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.     

Competition,legacy, and priority and the success of three invasive species

Competitive ability, the ability to generate legacy effects, and the potential to benefit from priority, individually or interactively, are traits that may increase the invasive potential of plants. In this project we examine these three traits in three invasive species (Agropyron cristatum, Bromus tectorum, and Taeniatherum caput-medusae). Specifically in this study, we examine competitive effects of these invasive species, the ability of these invasive species to generate legacy effects (as plant–soil feedback), and the potential of these three species benefit from priority (being sown concurrently, 30 days before, and 30 days after the restoration species) in a greenhouse study using field collected soil. Our results suggest that all three invasive species can benefit from priority and all three have high competitive ability. However, only A. cristatum benefited from legacy effects of plant–soil feedback.     

Intra and interspecific competition among invasive and native species during early stages of plant growth

Plant competition is a primary ecological process limiting grassland restoration success. Appropriate restoration techniques require an understanding of the degree to which intra and interspecific competition control invasive and native plant growth. The objective of this study was to determine how the intensity of intra and interspecific competition changes during early stages of plant growth. Two invasive (Bromus tectorum and Taeniatherum caput-medusae) and two native (Pseudoroegneria spicata and Poa secunda) species were grown in a diallel competition experiment, either alone or in 1:1 binary combinations and exposed to two levels of N (no N or 400 mg N kg⁻¹ soil added) in a greenhouse. Total biomass for each species was quantified over four harvests and competitive effects were calculated. Our results show that the relative magnitude of intra and interspecific competition changes through time. Intraspecific competition was intense for native species at the initial harvests and therefore important in contributing to the outcome of final size of native species seedlings. Interestingly, bluebunch wheatgrass imposed interspecific competition on annual grasses at the first two harvests and appeared to be a better competitor than Sandberg’s bluegrass. We found that fast growing invasive species became more competitive compared to slow growing native species with increasing N and appear to establish a positive feedback mechanism between size and resource uptake. Opportunities to improve restoration success exist from determining the optimum combination of density, species proportion, and their spatial arrangement in various ecosystems and environments.     

Invasive plants decrease microbial capacity to nitrify and denitrify compared to native California grassland communities

Exotic plant invasions are a major driver of global environmental change that can significantly alter the availability of limiting nutrients such as nitrogen (N). Beginning with European colonization of California, native grasslands were replaced almost entirely by annual exotic grasses, many of which are now so ubiquitous that they are considered part of the regional flora (“naturalized”). A new wave of invasive plants, such as Aegilops triuncialis (Barb goatgrass) and Elymus caput-medusae (Medusahead), continue to spread throughout the state today. To determine whether these new-wave invasive plants alter soil N dynamics, we measured inorganic N pools, nitrification and denitrification potentials, and possible mediating factors such as microbial biomass and soil pH in experimental grasslands comprised of A. triuncialis and E. caput-medusae. We compared these measurements with those from experimental grasslands containing: (1) native annuals and perennials and (2) naturalized exotic annuals. We found that A. triuncialis and E. caput-medusae significantly reduced ion-exchange resin estimates of nitrate (NO₃ ^?) availability as well as nitrification and denitrification potentials compared to native communities. Active microbial biomass was also lower in invaded soils. In contrast, potential measurements of nitrification and denitrification were similar between invaded and naturalized communities. These results suggest that invasion by A. triuncialis and E. caput-medusae may significantly alter the capacity for soil microbial communities to nitrify or denitrify, and by extension alter soil N availability and rates of N transformations during invasion of remnant native-dominated sites.     

Promoting Native Vegetation and Diversity in Exotic Annual Grass Infestations

Exotic plant invasions are especially problematic because reestablishment of native perennial vegetation is rarely successful. It may be more appropriate to treat exotic plant infestations that still have some remaining native vegetation. We evaluated this restoration strategy by measuring the effects of spring burning, fall burning, fall applied imazapic, spring burning with fall applied imazapic, and fall burning with fall applied imazapic on the exotic annual grass, medusahead (Taeniatherum caput‐medusae (L.) Nevski), and native vegetation at six sites in Oregon for 2 years post‐treatment. Medusahead infestations included in this study had some residual native perennial bunchgrasses and forbs. Burning followed by imazapic application provided the best control of medusahead and resulted in the greatest increases in native perennial vegetation. However, imazapic application decreased native annual forb cover the first year post‐treatment and density the first and second year post‐treatment. The spring burn followed by imazapic application produced an almost 2‐fold increase in plant species diversity compared to the control. The fall burn followed by imazapic application also increased diversity compared to the control. Results of this study indicate that native plants can be promoted in medusahead invasions; however, responses vary by plant functional group and treatment. Our results compared to previous research suggest that restoration of plant communities invaded by exotic annual grass may be more successful if efforts focus on areas with some residual native perennial vegetation. Thus, invasive plant infestations with some native vegetation remaining should receive priority for restoration efforts over near monocultures of invasive plant species.     

Containment and spot eradication of a highly destructive,invasive plant pathogen (<Emphasis Type="Italic">Phytophthora cinnamomi</Emphasis>) in natural ecosystems

The invasive plant pathogen Phytophthora cinnamomi (Stramenopila, Oomycota) has been introduced into 15 of the 25 global biodiversity hotspots, threatening susceptible rare flora and degrading plant communities with severe consequences for fauna. We developed protocols to contain or eradicate P. cinnamomi from spot infestations in threatened ecosystems based on two assumptions: in the absence of living hosts, P. cinnamomi is a weakly competitive saprotroph; and in the ecosystems we treated, the transmission of the pathogen occurs mainly by root-to-root contact. At two P. cinnamomi-infested sites differing in climate and vegetation types, we applied increasingly robust treatments including vegetation (host) destruction, fungicides, fumigation and physical root barriers. P. cinnamomi was not recovered at three assessments of treated plots 6–9 months after treatments. Given the high rates of recovery of P. cinnamomi from untreated infested soil and the sampling frequency, the probability of failing to detect P. cinnamomi in treated soil was <0.0003. The methods described have application in containing large infestations, eradicating small infestations and protecting remnant populations of threatened 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.     

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.     

Invasion resistance and persistence: established plants win,even with disturbance and high propagule pressure

Disturbances and propagule pressure are key mechanisms in plant community resistance to invasion, as well as persistence of invasions. Few studies, however, have experimentally tested the interaction of these two mechanisms. We initiated a study in a southwestern ponderosa pine (Pinus ponderosa Laws.)/bunch grass system to determine the susceptibility of remnant native plant communities to cheatgrass (Bromus tectorum L.) invasion, and persistence of cheatgrass in invaded areas. We used a 2 × 2 factorial design consisting of two levels of aboveground biomass removal and two levels of reciprocal seeding. We seeded cheatgrass seeds in native plots and a native seed mixture in cheatgrass plots. Two biomass removal disturbances and sowing seeds over 3 years did not reverse cheatgrass dominance in invaded plots or native grass dominance in non-invaded native plots. Our results suggest that two factors dictated the persistence of the resident communities. First, bottlebrush squirreltail (Elymus elymoides (Raf.) Swezey) was the dominant native herbaceous species on the study site. This species is typically a poor competitor with cheatgrass as a seedling, but is a strong competitor when mature. Second, differences in pretreatment levels of plant-available soil nitrogen and phosphorus may have favored the dominant species in each community. Annual species typically require higher levels of plant-available soil nutrients than perennial plants. This trend was observed in the annual cheatgrass community and perennial native community. Our study shows that established plants and soil properties can buffer the influences of disturbance and elevated propagule pressure on cheatgrass invasion.     

An invasive wetland grass primes deep soil carbon pools

Understanding the processes that control deep soil carbon (C) dynamics and accumulation is of key importance, given the relevance of soil organic matter (SOM) as a vast C pool and climate change buffer. Methodological constraints of measuring SOM decomposition in the field prevent the addressing of real‐time rhizosphere effects that regulate nutrient cycling and SOM decomposition. An invasive lineage of Phragmites australis roots deeper than native vegetation (Schoenoplectus americanus and Spartina patens) in coastal marshes of North America and has potential to dramatically alter C cycling and accumulation in these ecosystems. To evaluate the effect of deep rooting on SOM decomposition we designed a mesocosm experiment that differentiates between plant‐derived, surface SOM‐derived (0–40 cm, active root zone of native marsh vegetation), and deep SOM‐derived mineralization (40–80 cm, below active root zone of native vegetation). We found invasive P. australis allocated the highest proportion of roots in deeper soils, differing significantly from the native vegetation in root : shoot ratio and belowground biomass allocation. About half of the CO₂ produced came from plant tissue mineralization in invasive and native communities; the rest of the CO₂ was produced from SOM mineralization (priming). Under P. australis, 35% of the CO₂ was produced from deep SOM priming and 9% from surface SOM. In the native community, 9% was produced from deep SOM priming and 44% from surface SOM. SOM priming in the native community was proportional to belowground biomass, while P. australis showed much higher priming with less belowground biomass. If P. australis deep rooting favors the decomposition of deep‐buried SOM accumulated under native vegetation, P. australis invasion into a wetland could fundamentally change SOM dynamics and lead to the loss of the C pool that was previously sequestered at depth under the native vegetation, thereby altering the function of a wetland as a long‐term C sink.     

Non-native grass invasion alters native plant composition in experimental communities

Invasions of non-native species are considered to have significant impacts on native species, but few studies have quantified the direct effects of invasions on native community structure and composition. Many studies on the effects of invasions fail to distinguish between (1) differential responses of native and non-native species to environmental conditions, and (2) direct impacts of invasions on native communities. In particular, invasions may alter community assembly following disturbance and prevent recolonization of native species. To determine if invasions directly impact native communities, we established 32 experimental plots (27.5 m²) and seeded them with 12 native species. Then, we added seed of a non-native invasive grass (Microstegium vimineum) to half of the plots and compared native plant community responses between control and invaded plots. Invasion reduced native biomass by 46, 64, and 58%, respectively, over three growing seasons. After the second year of the experiment, invaded plots had 43% lower species richness and 38% lower diversity as calculated from the Shannon index. Nonmetric multidimensional scaling ordination showed a significant divergence in composition between invaded and control plots. Further, there was a strong negative relationship between invader and native plant biomass, signifying that native plants are more strongly suppressed in densely invaded areas. Our results show that a non-native invasive plant inhibits native species establishment and growth following disturbance and that native species do not gain competitive dominance after multiple growing seasons. Thus, plant invaders can alter the structure of native plant communities and reduce the success of restoration efforts.     

Effect of an invasive grass on ambient rates of decomposition and microbial community structure: a search for causality

In situ decomposition of above and belowground plant biomass of the native grass species Andropogon glomeratus (Walt.) B.S.P. and exotic Imperata cylindrica (L.) Beauv. (cogongrass) was investigated using litter bags over the course of a 12 month period. The above and belowground biomass of the invasive I. cylindrica always decomposed faster than that of the native A. glomeratus. Also, belowground biomass of both species decomposed at a consistently faster rate when placed within an invaded area consisting of a monotypic stand of I. cylindrica as opposed to within a native plant assemblage. However, there was no similar such trend observed in the aboveground plant material. The microbial communities associated with the invaded sites often differed from those found in the native vegetation and provide a possible causal mechanism by which to explain the observed differences in decomposition rates. The microbial communities differed not only compositionally, as indicated by ordination analyses, but also functionally with respect to enzymatic activity essential to the decomposition process. This study supports the growing consensus that invasive plant species alter normal ecological processes and highlights a possible mechanism (alteration of microbial assemblages) by which I. cylindrica may alter an ecosystem process (decomposition).     

Sonoran Desert Ecosystem transformation by a C4 grass without the grass/fire cycle

Aim Biological invasions facilitate ecosystem transformation by altering the structure and function, diversity, dominance and disturbance regimes. A classic case is the grass–fire cycle in which grass invasion increases the frequency, scale and/or intensity of wildfires and promotes the continued invasion of invasive grasses. Despite wide acceptance of the grass–fire cycle, questions linger about the relative roles that interspecific plant competition and fire play in ecosystem transformations. Location Sonoran Desert Arizona Upland of the Santa Catalina Mountains, Arizona, USA. Methods We measured species cover, density and saguaro (Carnegiea gigantea) size structure along gradients of Pennisetum ciliare invasion at 10 unburned/ungrazed P. ciliare patches. Regression models quantified differences in diversity, cover and density with respect to P. ciliare cover, and residence time and a Fisher’s exact test detected demographic changes in saguaro populations. Because P. ciliare may have initially invaded locations that were both more invasible and less diverse, we ran analyses with and without the plots in which initial infestations were located. Results Richness and diversity decreased with P. ciliare cover as did cover and density of most dominant species. Richness and diversity declined with increasing time since invasion, suggesting an ongoing transformation. The proportion of old‐to‐young Carnegiea gigantea was significantly lower in plots with dominant P. ciliare cover. Main conclusions Rich desert scrub (15–25 species per plot) was transformed into depauperate grassland (2–5 species per plot) within 20 years following P. ciliare invasion without changes to the fire regime. While the onset of a grass–fire cycle may drive ecosystem change in the later stages and larger scales of grass invasions of arid lands, competition by P. ciliare can drive small‐scale transformations earlier in the invasion. Linking competition‐induced transformation rates with spatially explicit models of spread may be necessary for predicting landscape‐level impacts on ecosystem processes in advance of a grass–fire cycle.     

Competitive effects and responses of the invasive grass Eragrostis plana in Río de la Plata grasslands

The ability of an invasive species to establish is mostly determined by its biotic interactions with native species from the recipient community. Here, we evaluate the competitive effects and responses of the invasive Eragrostis plana when interacting with native species, in order to identify possible mechanisms driving invasion in Río de la Plata grasslands. A pairwise competition experiment was performed consisting of treatments that varied in the identity of neighbour plant species: (i) control (no interaction); (ii) intraspecific interaction; (iii) interspecific interaction between native and invasive species; and (iv) interspecific interaction between two co‐occurring native species. Data analysis was separated into the effect of E. plana on the performance of three native perennial grasses (target species: Aristida laevis, Eragrostis neesii and Paspalum notatum) and the response of E. plana to natives (target species: E. plana). Separately for each target species, components of plant performance were compared between neighbouring species treatments. We found that the strength of competitive interactions depended on both target and neighbour species identity. Regarding natives, interspecific competition was stronger than intraspecific. Native species showed distinctive responses to whether the neighbour was the invasive or a co‐occurring native (Eragrostis lugens). Competition between E. plana and native species was stronger than between co‐occurring natives. We demonstrated E. plana had a greater negative effect on native's species performance than the native congener E. lugens. Regarding E. plana, intraspecific competition was stronger than interspecific, and its response was positive or neutral when interacting with natives, suggesting its high tolerance to grow in competition with neighbours. We conclude E. plana's negative effects on native species performance, and its positive or neutral responses to neighbouring native plants demonstrate its strong competitive ability in the recipient community. This may explain its invasion success in southern Brazil and in the encompassing Río de la Plata grasslands. Abstract in Spanish is available with online material.     

Multiple scale analysis of factors influencing the distribution of an invasive aquatic grass

With the expected increase in the spread of invasive species, examination of factors controlling distributions at multiple spatial scales and ecological modelling of their potential distributions are important analyses for informed decision-making. The scale-dependence of mechanisms influencing invasion by non-native species has been shown previously, indicating that studies of key factors affecting invasive species distributions at multiple spatial scales are critical for successful management. Freshwater systems are particularly vulnerable to invasive species, yet few studies have examined the environmental factors influencing distributions of invasive species at multiple spatial scales. We examined the effect of environmental variables on the predicted distribution of the invasive aquatic grass Glyceria maxima over continental, regional and local scales in Australia. We undertook an initial critical evaluation of which predictor variables were most appropriate to use at each scale, largely considering prior knowledge. On a continental scale, climatic, topographic and hydrological variables predicted well the potential distribution of G. maxima, identifying temperate regions as most susceptible to invasion. The regional analysis found that dense, woody, riparian vegetation has a strong negative impact on the occurrence of G. maxima, especially at intermediate elevations. The invasive grass was found less often on biotite granite and on fluvial geology. At a local scale, occurrence of G. maxima was related positively to soil phosphorus and nitrogen, and negatively related to soil organic carbon. The identification of key factors affecting invasive species distributions at multiple spatial scales will inform prevention schemes, assist targeted field sampling for the development of monitoring programs, and allow prioritization of control methods.     

Temperature and functional traits influence differences in nitrogen uptake capacity between native and invasive grasses

Performance differences between native and exotic invasive plants are often considered static, but invasive grasses may achieve growth advantages in western North America shrublands and steppe under only optimal growing conditions. We examine differences in N uptake and several morphological variables that influence uptake at temperatures between 5 and 25 °C. We contrast two native perennial grasses in western North America: Elymus elymoides and Pseudoroegneria spicata; two invasive annual grasses: Bromus tectorum and Taeniatherum caput-medusae; and one highly selected non-native perennial grass: Agropyron cristatum. The influence of temperature on N uptake is poorly characterized, yet these invasive annual grasses are known to germinate in warm soils in the autumn, and both experience cool soils during the short growing season following snowmelt in the spring. To further explore the influence of temperature on the correlation between morphological variables and N uptake, our data are applied to a previously published path model and one proposed here. Differences in N uptake between native and invasive grasses were small at the lowest temperature, but were large at the highest temperature. At lower temperatures, uptake of N by annuals and perennials was correlated with leaf N and mass. At higher temperatures, uptake by annuals was correlated only with these leaf traits, but uptake by perennials was correlated with these leaf traits as well as root N and mass. Consequently, our results imply that annual grasses face fewer morphological constraints on N uptake than perennial grasses, and annual grasses may gain further advantage in warmer temperature conditions or during more frequent warm periods.     

Effects of the Epichloë fungal endophyte symbiosis with Schedonorus pratensis on host grass invasiveness

Initial studies of grass–endophyte mutualisms using Schedonorus arundinaceus cultivar Kentucky‐31 infected with the vertically transmitted endophyte Epichloë coenophiala found strong, positive endophyte effects on host‐grass invasion success. However, more recent work using different cultivars of S. arundinaceus has cast doubt on the ubiquity of this effect, at least as it pertains to S. arundinaceus–E. coenophiala. We investigated the generality of previous work on vertically transmitted Epichloë‐associated grass invasiveness by studying a pair of very closely related species: S. pratensis and E. uncinata. Seven cultivars of S. pratensis and two cultivars of S. arundinaceus that were developed with high‐ or low‐endophyte infection rate were broadcast seeded into 2 × 2‐m plots in a tilled, old‐field grassland community in a completely randomized block design. Schedonorus abundance, endophyte infection rate, and co‐occurring vegetation were sampled 3, 4, 5, and 6 years after establishment, and the aboveground invertebrate community was sampled in S. pratensis plots 3 and 4 years after establishment. Endophyte infection did not enable the host grass to achieve high abundance in the plant community. Contrary to expectations, high‐endophyte S. pratensis increased plant richness relative to low‐endophyte cultivars. However, as expected, high‐endophyte S. pratensis marginally decreased invertebrate taxon richness. Endophyte effects on vegetation and invertebrate community composition were inconsistent among cultivars and were weaker than temporal effects. The effect of the grass–Epichloë symbiosis on diversity is not generalizable, but rather specific to species, cultivar, infection, and potentially site. Examining grass–endophyte systems using multiple cultivars and species replicated among sites will be important to determine the range of conditions in which endophyte associations benefit host grass performance and have subsequent effects on co‐occurring biotic communities.     

Postfire vegetation dynamics in a sagebrush steppe in southeastern Idaho,USA

Postburn vegetation in an Artemisia tripartita and A. tridentata sagebrush community one year after the burn was compared with unburned vegetation. While the vegetal cover amounted to 38.1% of the total area on the burned site, it was 91.1% on the unburned site. Dominance-diversity curves for plant communities on both sites approach the niche pre-emption hypothesis or geometric series. Cover values and soil residual propagule data were used to suggest mechanisms of persistence of the more prominent species through fire using Noble & Slatyer's (1980) Vital Attributes model. The first year postfire vegetation was dominated by forbs and grasses with vegetative and propagule storage mechanisms of persistence. Such information on succession mechanisms should be of benefit to range managers.     

Soil pH influences patterns of plant community composition after restoration with native‐based seed mixes

Reclamation of highly disturbed lands typically includes establishing fast‐growing, non‐native plants to achieve rapid ground cover for erosion control. Establishing native plant communities could achieve ecosystem functions beyond soil erosion, such as providing wildlife habitat. Pipelines, or other disturbed corridors through a landscape, present unique challenges for establishing native plant communities given the heterogeneity of soil environments and invasive plant propagule pressure. We created two structural equation models to address multiple related hypotheses about the influence of soil pH on plant community composition (current diversity and vegetative cover of the original restoration seed mix and background flora, and invasive plant density during mix establishment and current density) of a highly disturbed landscape corridor restored with native species. To test our hypotheses we conducted a plant survey on a gas pipeline crossing two state forests in the north‐central Appalachians that had been seeded with a native‐based mixture 8 years prior. Low soil pH was a strong predictor of density of the invasive annual plant, Microstegium vimineum, and had resulted in lower species diversity and cover of the seeded mix. Overall, our data provide evidence that native‐based grass and forb mixtures can establish and persist on a wide range of soil environments and thrive in competition with invasive plants in moderately acidic to neutral soils. Advancing knowledge on restoration methods using native species is essential to improving restoration practice norms to incorporate multifunctional ecological goals.     

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.