to hold soil in place until native plants are reestablished. Critics argue that seeded grasses compete with native vegetation and do not effectively reduce erosion. Few data exist on the effectiveness of erosion control; less than half of the studies I reviewed showed reduced sediment movement with seeding. In all vegetation types, successful growth of seeded grasses—enough to affect erosion—appears to displace native or naturalized species, including shrub and tree seedlings. Due to the competitiveness of seeded grasses, they are used to attempt suppression of noxious weeds in some postfire seeding operations. In burned sagebrush range, postfire seeding is frequently used to replace non-native cheatgrass (  Bromus tectorum ) with native or introduced bunchgrasses, with at least short-term success. In recent years, native species and sterile cereal grains have increasingly been used for seeding. Use of aerially applied straw mulch has increased as well, with the risk of weed introduction from contaminated bales. More research on the effectiveness and ecosystem impacts of these alternatives is needed.     

Long-term population dynamics of seeded plants in invaded grasslands

In recent decades, dozens of studies have involved attempts to introduce native and desirable nonnative plant species into grasslands dominated by invasive weeds. The newly introduced plants have proved capable of establishing, but because they are rarely monitored for more than four years, it is unknown if they have a high likelihood of persisting and suppressing invaders for the long-term. Beyond invaded grasslands, this lack of long-term monitoring is a general problem plaguing efforts to reintroduce a range of taxa into a range of ecosystems. We introduced species from seed and then periodically measured plant abundances for nine years at one site and 15 years at a second site. To our knowledge, our 15-year data are the longest to date from a seeding experiment in invaded, never-cultivated grassland. At one site, three seeded grasses maintained high densities for three or more years, but then all or nearly all individuals died. At the second site, one grass performed similarly, but two other grasses proliferated and at least one greatly suppressed the dominant invader (Centaurea maculosa). In one study, our point estimate suggests that the seeded grass Thinopyrum intermedium reduced C. maculosa biomass by 93% 15 years after seeding. In some cases, data from three and fewer years after seeding falsely suggested that seeded species were capable of persisting within the invaded grassland. In other cases, data from as late as nine years after seeding falsely suggested seeded populations would not become large enough to suppress the invader. These results show that seeded species sometimes persist and suppress invaders for long periods, but short-term data cannot predict if, when, or where this will occur. Because short-term data are not predictive of long-term seeded species performances, additional long-term data are needed to identify effective practices, traits, and species for revegetating invaded grasslands.     

Reclamation techniques in southwestern wyoming

Bridger Coal Company operates a 5.8 million tpy surface coal mine thrity five miles northeast of Rock Springs. Wyoming. Approximately 20.000 acres are under permit, with disturbance over the life of the mine projected to reach 10,000 acres. Located on the western rim of the continental divide, the mine receives less than 8.5 inches of precipitation annually. Soils in the area are coarse-textured. and problems associated with elevated salinity and sodicity arc encountered.A variety of common reclamation techniques have been modified to reflect these conditions. Soil horizons are segregated during salvage operations (the surface six inches as topsoil and the balance as subsoil). Unsuitable materials are not salvaged. Direct application of soil is used (over 130 acres in 1983) to maximize native plant regeneration and conserve soil fertility. Inter-seeding of seeding failures has proven to be significantly more successful than chisel plowing and reseeding. Broadcast seeding has been ineffective because of strong winds, and a no till drill has been modified to handle diverse seed mixes and rock conditions. The utility of fertilization under typically xeric moisture regimes is being evaluated. A research project has been initiated to assess establishment of a predominately native, diverse seed mix under irrigation, as well as to determine irrigation rates and duration.     

Reclamation techniques in southwestern wyoming

Bridger Coal Company operates a 5.8 million tpy surface coal mine thrity five miles northeast of Rock Springs. Wyoming. Approximately 20.000 acres are under permit, with disturbance over the life of the mine projected to reach 10,000 acres. Located on the western rim of the continental divide, the mine receives less than 8.5 inches of precipitation annually. Soils in the area are coarse-textured. and problems associated with elevated salinity and sodicity arc encountered. A variety of common reclamation techniques have been modified to reflect these conditions. Soil horizons are segregated during salvage operations (the surface six inches as topsoil and the balance as subsoil). Unsuitable materials are not salvaged. Direct application of soil is used (over 130 acres in 1983) to maximize native plant regeneration and conserve soil fertility. Inter-seeding of seeding failures has proven to be significantly more successful than chisel plowing and reseeding. Broadcast seeding has been ineffective because of strong winds, and a no till drill has been modified to handle diverse seed mixes and rock conditions. The utility of fertilization under typically xeric moisture regimes is being evaluated. A research project has been initiated to assess establishment of a predominately native, diverse seed mix under irrigation, as well as to determine irrigation rates and duration.     

Plant biomass allocation strategies of the dominant species in an alpine meadow of northwestern Sichuan,China北大核心CSCD

Plant biomass partitioning is an important driver of whole-plant net carbon gain, as biomass allocation could directly affect plant's future growth and reproduction. Alpine meadow in the northwestern Sichuan was impressed by the abundant community structure and species diversity. This study on biomass allocation pattern of different functional types and lifeforms might help understand plant life-history strategy of alpine meadow plants. We investigated 72 dominant herbaceous species for their compartments, biomass, and morphological traits during 2012-2014. These plants were sampled from natural grassland, disturbed grassland, and wintergreen grassland; they belonged to three functional types (grass, sedge, and forb) and two lifeforms (annual and perennial). The scaling relationships between functional traits of these plants were analyzed using Model type II regression method to estimate the parameters of the allometric equations. (1) Biomass allocation proportion of components significantly differed among grasses, sedges, and forbs owing to phylogeny: grasses had the highest stem biomass percentage, sedges had higher root biomass percentage, and forbs had higher leaf biomass percentage, but the scaling relationships were not significantly different, and isometric scaling was noted between biomass components for the three functional types. (2) Moreover, plant lifeforms affected the biomass allocation proportion of components, owing to the shorter or longer turnover rate and investment strategy between annual and perennial species. Annuals allocated more biomass to the stem and reproduction organs, but perennials invested more biomass to the leaves and roots. (3) In addition, plants from different grassland types differed in both biomass and morphology traits. Moreover, forbs from natural grassland and wintergreen grassland had higher leaf and reproductive biomass, but those from disturbed grasslands had higher stem biomass. Our results suggest that the functional type and lifeform decide the inherent scaling relationships between components of plants, but anthropogenic disturbance significantly impacted the quantity of component biomass. This study has important theoretical and practical significance to understand the response of alpine plants to climate change and anthropogenic disturbance as well as to help in the scientific management of alpine meadow. © 2018 Science Press. All rights reserved.     

Seed and establishment limitation contribute to long-term native forb declines in California grasslands

The effects of exotic species invasions on biodiversity vary with spatial scale, and documentation of local-scale changes in biodiversity following invasion is generally lacking. Coupling long-term observations of local community dynamics with experiments to determine the role played by exotic species in recruitment limitation of native species would inform both our understanding of exotic impacts on natives at local scales and regional-scale management efforts to promote native persistence. We used field experimentation to quantify propagule and establishment limitation in a suite of native annual forbs in a California reserve, and compared these findings to species abundance trends within the same sites over the past 48 years. Observations at 11 paired sites (inside and outside the reserve) indicated that exotic annual plants have continued to increase in abundance over the past 48 years. This trend suggests the system has not reached equilibrium > 250 years after exotic species began to spread, and 70 years after livestock grazing ceased within the reserve. Long-term monitoring observations also indicated that six native annual forb species went extinct from more local populations than were colonized. To determine the potential role of exotic species in these native plant declines, we added seed of these species into plots adjacent to monitoring sites where plant litter and live grass competition were removed. Experimental results suggest both propagule and establishment limitation have contributed to local declines observed for these native forbs. Recruitment was highest at sites that had current or historical occurrences of the seeded species, and in plots where litter was removed. Grazing history (i.e., location within or outside the reserve) interacted with exotic competition removal, such that removal of live grass competition increased recruitment in more recently grazed sites. Abundance of forbs was positively related to recruitment, while abundance of exotic forbs was negatively related. Thus, exotic competition is likely only one factor contributing to local declines of native species in invaded ecosystems, with a combination of propagule limitation, site quality, and land use history also playing important and interactive roles in native plant recruitment.     

Compensatory responses to loss of warming-sensitive plant species

Climate warming-induced plant species loss is likely to be nonrandom and based on species-specific susceptibility to changing climate. We examined the ecological consequences of losing shallow-rooted forbs, a group of species we predict to be adversely affected by climate change based on their response to experimental warming. After three years of experimental species removal, tap-rooted forbs and grasses were able to fully compensate for the loss of shallow-rooted forbs with increased biomass production. Moreover, the remaining plant community yielded a larger biomass response to nitrogen addition when shallow-rooted forbs were removed, possibly because removal led to increased soil moisture. We conclude that, although shallow-rooted forbs share a common response to warming, their loss did not affect community-level biomass. However, the loss of shallow-rooted forbs could result in increased sensitivity to perturbations, such as changing nutrient availability. Our results demonstrate that realistic, nonrandom scenarios of species loss do not necessarily follow the general pattern of decreased productivity and dampened response to nitrogen addition with species loss that is predicted by theory and many experimental results. Further examinations of nonrandom species loss in other ecosystems are needed to further improve our understanding of the consequences of human-driven species loss.     

Recruitment limitation constrains local species richness and productivity in dry grassland

Species coexistence and local-scale species richness are limited by the availability of seeds and microsites for germination and establishment. We conducted a seed addition experiment in seminatural grassland at three sites in southern Switzerland and repeated the experiment in two successive years to evaluate various circumstances under which seed limitation and establishment success affect community functioning. A collection of 144,000 seeds of 22 meadow species including grasses and forbs of local provenance was gathered, and seeds were individually sown in a density that resembled natural seed rain. The three communities were seed limited. Three years after sowing, single species varied in emergence (0-50%), survival (0-69%), and establishment rates (0-27%). One annual and 13 perennial species reached reproductive stage. Low establishment at one site and reduced growth at another site indicated stronger microsite limitation compared to the third site. Recruitment was influenced by differences in abiotic environmental conditions between sites (water availability, soil minerals) and by within-site differences in biotic interaction (competition). At the least water-limited site, sowing resulted in an increase in phytomass due to establishment of short-lived perennials in the second and third years after sowing. This increase persisted over the following two years due to establishment of longer-lived perennials. After sowing in a wetter year with higher phytomass, however, productivity did not increase, because higher intensity of competition in an early phase of establishment resulted in less vigorous plants later on. Due to the generally favorable weather conditions during this study, sowing year had a small effect on numbers of established individuals over all species. Recruitment limitation can thus constrain local-scale species richness and productivity, either by a lack of seeds or by reduced seedling growth, likely due to competition from the established vegetation.     

Vegetation pattern related tc grazing pressure in alpine meadows of Nanda Devi Biosphere Reserve

The present study aims to analyze the interaction of prevailing biotic pressure on plant species diversity in Nanda Devi Biosphere Reserve (NDBR) which lies in northern part of Uttaranchal hills between 79 degrees 40'E to 80 degrees 05'E longitude and 30 degrees 17' N to 30 degrees 41'N latitude and covers an area of 2236.7 km2. A total of 75 species has been found which included the herbaceous plants viz., grasses, sedges and forbs. Generally, the plants have a short life span of 3-4 months. However, few species persist throughout the growth period i.e. May-October. Phytosociological study performed in plots of varying slope and grazing pressure intensity revealed that the dominant grasses were Danthonia cachemyriana and Poa annua and dominant forbs were Trachydium roylei and Geum elatum in all the plots. Grasses were abundant on west facing slopes while forbs preferred the even topography of east facing meadows. The grasses and sedges together had optimum density during July and August. In general, short lived species exhibited more diversity for one or two months whilst the long lived species exhibited optimum diversity althrough the snow free period. The species diversity is maximum (100%) in moderately grazed bughiyals i.e. Pacchu and minimum in intensively grazed bughiyals i.e. Martoli. The species distribution among the plots was 60-90% contagious and 11.2-38.0% randomirrespective of grazing pressure, thus highlighting the significance of grazing pressure in management of alpine meadows.     

Grassland invader responses to realistic changes in native species richness.

The importance of species richness for repelling exotic plant invasions varies from ecosystem to ecosystem. Thus, in order to prioritize conservation objectives, it is critical to identify those ecosystems where decreasing richness will most greatly magnify invasion risks. Our goal was to determine if invasion risks greatly increase in response to common reductions in grassland species richness. We imposed treatments that mimic management-induced reductions in grassland species richness (i.e., removal of shallow- and/or deep-rooted forbs and/or grasses and/or cryptogam layers). Then we introduced and monitored the performance of a notorious invasive species (i.e., Centaurea maculosa). We found that, on a per-gram-of-biomass basis, each resident plant group similarly suppressed invader growth. Hence, with respect to preventing C. maculosa invasions, maintaining overall productivity is probably more important than maintaining the productivity of particular plant groups or species. But at the sites we studied, all plant groups may be needed to maintain overall productivity because removing forbs decreased overall productivity in two of three years. Alternatively, removing forbs increased productivity in another year, and this led us to posit that removing forbs may inflate the temporal productivity variance as opposed to greatly affecting time-averaged productivity. In either case, overall productivity responses to single plant group removals were inconsistent and fairly modest, and only when all plant groups were removed did C. maculosa growth increase substantially over a no-removal treatment. As such, it seems that intense disturbances (e.g., prolonged drought, overgrazing) that deplete multiple plant groups may often be a prerequisite for C. maculosa invasion.     

Use of hold-gro erosion control fabric in the establishment of plant species on coal mine soil

Experiments were conducted on the Black Mesa Coal Mine, Kayenta, Arizona in 1977 and 1978 to study the effectiveness of Hold-Gro Erosion Control Fabric (a product from the Gulf States Paper Corporation, Tuscaloosa, Alabama) in the establishment of plants on coal mine soil following the surface mining of coal. Four plant species were planted: (1) spring barley (Horduem vulgare L.), an annual grass (2) crested wheatgrass (Agropyron cristatum L.), a perennial grass (3) alfalfa (lucerne) (Medicago sativa L.), a perennial legume and (4) fourwing saltbush (Atriplex canescens Pursh.), a perennial shrub. Seeds of each plant species were planted in reclaimed coal mine soil in the spring of the year by both broadcast seeding (conventional culture) and the incorporation of seeds in Hold-Gro Erosion Control Fabric. Average numbers of seedlings established and percent ground cover for all species studied were higher in areas where conventional culture was used than they were in areas where seeds were incorporated in Hold-Gro Erosion Control Fabric. The incorporation of seeds in Hold-Gro Erosion Control Fabric in the establishment of plant species on coal mine soil was not an effective cultural practice in the southwestern United States. The authors gratefully acknowledge the assistance of M.A. Norem with portions of the research involving this contribution. The authors also express their appreciation to the Gulf States Paper Corporation in Tuscaloosa. Alabama for providing the Hold-Gro Erosion Control Fabrie used in this research. Contribution from the Arizona Agr. Exp. Stn., University of Arizona, Tucson, Arizona 85721. Approved for publication as Arizona Agr. Exp. Stn. Research Contribution No. 3767.     

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

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

Reclamation of bentonite mined lands in the northern great plains

Ninety percent of the nation's supply of bentonite is mined in Montana, South Dakota and Wyoming. These lands are difficult to reclaim because of the chemical and physical properties of the soil/spoil material and the arid/semiarid climate of the area. Replacement of the limited topsoil available has shown some benefit but supplies are generally inadequate. The use of inorganic amendments, such as sulfuric acid, gypsum, calcium chloride, vermiculite and perlite, have not shown consistent benefits in plant establishment and growth. Organic amendments; sawmill wood residues, straw and manure, have provided the greatest benefits in the reclamation of bentonite mined lands. Cultural and management practices are important in determining the long-term success of these reclaimed lands. Reclamation technology development for bentonite mined lands is recent and limited and the refinement and application of such technology will depend on increased activity and cooperation among researchers, regulators and the mining industry.     

若尔盖西部草甸的基本类型、特点及近几十年来的变化

2000年8月对若尔盖西部草地的调查，确定了该地18个草丛类型（不包括典型的沼泽植被）。经群丛间Pearson最大相关系数配合生境作图，将该地区的草地群丛分成3大类，即沼生草地，中生性草甸和旱生性草地。各群丛的SW指数，物种丰富度，地上生物量，裸地面积按照生境序列作图，显示中性草甸具有最高的物种丰富度和地上生物量。群丛C和J分别是沼生和旱生植被与中生性草甸之间的拐点式过渡类型，其物种丰富度明显比周期的群丛低，生境转换关键种分别为鹅绒委陵菜和垂穗披碱草。在草地物种组成上，杂类草占81．3％，莎草类仅占6．8％；群丛建群种方面，杂类草占61％，莎草类占28％；在次优势种方面，杂类草和禾草所占比重都有所增加。草原成分出现在丛中生至旱生的群丛中，在有些群丛中甚至成为次优势种。沼泽消退，黄河河道附近沙化以及过牧的阳坡山地生长带刺的旱生化植被，说明当地正经历着干旱化的过程，原因不仅有自然的还有人为的，特别是草地的过度使用。最后对当地草场的管理及发展提出了一些建议。图3表2参20     

Effect of Vertebrate Grazing on Plant and Insect Community Structure

Abstract: We compared species diversity of plants and insects among grazed and ungrazed areas of Ponderosa pine–grassland communities in Arizona. Plant species richness was higher in two of three grassland communities that were grazed by native elk and deer and domestic cattle than in ungrazed areas inside a series of three large (approximately 40-ha) grazing exclosures. Similarly, plant species richness was higher in grazed areas relative to ungrazed areas at one of two series of smaller (approximately 25-m²) and short-term exclosure sites. Evenness of plant distribution, however, was greater inside ungrazed long-term exclosures but was reduced inside ungrazed short-term exclosures relative to grazed areas. Relative abundances of forbs, grasses, trees, and shrubs, and native and introduced plants did not differ between the long- and short-term grazing exclosures and their grazed counterparts. Relative abundances of some plant species changed when grazers were excluded, however. In contrast, insect species richness was not different between grazed and ungrazed habitats, although insect abundance increased 4- to 10-fold in ungrazed vegetation. Our results suggest that vertebrate grazing may increase plant richness, even in nutrient-poor, semi-arid grasslands, but may decrease insect abundances.     

The adaptive value of remnant native plants in invaded communities: an example from the Great Basin

Changes in the species composition of biotic communities may alter patterns of natural selection occurring within them. Native perennial grass species in the Intermountain West are experiencing a shift in the composition of interspecific competitors from primarily perennial species to an exotic, annual grass. Thus traits that confer an advantage to perennial grasses in the presence of novel annual competitors may evolve in invaded communities. Here I show that such traits are apparent in populations of a native perennial grass, big squirreltail (Elymus multisetus M.E. Jones), exposed to cheatgrass (Bromus tectorum L.) competitors. Dormant big squirreltail plants were collected from cheatgrass-invaded and uninvaded sites near Bordertown, California, USA, a mid-elevation (1600 m) sagebrush community, and transplanted into pots in a greenhouse. Individual plants were split into equal halves. One half was grown with competition from cheatgrass, and the other half was grown without competition. Plants collected from invaded sites responded more quickly to watering, growing more leaves in the first 10 days after transplanting. In addition, big squirreltail plants collected from invaded areas experienced a smaller decrease in plant size when grown with competition than did plants collected from uninvaded areas. Accordingly, while there were fewer big squirreltail individuals in the invaded sites, they were more competitive with cheatgrass than were the more abundant conspecifics in nearby uninvaded areas. It is possible that annual grasses were the selective force that caused these population differences, which may contribute to the long-term persistence of the native populations. While it is tempting to restore degraded areas to higher densities of natives (usually done by bringing in outside seed material), such actions may impede long-term adaptation to new conditions by arresting or reversing the direction of ongoing natural selection in the resident population. If hot spots of rapid evolutionary change can be identified within invaded systems, these areas should be managed to promote desirable change and could serve as possible sources of restoration material or reveal traits that should be prioritized during the development of restoration seed material.     

Long-term dynamics of biotic and abiotic resistance to exotic species invasion in restored vernal pool plant communities

Invasion of native ecosystems by exotic species can seriously threaten native biodiversity, alter ecosystem function, and inhibit conservation. Moreover, restoration of native plant communities is often impeded by competition from exotic species. Exotic species invasion may be limited by unfavorable abiotic conditions and by competition with native species, but the relative importance of biotic and abiotic factors remains controversial and may vary during the invasion process. We used a long-term experiment involving restored vernal pool plant communities to characterize the temporal dynamics of exotic species invasion, and to evaluate the relative support for biotic and abiotic factors affecting invasion resistance. Experimental pools (n=256) were divided among controls and several seeding treatments. In most treatments, native vernal pool species were initially more abundant than exotic species, and pools that initially received more native seeds exhibited lower frequencies of exotic species over time. However, even densely seeded pools were eventually dominated by exotic species, following extreme climatic events that reduced both native and exotic plant densities across the study site. By the sixth year of the experiment, most pools supported more exotics than native vernal pool species, regardless of seeding treatment or pool depth. Although deeper pools were less invaded by exotic species, two exotics (Hordeum marinum and Lolium multiflorum) were able to colonize deeper pools as soon as the cover of native species was reduced by climatic extremes. Based on an information-theoretic analysis, the best model of invasion resistance included a nonlinear effect of seeding treatment and both linear and nonlinear effects of pool depth. Pool depth received more support as a predictor of invasion resistance, but seeding intensity was also strongly supported in multivariate models of invasion, and was the best predictor of resistance to invasion by H. marinum and L. multilorum. We conclude that extreme climatic events can facilitate exotic species invasions by both reducing abiotic constraints and weakening biotic resistance to invasion.