Plant species' origin predicts dominance and response to nutrient enrichment and herbivores in global grasslands

Genotype diversity enhances invasion resistance of native plants via soil biotic feedbacks

Although native species diversity is frequently reported to enhance invasion resistance, within-species diversity of native plants can also moderate invasions. While the positive diversity-invasion resistance relationship is often attributed to competition, indirect effects mediated through plant-soil feedbacks can also influence the relationship. We manipulated the genotypic diversity of an endemic species, Scirpus mariqueter, and evaluated the effects of abiotic versus biotic feedbacks on the performance of a global invader, Spartina alterniflora. We found that invader performance on live soils decreased non-additively with genotypic diversity of the native plant that trained the soils, but this reversed when soils were sterilized to eliminate feedbacks through soil biota. The influence of soil biota on the feedback was primarily associated with increased levels of microbial biomass and fungal diversity in soils trained by multiple-genotype populations. Our findings highlight the importance of plant-soil feedbacks mediating the positive relationship between genotypic diversity and invasion resistance.

Establishment from seed is more important for exotic than for native plant species

Climate change has initiated movement of both native and non-native (exotic) species across the landscape. Exotic species are hypothesized to establish from seed more readily than comparable native species. We tested the hypothesis that seed limitation is more important for exotic species than native grassland species. We compared seed limitation and invasion resistance over three growing seasons between 18 native and 18 exotic species, grown in both monocultures and mixtures in a field experiment. Half of the plots received a seed mix of the contrasting treatment (i.e., exotic species were seeded into native plots, and native species were seeded into exotic plots), and half served as controls. We found that (1) establishment in this perennial grassland is seed limited, (2) establishment from seed is greater in exotic than native species, and (3) community resistance to seedling establishment was positively related to diversity of extant species, but only in native communities. Native-exotic species diversity and composition differences did not converge over time. Our results imply that native to exotic transformations occur when diversity declines in native vegetation and exotic seeds arrive from adjacent sites, suggesting that managing for high diversity will reduce transformations to exotic dominance.

Opposite effects of nutrient enrichment and an invasive snail on the growth of invasive and native macrophytes

Many ecosystems are now co-invaded by alien plant and herbivore species. The evolutionary naivety of native plants to alien herbivores can make the plants more susceptible to the detrimental effects of herbivory than co-occurring invasive plants, in accordance with the apparent competition hypothesis. Moreover, the invasional meltdown hypothesis predicts that in multiply invaded ecosystems, invasive species can facilitate each other's impacts on native communities. Although there is growing empirical support for these hypotheses, facilitative interactions between invasive plants and herbivores remain underexplored in aquatic ecosystems. Many freshwater ecosystems are co-invaded by aquatic macrophytes and mollusks and simultaneously experience nutrient enrichment. However, the interactive effects of these ecological processes on native macrophyte communities remain an underexplored area. To test these effects, we performed a freshwater mesocosm experiment in which we grew a synthetic native community of three macrophyte species under two levels of invasion by an alien macrophyte Myriophyllum aquaticum (invasion vs. no invasion) and fully crossed with two levels of nutrient enrichment (enrichment vs. no enrichment) and herbivory by an invasive snail Pomacea canaliculata (herbivory vs. no herbivory). In line with the invasional meltdown and apparent competition hypotheses, we found that the proportional aboveground biomass yield of the invasive macrophyte, relative to that of the native macrophyte community, was significantly greater in the presence of the invasive herbivore. Evidence of a reciprocal facilitative effect of the invasive macrophyte on the invasive herbivore is provided by results showing that the herbivore produced greater egg biomass in the presence versus in the absence of M. aquaticum. However, nutrient enrichment reduced the mean proportional aboveground biomass yield of the invasive macrophyte. Our results suggested that herbivory by invader P. canaliculata may enhance the invasiveness of M. aquaticum. However, nutrient enrichment of habitats that already harbor M. aquaticum may slow down the invasive spread of the macrophyte. Broadly, our study underscores the significance of considering several factors and their interactions when assessing the impact of invasive species, especially considering that many habitats experience co-invasion by plants and herbivores and simultaneously undergo various other disturbances, including nutrient enrichment.

Nutrient enrichment promotes invasion success of alien plants via increased growth and suppression of chemical defenses

In support of the prediction of the enemy release hypothesis regarding a growth-defense trade-off, invasive alien plants often exhibit greater growth and lower anti-herbivory defenses than native plants. However, it remains unclear how nutrient enrichment of invaded habitats may influence competitive interactions between invasive alien and co-occurring native plants, as well as production of anti-herbivore defense compounds, growth-promoting hormones, and defense-regulating hormones by the two groups of plants. Here, we tested whether: (i) nutrient enrichment causes invasive alien plants to produce greater biomass and lower concentrations of the defense compounds flavonoids and tannins than native plants; and (ii) invasive alien plants produce lower concentrations of a defense-regulating hormone jasmonic acid (JA) and higher concentrations of a growth-promoting hormone gibberellic acid (GA3). In a greenhouse experiment, we grew five congeneric pairs of invasive alien and native plant species under two levels each of nutrient enrichment (low vs. high), simulated herbivory (leaf clipping vs. no-clipping), and competition (alone vs. competition) in 2.5-L pots. In the absence of competition, high-nutrient treatment induced a greater increase in total biomass of invasive alien species than that of native species, whereas the reverse was true under competition as native species benefitted more from nutrient enrichment than invasive alien species. Moreover, high-nutrient treatment caused a greater increase in total biomass of invasive alien species than that of native species in the presence of simulated herbivory. Competition induced higher production of flavonoids and tannins. Simulated herbivory induced higher flavonoid expression in invasive alien plants under low-nutrient than high-nutrient treatments. However, flavonoid concentrations of native plants did not change under nutrient enrichment and simulated herbivory treatments. Invasive alien plants produced higher concentrations of GA3 than native plants. Taken together, these results suggest that impact of nutrient enrichment on growth of invasive alien and co-occurring native plants may depend on the level of competition that they experience. Moreover, invasive alien plants might adjust their flavonoid-based defense more efficiently than native plants in response to variation in soil nutrient availability and herbivory pressure. Our findings suggest that large-scale efforts to reduce nutrient enrichment of invaded habitats may help to control future invasiveness of target alien plant species.

Robust Zero Modes in Non-Hermitian Systems without Global Symmetries

We present an approach to achieve zero modes in lattice models that do not rely on any symmetry or topology of the bulk, which are robust against disorder in the bulk of any type and strength. Such symmetry-free zero modes (SFZMs) are formed by attaching a single site or small cluster with zero mode(s) to the bulk, which serves as the "nucleus" that expands to the entire lattice. We identify the requirements on the couplings between this boundary and the bulk, which reveals that this approach is intrinsically non-Hermitian. We then provide several examples with either an arbitrary or structured bulk, forming spectrally embedded zero modes in the bulk continuum, midgap zero modes, and even restoring the "zeroness" of coupling or disorder-shifted topological corner states. Focusing on viable realizations using photonic lattices, we show that the resulting SFZM can be observed as the single lasing mode when optical gain is applied to the boundary.

Nitrogen availability constrains grassland plant diversity in response to grazing

Grassland plant diversity has been observed with divergent responses in grazing experiments around the world. However, the dominant role of nitrogen (N) availability in controlling this global variation has not been well explored, impeding our capacity to formulate effective strategies for preserving grassland plant diversity. Here, we synthesized data from 306 grazing experiments that measured plant diversity and soil N content across global grasslands. Overall, grazing reduced plant diversity by 7.63 %, with substantial variations observed across the dataset. Our study revealed that these contrasting effects were best explained by soil N change. Plant diversity under enhanced soil N showed a strong increase in response to grazing. We found that lower grazing intensity and higher background N deposition could collectively enhance soil N, thereby promoting diversity. These results suggest that while avoiding high grazing intensity is crucial in maintaining biodiversity of grazed grasslands, it alone is not sufficient. In regions with lower N deposition (< 500 mg N m-2 yr-1), additional management strategies that target improving soil fertility are needed. Our analysis propounds that local environmental conditions should be incorporated into decision-making of grassland biodiversity conservation, or ignoring this may lead to counterproductive impacts.

Multidimensional responses of grassland stability to eutrophication

Eutrophication usually impacts grassland biodiversity, community composition, and biomass production, but its impact on the stability of these community aspects is unclear. One challenge is that stability has many facets that can be tightly correlated (low dimensionality) or highly disparate (high dimensionality). Using standardized experiments in 55 grassland sites from a globally distributed experiment (NutNet), we quantify the effects of nutrient addition on five facets of stability (temporal invariability, resistance during dry and wet growing seasons, recovery after dry and wet growing seasons), measured on three community aspects (aboveground biomass, community composition, and species richness). Nutrient addition reduces the temporal invariability and resistance of species richness and community composition during dry and wet growing seasons, but does not affect those of biomass. Different stability measures are largely uncorrelated under both ambient and eutrophic conditions, indicating consistently high dimensionality. Harnessing the dimensionality of ecological stability provides insights for predicting grassland responses to global environmental change.

Soil microbes mediate the effects of resource variability on plant invasion

A fundamental question in ecology is which species will prevail over others amid changes in both environmental mean conditions and their variability. Although the widely accepted fluctuating resource hypothesis predicts that increases in mean resource availability and variability therein will promote nonnative plant invasion, it remains unclear to what extent these effects might be mediated by soil microbes. We grew eight invasive nonnative plant species as target plants in pot-mesocosms planted with five different synthetic native communities as competitors, and assigned them to eight combinations of two nutrient-fluctuation (constant vs. pulsed), two nutrient-availability (low vs. high) and two soil-microbe (living vs. sterilized) treatments. We found that when plants grew in sterilized soil, nutrient fluctuation promoted the dominance of nonnative plants under overall low nutrient availability, whereas the nutrient fluctuation had minimal effect under high nutrient availability. In contrast, when plants grew in living soil, nutrient fluctuation promoted the dominance of nonnative plants under high nutrient availability rather than under low nutrient availability. Analysis of the soil microbial community suggests that this might reflect that nutrient fluctuation strongly increased the relative abundance of the most dominant pathogenic fungal family or genus under high nutrient availability, while decreasing it under low nutrient availability. Our findings are the first to indicate that besides its direct effect, environmental variability could also indirectly affect plant invasion via changes in soil microbial communities.

Globally consistent response of plant microbiome diversity across hosts and continents to soil nutrients and herbivores

All multicellular organisms host a diverse microbiome composed of microbial pathogens, mutualists, and commensals, and changes in microbiome diversity or composition can alter host fitness and function. Nonetheless, we lack a general understanding of the drivers of microbiome diversity, in part because it is regulated by concurrent processes spanning scales from global to local. Global-scale environmental gradients can determine variation in microbiome diversity among sites, however an individual host's microbiome also may reflect its local micro-environment. We fill this knowledge gap by experimentally manipulating two potential mediators of plant microbiome diversity (soil nutrient supply and herbivore density) at 23 grassland sites spanning global-scale gradients in soil nutrients, climate, and plant biomass. Here we show that leaf-scale microbiome diversity in unmanipulated plots depended on the total microbiome diversity at each site, which was highest at sites with high soil nutrients and plant biomass. We also found that experimentally adding soil nutrients and excluding herbivores produced concordant results across sites, increasing microbiome diversity by increasing plant biomass, which created a shaded microclimate. This demonstration of consistent responses of microbiome diversity across a wide range of host species and environmental conditions suggests the possibility of a general, predictive understanding of microbiome diversity.

Invasive and Native Plants Differentially Respond to Exogenous Phosphorus Addition in Root Growth and Nutrition Regulated by Arbuscular Mycorrhizal Fungi

Plant invasion has severely damaged ecosystem stability and species diversity worldwide. The cooperation between arbuscular mycorrhizal fungi (AMF) and plant roots is often affected by changes in the external environment. Exogenous phosphorus (P) addition can alter the root absorption of soil resources, thus regulating the root growth and development of exotic and native plants. However, it remains unclear how exogenous P addition regulates the root growth and development of exotic and native plants mediated by AMF, affecting the exotic plant invasion. In this experiment, the invasive plant Eupatorium adenophorum and native plant Eupatorium lindleyanum were selected and cultured under intraspecific (Intra-) competition and interspecific (Inter-) competition conditions, involving inoculation with (M⁺) and without AMF (M^-) and three different levels of P addition including no addition (P₀), addition with 15 mg P kg^-1 soil (P₁₅), and addition with 25 mg P kg^-1 soil (P₂₅) for the two species. Root traits of the two species were analyzed to study the response of the two species' roots to AMF inoculation and P addition. The results showed that AMF significantly promoted the root biomass, length, surface area, volume, tips, branching points, and carbon (C), nitrogen (N), and P accumulation of the two species. Under M⁺ treatment, the Inter- competition decreased the root growth and nutrient accumulation of invasive E. adenophorum but increased the root growth and nutrient accumulation of native E. lindleyanum relative to the Intra- competition. Meanwhile, the exotic and native plants responded differently to P addition, exhibiting root growth and nutrient accumulation of invasive E. adenophorum increased with P addition, whereas native E. lindleyanum reduced with P addition. Further, the root growth and nutrition accumulation of native E. lindleyanum were higher than invasive E. adenophorum under Inter- competition. In conclusion, exogenous P addition promoted the invasive plant but reduced the native plant in root growth and nutrient accumulation regulated by AMF, although the native plant outcompeted the invasive plant when the two species competed. The findings provide a critical perspective that the anthropogenic P fertilizer addition might potentially contribute to the successful invasion of exotic plants.

Physiology and transcriptome analysis of the response mechanism of Solidago canadensis to the nitrogen addition environment

Solidago canadensis is an invasive plant that can adapt to variable environmental conditions. To explore the molecular mechanism of the response to nitrogen (N) addition conditions in S. canadensis, physiology and transcriptome analysis were performed with samples that cultured by natural and three N level conditions. Comparative analysis detected many differentially expressed genes (DEGs), including the function of plant growth and development, photosynthesis, antioxidant, sugar metabolism and secondary metabolism pathways. Most genes encoding proteins involved in plant growth, circadian rhythm and photosynthesis were upregulated. Furthermore, secondary metabolism-related genes were specifically expressed among the different groups; for example, most DEGs related to phenol and flavonoid synthesis were downregulated in the N-level environment. Most DEGs related to diterpenoid and monoterpenoid biosynthesis were upregulated. In addition, many physiological responses, such as antioxidant enzyme activities and chlorophyll and soluble sugar contents, were elevated by the N environment, which was consistent with the gene expression levels in each group. Collectively, our observations indicated that S. canadensis may be promoted by N deposition conditions with the alteration of plant growth, secondary metabolism and physiological accumulation.

The multiscale feedback theory of biodiversity

Plants and their environments engage in feedback loops that not only affect individuals, but also scale up to the ecosystem level. Community-level negative feedback facilitates local diversity, while the ability of plants to engineer ecosystem-wide conditions for their own benefit enhances local dominance. Here, we suggest that local and regional processes influencing diversity are inherently correlated: community-level negative feedback predominates among large species pools formed under historically common conditions; ecosystem-level positive feedback is most apparent in historically restricted habitats. Given enough time and space, evolutionary processes should lead to transitions between systems dominated by positive and negative feedbacks: species-poor systems should become richer due to diversification of dominants and adaptation of subordinates; however, new monodominants may emerge due to migration or new adaptations.

Evolutionary effects of nitrogen are not easily predicted from ecological responses

PREMISE

Anthropogenic nitrogen (N) addition alters the abiotic and biotic environment, potentially leading to changes in patterns of natural selection (i.e., trait-fitness relationships) and the opportunity for selection (i.e., variance in relative fitness). Because N addition favors species with light acquisition strategies (e.g., tall species), we predicted that N would strengthen selection favoring those same traits. We also predicted that N could alter the opportunity for selection via its effects on mean fitness and/or competitive asymmetries.

METHODS

We quantified the strength of selection and the opportunity for selection in replicated populations of the annual grass Setaria faberi (giant foxtail) growing in a long-term N addition experiment. We also correlated these population-level parameters with community-level metrics to identify the proximate causes of N-mediated evolutionary effects.

RESULTS

N addition increased aboveground productivity, light asymmetry, and reduced species diversity. Contrary to expectations, N addition did not strengthen selection for trait values associated with higher light acquisition such as greater height and specific leaf area (SLA); rather, it strengthened selection favoring lower SLA. Light asymmetry and species diversity were associated with selection for height and SLA, suggesting a role for these factors in driving N-mediated selection. The opportunity for selection was not influenced by N addition but was negatively associated with species diversity.

CONCLUSIONS

Our results indicate that anthropogenic N enrichment can affect evolutionary processes, but that evolutionary changes in plant traits within populations are unlikely to parallel the shifts in plant traits observed at the community level.

Soil disturbance and invasion magnify CO2 effects on grassland productivity, reducing diversity

Climate change, disturbance, and plant invasion threaten grassland ecosystems, but their combined and interactive effects are poorly understood. Here, we examine how the combination of disturbance and plant invasion influences the sensitivity of mixed-grass prairie to elevated carbon dioxide (eCO₂ ) and warming. We established subplots of intact prairie and disturbed/invaded prairie within a free-air CO₂ enrichment (to 600 ppmv) by infrared warming (+1.5°C day, 3°C night) experiment and followed plant and soil responses for 5 years. Elevated CO₂ initially led to moderate increases in biomass and plant diversity in both intact and disturbed/invaded prairie, but these effects shifted due to strong eCO₂ responses of the invasive forb Centaurea diffusa. In the final 3 years, biomass responses to eCO₂ in disturbed/invaded prairie were 10 times as large as those in intact prairie (+186% vs. +18%), resulting in reduced rather than increased plant diversity (-17% vs. +10%). At the same time, warming interacted with disturbance/invasion and year, reducing the rate of topsoil carbon recovery following disturbance. The strength of these interactions demonstrates the need to incorporate disturbance into predictions of climate change effects. In contrast to expectations from studies in intact ecosystems, eCO₂ may threaten plant diversity in ecosystems subject to soil disturbance and invasion.

Increases in multiple resources promote competitive ability of naturalized non-native plants

Invasion by non-native plants is frequently attributed to increased resource availability. Still, our understanding is mainly based on effects of single resources and on plants grown without competition despite the fact that plants rely on multiple resources and usually grow in competition. How multiple resources affects competition between native and non-native plants remains largely unexplored. Here, with two similar common garden experiments, one in China and one in Germany, we tested whether nutrient and light availabilities affected the competitive outcomes, in terms of biomass production, between native and naturalized non-native plants. We found that under low resource availability or with addition of only one type of resource non-natives were not more competitive than natives. However, with a joint increase of nutrients and light intensity, non-natives were more competitive than natives. Our finding indicates that addition of multiple resources could greatly reduce the niche dimensionality (i.e. number of limiting factors), favoring dominance of non-native species. It also indicates that habitats experiencing multiple global changes might be more vulnerable to plant invasion.

Comparing how light and nutrient availability affect biomass production in garden experiments in China and Germany shows that with a joint increase of nutrients and light intensity, non-native plants outcompete natives.

Responses of invasive and native plants to different forms and availability of phosphorus

PREMISE

Many studies have assessed the various responses of alien plants to changes in overall nutrient or different nitrogen (N) availabilities. However, in natural soils, nutrients are present as different elements (e.g., N and phosphorus [P]) and forms (e.g., inorganic and organic). Few studies have explored whether invasive and native species differ in their responses to varying P availability and forms.

METHODS

We grew five taxonomically related pairs of common herbaceous, invasive and native species alone or in competition under six different conditions of P availability or forms and assessed their growth performance.

RESULTS

Invasive species overall did not produce more biomass than native species did in the various P conditions. However, the biomass response to organic forms of P was, relative to the response to inorganic forms of P, stronger for the invasive species than that for the native species and agreed with invasive species mainly allocating biomass to the root system under organic P conditions.

CONCLUSIONS

While invasive species were not more promiscuous than the native species, they took great advantage of the organic P forms. Therefore, the invasion risk of alien species may increase in habitats with more organic P sources.

FREQUENCY AND DEGREE OF DOMINATION OF ALIEN AND NATIVE SPECIES IN SYNANTHROPIC PLANT COMMUNITIES OF THE SOUTH OF RUSSIA

Despite the steady interest of biologists in the problem of invasions, the role of alien dominants in the formation of the vegetation cover of recipient regions has not yet been quantified. We compared the frequency and degree of dominance of alien and native plant species in nine sites of synanthropic vegetation in the vicinity of several settlements in the Republic of Adygea and the Krasnodar Territory (the Western Caucasus: the basins of the Belaya, Tuapse, and Agoy rivers; the Kuban-Azov Lowland). Within them, 1950 to 3683 (24847 in total) accounting plots of 1 m were established, on which the projective cover of the dominant species was estimated. The results showed that in the studied sites of synanthropic communities about 10% of the dominants identified were alien species. Compared to native dominants, they are characterized, on average (per species), by a slightly higher frequency of dominance and achievement of a coverage of more than 80% in most sites. At the same time, on average, for all sites, alien plant species dominate in 12% of the accounting plots (in different sites from 2 to 28%), and they reach coverages of more than 80% only on 2.9% of plots (0.04-7.7%). The species similarity between complexes of alien dominants in different sites is, on average, higher than the similarity of complexes of aboriginal dominants. This means that the strengthening of the positions of alien species leads to an increase in the homogeneity of synanthropic vegetation in southern Russia.

Evolutionary history of grazing and resources determine herbivore exclusion effects on plant diversity

Ecological models predict that the effects of mammalian herbivore exclusion on plant diversity depend on resource availability and plant exposure to ungulate grazing over evolutionary time. Using an experiment replicated in 57 grasslands on six continents, with contrasting evolutionary history of grazing, we tested how resources (mean annual precipitation and soil nutrients) determine herbivore exclusion effects on plant diversity, richness and evenness. Here we show that at sites with a long history of ungulate grazing, herbivore exclusion reduced plant diversity by reducing both richness and evenness and the responses of richness and diversity to herbivore exclusion decreased with mean annual precipitation. At sites with a short history of grazing, the effects of herbivore exclusion were not related to precipitation but differed for native and exotic plant richness. Thus, plant species' evolutionary history of grazing continues to shape the response of the world's grasslands to changing mammalian herbivory.

ALIEN AND NATIVE DOMINANTS HAVE A SIMILAR EFFECT ON THE SPECIES RICHNESS OF SYNANTHROPIC PLANT COMMUNITIES OF THE WESTERN CAUCASUS

It remains unclear whether alien dominants, on average, have a stronger effect on the species richness of plant communities than native ones. We examined this issue on the example of 20 areas of synanthropic plant communities dominated by species of different biogeographic origin (the study area is the Western Caucasus, the Belaya River valley, 190-680 m above the sea level). Within each of them, samples of aboveground biomass were taken from 25-30 plots of 0.25 m with different coverings of dominants, which were then sorted by species and weighed. Analysis of the data has shown: 1) the average species richness of samples with a similar degree of dominance of alien and native species differs mainly insignificantly; 2) the close relationship between the degree of dominance of alien species and species richness is, on average, about the same as between the degree of dominance of native species and species richness; 3) the relationship between these characteristics in most cases can be satisfactorily explained on the basis of "energy-diversity" hypothesis; 4) the share of synanthropic plant species in communities with high participation of both alien and aboriginal dominants is not higher than in communities with low participation of these dominants. On the whole, our results indicate a similar and predominantly nonselective nature of the impact of alien and native dominants on accompanying species of communities.

Nitrogen deposition and climate: an integrated synthesis

Human activities have more than doubled reactive nitrogen (N) deposited in ecosystems, perturbing the N cycle and considerably impacting plant, animal, and microbial communities. However, biotic responses to N deposition can vary widely depending on factors including local climate and soils, limiting our ability to predict ecosystem responses. Here, we synthesize reported impacts of elevated N on grasslands and draw upon evidence from the globally distributed Nutrient Network experiment (NutNet) to provide insight into causes of variation and their relative importance across scales. This synthesis highlights that climate and elevated N frequently interact, modifying biotic responses to N. It also demonstrates the importance of edaphic context and widespread interactions with other limiting nutrients in controlling biotic responses to N deposition.

Functional traits explain the consistent resistance of biodiversity to plant invasion under nitrogen enrichment

Elton's biotic resistance hypothesis, which posits that diverse communities should be more resistant to biological invasions, has received considerable experimental support. However, it remains unclear whether such a negative diversity-invasibility relationship would persist under anthropogenic environmental change. By using the common ragweed (Ambrosia artemisiifolia) as a model invader, our 4-year grassland experiment demonstrated consistently negative relationships between resident species diversity and community invasibility, irrespective of nitrogen addition, a result further supported by a meta-analysis. Importantly, our experiment showed that plant diversity consistently resisted invasion simultaneously through increased resident biomass, increased trait dissimilarity among residents, and increased community-weighted means of resource-conservative traits that strongly resist invasion, pointing to the importance of both trait complementarity and sampling effects for invasion resistance even under resource enrichment. Our study provides unique evidence that considering species' functional traits can help further our understanding of biotic resistance to biological invasions in a changing environment.

Native herbivores indirectly facilitate the growth of invasive Spartina in a eutrophic saltmarsh

Current theory (e.g., consumer-controlled theory) predicts that nutrient enrichment typically amplifies herbivory and thereby suppresses the growth and expansion of invasive plants. Herbivores can facilitate plant regrowth in the native community by stimulating complementary growth or ameliorating habitat conditions (e.g., by increasing soil oxygen and nutrient availability), but whether they have similar positive effects on invasive plants, especially under nutrient enrichment, remains unknown. Using a field nitrogen (N)-enrichment X crab exclusion experiment, we evaluated and compared the effects of both N enrichment and crab herbivory on the growth performance of a global invasive cordgrass, Spartina alterniflora, and a co-occurring native plant, Phragmites australis. We found that crabs consistently suppressed P. australis by decreasing density and aboveground biomass regardless of N enrichment. In contrast, for S. alterniflora, the negative effects of crabs under ambient N were replaced by positive effects under N enrichment, with crabs stimulating complementary increases in density and aboveground biomass. The differing effects between the N treatments were driven by crab burrowing activity, which increased soil N availability, and the nutrient-use efficiency of S. alterniflora. Our findings reveal that native herbivores can have opposing effects on native and invasive plants, which broadens our understanding of how exotic plants can achieve dominance in a changing world. This article is protected by copyright. All rights reserved.

Vulnerability of grassland seed banks to resource-enhancing global changes

Soil seed banks represent reservoirs of diversity in the soil that may increase resilience of communities to global changes. Two global change factors that can dramatically alter the composition and diversity of aboveground communities are nutrient enrichment and increased rainfall. In a full-factorial nutrient and rainfall addition experiment in an annual Californian grassland, we asked whether shifts in aboveground composition and diversity were reflected in belowground seed banks. Nutrient and rainfall additions increased exotic and decreased native abundances, while rainfall addition increased exotic richness, both in aboveground communities and seed banks. Under nutrient addition, forbs and short-statured plants were replaced by grasses and tall-statured species, both above and below ground, and whole-community responses to the treatments were similar. Structural equation models indicated that especially nutrient addition effects on seed banks were largely indirect via aboveground communities. However, rainfall addition also had a direct negative effect on native species richness and abundance of species with high specific leaf area (SLA) in seed banks, showing that seed banks are sensitive to the direct effects of temporary increases in rainfall. Our findings highlight the vulnerability of seed banks in annual, resource-poor grasslands to shifts in compositional and trait changes in aboveground communities and show how invasion of exotics and depletion of natives are critical for these above-belowground compositional shifts. Our findings suggest that seed banks have limited potential to buffer resource-poor annual grasslands from the community changes caused by resource enrichment.

Increased ploidy of Butomus umbellatus in introduced populations is not associated with higher phenotypic plasticity to N and P

Separate introductions or post-introduction evolution may lead to multiple invader genotypes or cytotypes that differ in growth rates, biomass or chemical profile responses (phenotype) to a range of environments. If the invader has high trait plasticity to a range of resource levels, then sediment N or P enrichment may enhance invasiveness. However, the ways in which ploidy, plasticity, and available N or P interact are unknown for most species despite the potential to explain spread and impacts by invaders with multiple introduced lineages. We conducted a common garden experiment with four triploid and six diploid populations of Butomus umbellatus, collected from across its invasive range in the USA. Plants were grown under different N or P nutrient levels (4, 40, 200, 400 mg L^-1 N; 0.4, 4, 40 mg L^-1 P) and we measured reaction norms for biomass, clonal reproduction and tissue chemistry. Contrary to our expectation, triploid B. umbellatus plants were less plastic to variation in N or P than diploid B. umbellatus in most measured traits. Diploid plants produced 172 % more reproductive biomass and 57 % more total biomass across levels of N, and 158 % more reproductive biomass and 33 % more total biomass across P than triploid plants. Triploid plants had lower shoot:root ratios and produced 30 % and 150 % more root biomass than diploid plants in response to increases in N and P, respectively. Tissue chemistry differed between cytotypes but plasticity was similar; N was 8 % higher and C:N ratio was 30 % lower in triploid than diploid plants across levels of N and plant parts, and N was 22 % higher and C:N ratio 27 % lower across levels of P and plant parts. Our results highlight differences in nutrient response between cytotypes of a widespread invader, and we call for additional field studies to better understand the interaction of nutrients and ploidy during invasion.

Temporal rarity is a better predictor of local extinction risk than spatial rarity

Spatial rarity is often used to predict extinction risk, but rarity can also occur temporally. Perhaps more relevant in the context of global change is whether a species is core to a community (persistent) or transient (intermittently present), with transient species often susceptible to human activities that reduce niche space. Using 5-12 yr of data on 1,447 plant species from 49 grasslands on five continents, we show that local abundance and species persistence under ambient conditions are both effective predictors of local extinction risk following experimental exclusion of grazers or addition of nutrients; persistence was a more powerful predictor than local abundance. While perturbations increased the risk of exclusion for low persistence and abundance species, transient but abundant species were also highly likely to be excluded from a perturbed plot relative to ambient conditions. Moreover, low persistence and low abundance species that were not excluded from perturbed plots tended to have a modest increase in abundance following perturbance. Last, even core species with high abundances had large decreases in persistence and increased losses in perturbed plots, threatening the long-term stability of these grasslands. Our results demonstrate that expanding the concept of rarity to include temporal dynamics, in addition to local abundance, more effectively predicts extinction risk in response to environmental change than either rarity axis predicts alone.

Negative effects of nitrogen override positive effects of phosphorus on grassland legumes worldwide

Anthropogenic nutrient enrichment is driving global biodiversity decline and modifying ecosystem functions. Theory suggests that plant functional types that fix atmospheric nitrogen have a competitive advantage in nitrogen-poor soils, but lose this advantage with increasing nitrogen supply. By contrast, the addition of phosphorus, potassium, and other nutrients may benefit such species in low-nutrient environments by enhancing their nitrogen-fixing capacity. We present a global-scale experiment confirming these predictions for nitrogen-fixing legumes (Fabaceae) across 45 grasslands on six continents. Nitrogen addition reduced legume cover, richness, and biomass, particularly in nitrogen-poor soils, while cover of non-nitrogen-fixing plants increased. The addition of phosphorous, potassium, and other nutrients enhanced legume abundance, but did not mitigate the negative effects of nitrogen addition. Increasing nitrogen supply thus has the potential to decrease the diversity and abundance of grassland legumes worldwide regardless of the availability of other nutrients, with consequences for biodiversity, food webs, ecosystem resilience, and genetic improvement of protein-rich agricultural plant species.

Increasing effects of chronic nutrient enrichment on plant diversity loss and ecosystem productivity over time

Human activities are enriching many of Earth's ecosystems with biologically limiting mineral nutrients such as nitrogen (N) and phosphorus (P). In grasslands, this enrichment generally reduces plant diversity and increases productivity. The widely demonstrated positive effect of diversity on productivity suggests a potential negative feedback, whereby nutrient-induced declines in diversity reduce the initial gains in productivity arising from nutrient enrichment. In addition, plant productivity and diversity can be inhibited by accumulations of dead biomass, which may be altered by nutrient enrichment. Over longer time frames, nutrient addition may increase soil fertility by increasing soil organic matter and nutrient pools. We examined the effects of 5-11 yr of nutrient addition at 47 grasslands in 12 countries. Nutrient enrichment increased aboveground live biomass and reduced plant diversity at nearly all sites, and these effects became stronger over time. We did not find evidence that nutrient-induced losses of diversity reduced the positive effects of nutrients on biomass; however, nutrient effects on live biomass increased more slowly at sites where litter was also increasing, regardless of plant diversity. This work suggests that short-term experiments may underestimate the long-term nutrient enrichment effects on global grassland ecosystems.

Adaptive associations among life history, reproductive traits, environment, and origin in the Wisconsin angiosperm flora

PREMISE

We tested 25 classic and novel hypotheses regarding trait-origin, trait-trait, and trait-environment relationships to account for flora-wide variation in life history, habit, and especially reproductive traits using a plastid DNA phylogeny of most native (96.6%, or 1494/1547 species) and introduced (87.5%, or 690/789 species) angiosperms in Wisconsin, USA.

METHODS

We assembled data on life history, habit, flowering, dispersal, mating system, and occurrence across open/closed/mixed habitats across species in the state phylogeny. We used phylogenetically structured analyses to assess the strength and statistical significance of associations predicted by our models.

RESULTS

Introduced species are more likely to be annual herbs, occupy open habitats, have large, visually conspicuous, hermaphroditic flowers, and bear passively dispersed seeds. Among native species, hermaphroditism is associated with larger, more conspicuous flowers; monoecy is associated with small, inconspicuous flowers and passive seed dispersal; and dioecy is associated with small, inconspicuous flowers and fleshy fruits. Larger flowers with more conspicuous colors are more common in open habitats, and in understory species flowering under open (spring) canopies; fleshy fruits are more common in closed habitats. Wind pollination may help favor dioecy in open habitats.

CONCLUSIONS

These findings support predictions regarding how breeding systems depend on flower size, flower color, and fruit type, and how those traits depend on habitat. This study is the first to combine flora-wide phylogenies with complete trait databases and phylogenetically structured analyses to provide powerful tests of evolutionary hypotheses about reproductive traits and their variation with geographic source, each other, and environmental conditions.

Grassland ecosystem recovery after soil disturbance depends on nutrient supply rate

Human disturbances alter the functioning and biodiversity of many ecosystems. These ecosystems may return to their pre-disturbance state after disturbance ceases; however, humans have altered the environment in ways that may change the rate or direction of this recovery. For example, human activities have increased supplies of biologically limiting nutrients, such as nitrogen (N) and phosphorus (P), which can reduce grassland diversity and increase productivity. We tracked the recovery of a grassland for two decades following an intensive agricultural disturbance under ambient and elevated nutrient conditions. Productivity returned to pre-disturbance levels quickly under ambient nutrient conditions, but nutrient addition slowed this recovery. In contrast, the effects of disturbance on diversity remained hidden for 15 years, at which point diversity began to increase in unfertilised plots. This work demonstrates that enrichment of terrestrial ecosystems by humans may alter the recovery of ecosystems and that disturbance effects may remain hidden for many years.

Disease in Invasive Plant Populations

Non-native invasive plants can establish in natural areas, where they can be ecologically damaging and costly to manage. Like cultivated plants, invasive plants can experience a relatively disease-free period upon introduction and accumulate pathogens over time. Diseases of invasive plant populations are infrequently studied compared to diseases of agriculture, forestry, and even native plant populations. We evaluated similarities and differences in the processes that are likely to affect pathogen accumulation and disease in invasive plants compared to cultivated plants, which are the dominant focus of the field of plant pathology. Invasive plants experience more genetic, biotic, and abiotic variation across space and over time than cultivated plants, which is expected to stabilize the ecological and evolutionary dynamics of interactions with pathogens and possibly weaken the efficacy of infectious disease in their control. Although disease is expected to be context dependent, the widespread distribution of invasive plants makes them important pathogen reservoirs. Research on invasive plant diseases can both protect crops and help manage invasive plant populations.

Community diversity outweighs effect of warming on plant colonization

Abiotic environmental change, local species extinctions and colonization of new species often co-occur. Whether species colonization is driven by changes in abiotic conditions or reduced biotic resistance will affect community functional composition and ecosystem management. We use a grassland experiment to disentangle effects of climate warming and community diversity on plant species colonization. Community diversity had dramatic impacts on the biomass, richness and traits of plant colonists. Three times as many species colonized the monocultures than the high diversity 17 species communities (~30 vs. 10 species), and colonists collectively produced 10 times as much biomass in the monocultures than the high diversity communities (~30 vs. 3 g/m² ). Colonists with resource-acquisitive strategies (high specific leaf area, light seeds, short heights) accrued more biomass in low diversity communities, whereas species with conservative strategies accrued most biomass in high diversity communities. Communities with higher biomass of resident C4 grasses were more resistant to colonization by legume, nonlegume forb and C3 grass colonists, but not by C4 grass colonists. Compared with effects of diversity, 6 years of 3°C-above-ambient temperatures had little impact on plant colonization. Warmed subplots had ~3 fewer colonist species than ambient subplots and selected for heavier seeded colonists. They also showed diversity-dependent changes in biomass of C3 grass colonists, which decreased under low diversity and increased under high diversity. Our findings suggest that species colonization is more strongly affected by biotic resistance from residents than 3°C of climate warming. If these results were extended to invasive species management, preserving community diversity should help limit plant invasion, even under climate warming.

Anthropogenic global shifts in biospheric N and P concentrations and ratios and their impacts on biodiversity, ecosystem productivity, food security, and human health

The availability of carbon (C) from high levels of atmospheric carbon dioxide (CO2 ) and anthropogenic release of nitrogen (N) is increasing, but these increases are not paralleled by increases in levels of phosphorus (P). The current unstoppable changes in the stoichiometries of C and N relative to P have no historical precedent. We describe changes in P and N fluxes over the last five decades that have led to asymmetrical increases in P and N inputs to the biosphere. We identified widespread and rapid changes in N:P ratios in air, soil, water, and organisms and important consequences to the structure, function, and biodiversity of ecosystems. A mass-balance approach found that the combined limited availability of P and N was likely to reduce C storage by natural ecosystems during the remainder of the 21st Century, and projected crop yields of the Millennium Ecosystem Assessment indicated an increase in nutrient deficiency in developing regions if access to P fertilizer is limited. Imbalances of the N:P ratio would likely negatively affect human health, food security, and global economic and geopolitical stability, with feedbacks and synergistic effects on drivers of global environmental change, such as increasing levels of CO2 , climatic warming, and increasing pollution. We summarize potential solutions for avoiding the negative impacts of global imbalances of N:P ratios on the environment, biodiversity, climate change, food security, and human health.

Effects of nutrient supply, herbivory, and host community on fungal endophyte diversity

The microbes contained within free-living organisms can alter host growth, reproduction, and interactions with the environment. In turn, processes occurring at larger scales determine the local biotic and abiotic environment of each host that may affect the diversity and composition of the microbiome community. Here, we examine variation in the diversity and composition of the foliar fungal microbiome in the grass host, Andropogon gerardii, across four mesic prairies in the central United States. Composition of fungal endophyte communities differed among sites and among individuals within a site, but was not consistently affected by experimental manipulation of nutrient supply to hosts (A. gerardii) or herbivore reduction via fencing. In contrast, mean fungal diversity was similar among sites but was limited by total plant biomass at the plot scale. Our work demonstrates that distributed experiments motivated by ecological theory are a powerful tool to unravel the multiscale processes governing microbial community composition and diversity.

Organic amendment additions to rangelands: A meta-analysis of multiple ecosystem outcomes

Interest in land application of organic amendments-such as biosolids, composts, and manures-is growing due to their potential to increase soil carbon and help mitigate climate change, as well as to support soil health and regenerative agriculture. While organic amendments are predominantly applied to croplands, their application is increasingly proposed on relatively arid rangelands that do not typically receive fertilizers or other inputs, creating unique concerns for outcomes such as native plant diversity and water quality. To maximize environmental benefits and minimize potential harms, we must understand how soil, water, and plant communities respond to particular amendments and site conditions. We conducted a global meta-analysis of 92 studies in which organic amendments had been added to arid, semiarid, or Mediterranean rangelands. We found that organic amendments, on average, provide some environmental benefits (increased soil carbon, soil water holding capacity, aboveground net primary productivity, and plant tissue nitrogen; decreased runoff quantity), as well as some environmental harms (increased concentrations of soil lead, runoff nitrate, and runoff phosphorus; increased soil CO₂ emissions). Published data were inadequate to fully assess impacts to native plant communities. In our models, adding higher amounts of amendment benefitted four outcomes and harmed two outcomes, whereas adding amendments with higher nitrogen concentrations benefitted two outcomes and harmed four outcomes. This suggests that trade-offs among outcomes are inevitable; however, applying low-N amendments was consistent with both maximizing benefits and minimizing harms. Short study time frames (median 1-2 years), limited geographic scope, and, for some outcomes, few published studies limit longer-term inferences from these models. Nevertheless, they provide a starting point to develop site-specific amendment application strategies aimed toward realizing the potential of this practice to contribute to climate change mitigation while minimizing negative impacts on other environmental goals.

The Ecology and Evolution of Alien Plants

We review the state of the art of alien plant research with emphasis on conceptual advances and knowledge gains on general patterns and drivers, biotic interactions, and evolution. Major advances include the identification of different invasion stages and invasiveness dimensions (geographic range, habitat specificity, local abundance) and the identification of appropriate comparators while accounting for propagule pressure and year of introduction. Developments in phylogenetic and functional trait research bear great promise for better understanding of the underlying mechanisms. Global patterns are emerging with propagule pressure, disturbance, increased resource availability, and climate matching as major invasion drivers, but species characteristics also play a role. Biotic interactions with resident communities shape invasion outcomes, with major roles for species diversity, enemies, novel weapons, and mutualists. Mounting evidence has been found for rapid evolution of invasive aliens and evolutionary responses of natives, but a mechanistic understanding requires tighter integration of molecular and phenotypic approaches. We hope the open questions identified in this review will stimulate further research on the ecology and evolution of alien plants.

The Neolithic Plant Invasion Hypothesis: the role of preadaptation and disturbance in grassland invasion

A long-standing hypothesis is that many European plants invade temperate grasslands globally because they are introduced simultaneously with pastoralism and cultivation, to which they are 'preadapted' after millennia of exposure dating to the Neolithic era ('Neolithic Plant Invasion Hypothesis' (NPIH)). These 'preadaptations' are predicted to maximize their performance relative to native species lacking this adaptive history. Here, we discuss the explanatory relevance of the NPIH, clarifying the importance of evolutionary context vs other mechanisms driving invasion. The NPIH makes intuitive sense given established connections between invasion and agricultural-based perturbation. However, tests are often incomplete given the need for performance contrasts between home and away ranges, while controlling for other mechanisms. We emphasize six NPIH-based predictions, centring on trait similarity of invaders between home vs away populations, and differing perturbation responses by invading and native plants. Although no research has integrated all six predictions, we highlight studies suggesting preadaptation influences on invasion. Given that many European grasslands are creations of human activity from the past, current invasions by these flora may represent the continuation of processes dating to the Neolithic. Ironically, European Neolithic-derived grasslands are becoming rarer, reflecting changes in management and illustrating the importance of human influences on these species.