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Abstract PremiseClimate change poses challenges to grasslands, including those of the North American Great Plains Region, where shifts in species distributions and fire dynamics are expected. Our present analysis focuses on remaining grasslands within this largely developed and agricultural region. The differential responses of C₄and C₃grass species to future climate conditions, particularly in habitat suitability and flammability, are critical for understanding ecosystem changes. MethodsWe used species distribution models to predict shifts in habitat suitability for 37 grass species under future climate scenarios and assessed flammability traits in a free‐air CO₂‐enrichment study, focusing on species' physiological responses to elevated CO₂, warming, and drought. ResultsOur models predicted that C₄species will retain higher habitat suitability, while C₃species will decline. Leaf‐level flammability analysis showed that species with higher water‐use efficiency under elevated CO will have lower flammability than under non‐elevated, potentially decreasing the predicted rate of fire spread when such species dominate. In contrast, species with higher growth rates but lower water‐use efficiency may be more flammable. Species‐specific responses varied within functional types. Anticipated shifts in species distributions suggest C₄species will become more dominant, potentially altering competitive dynamics and reducing C₃diversity. Changes in flammability under future conditions are expected to influence fire regimes, with a predicted decrease in mean community rate of spread due to the dominance of less‐flammable C₄species. ConclusionsThese findings highlight the need for adaptive fire management and conservation strategies to maintain biodiversity and ecosystem function in North American grasslands under climate change. 

Abstract Grasslands cover approximately a third of the Earth’s land surface and account for about a third of terrestrial carbon storage. Yet, we lack strong predictive models of grassland plant biomass, the primary source of carbon in grasslands. This lack of predictive ability may arise from the assumption of linear relationships between plant biomass and the environment and an underestimation of interactions of environmental variables. Using data from 116 grasslands on six continents, we show unimodal relationships between plant biomass and ecosystem characteristics, such as mean annual precipitation and soil nitrogen. Further, we found that soil nitrogen and plant diversity interacted in their relationships with plant biomass, such that plant diversity and biomass were positively related at low levels of nitrogen and negatively at elevated levels of nitrogen. Our results show that it is critical to account for the interactive and unimodal relationships between plant biomass and several environmental variables to accurately include plant biomass in global vegetation and carbon models. 

Abstract Global environmental change is causing a decline in biodiversity with profound implications for ecosystem functioning and stability. It remains unclear how global change factors interact to influence the effects of biodiversity on ecosystem functioning and stability. Here, using data from a 24-year experiment, we investigate the impacts of nitrogen (N) addition, enriched CO₂(eCO₂), and their interactions on the biodiversity-ecosystem functioning relationship (complementarity effects and selection effects), the biodiversity-ecosystem stability relationship (species asynchrony and species stability), and their connections. We show that biodiversity remains positively related to both ecosystem productivity (functioning) and its stability under N addition and eCO₂. However, the combination of N addition and eCO₂diminishes the effects of biodiversity on complementarity and selection effects. In contrast, N addition and eCO₂do not alter the relationship between biodiversity and either species asynchrony or species stability. Under ambient conditions, both complementarity and selection effects are negatively related to species asynchrony, but neither are related to species stability; these links persist under N addition and eCO₂. Our study offers insights into the underlying processes that sustain functioning and stability of biodiverse ecosystems in the face of global change. 

Abstract Climate change poses a growing threat to many ecosystems, including grasslands, which are a current priority for conservation due to their vulnerability to interacting threats from human activity.North American grasslands are expected to experience warmer temperatures and more frequent and severe droughts in the coming decades, with potential consequences for native biodiversity.We conducted an experiment at Cedar Creek Ecosystem Science Reserve, Minnesota, USA, to investigate how warming and drought treatments affected grassland plant community structure over 6 years in plots planted with species mixtures.Warming consistently reduced plant species richness with its effects on Shannon diversity (which additionally considers species' relative abundances) and dominance varying across years. These warming‐by‐year interactions were likely driven by temporal variability in environmental conditions and species‐specific responses. Notably, legumes consistently showed positive responses to warming.Drought alone had minimal direct effects on species richness and diversity but reduced variability in diversity responses over time, suggesting greater stability of diversity under drought conditions.Synthesis. This study underscores the important role of warming in reducing species richness, altering diversity and reshaping functional group composition in grassland ecosystems. While temporal variability influenced the magnitude of warming effects on diversity, legumes' positive responses highlight the importance of functional group dynamics in potentially buffering against species loss. Long‐term experiments that allow consideration of interannual variability are essential for improving predictions of ecosystem responses and informing adaptive management strategies aimed at sustaining biodiversity and ecosystem functioning in grasslands. 

ABSTRACT AimsThe community composition of native and alien plant species is influenced by the environment (e.g., nutrient addition and changes in temperature or precipitation). A key objective of our study is to understand how differences in the traits of alien and native species vary across diverse environmental conditions. For example, the study examines how changes in nutrient availability affect community composition and functional traits, such as specific leaf area and plant height. Additionally, it seeks to assess the vulnerability of high‐nutrient environments, such as grasslands, to alien species colonization and the potential for alien species to surpass natives in abundance. Finally, the study explores how climatic factors, including temperature and precipitation, modulate the relationship between traits and environmental conditions, shaping species success. LocationIn our study, we used data from a globally distributed experiment manipulating nutrient supplies in grasslands worldwide (NutNet). MethodsWe investigate how temporal shifts in the abundance of native and alien species are influenced by species‐specific functional traits, including specific leaf area (SLA) and leaf nutrient concentrations, as well as by environmental conditions such as climate and nutrient treatments, across 17 study sites. Mixed‐effects models were used to assess these relationships. ResultsAlien and native species increasing in their abundance did not differ in their leaf traits. We found significantly lower specific leaf area (SLA) with an increase in mean annual temperature and lower leaf Potassium with mean annual precipitation. For trait–environment relationships, when compared to native species, successful aliens exhibited an increase in leaf Phosphorus and a decrease in leaf Potassium with an increase in mean annual precipitation. Finally, aliens' SLA decreased in plots with higher mean annual temperatures. ConclusionsTherefore, studying the relationship between environment and functional traits may portray grasslands' dynamics better than focusing exclusively on traits of successful species, per se. 

ABSTRACT Nutrient enrichment has decreased the diversity and abundance of wildflower species, raising questions about whether nutrient enrichment can further decrease the diversity and abundance of pollinators that rely on wildflowers. Whether the effects of nutrient enrichment on plant–pollinator interactions differ by nutrient type remains an open question. Moreover, plant family and flower color, two core axes of pollination niches, may further mediate how wildflowers and their pollinators respond to nutrient enrichment. We tested these questions using a nutrient addition experiment replicated at three grasslands in California, a global plant diversity hotspot. We found that adding nitrogen increased the floral abundance of Asteraceae, while decreasing that of Fabaceae, Geraniaceae, Iridaceae, and Euphorbiaceae. Adding phosphorus and potassium in the absence of nitrogen produced the opposite effects. Pollinator abundance and composition varied strongly by floral family, suggesting that these differing responses to nutrient addition by floral family may alter pollinator community composition. Nitrogen addition decreased the abundance of native blue, native green, and exotic pink flowers, while increasing the abundance of native and exotic yellow and exotic purple flowers. Consequently, nitrogen addition increased pollinator abundance on purple flowers, while decreasing pollinator abundance on pink flowers. Purple and yellow Asteraceae species, which increased under nitrogen enrichment, acted as core hubs in structuring the plant–pollinator network.Synthesis:Our findings suggest that the type of nutrient, plant family, and flower color modulate how plant–pollinator interactions respond to eutrophication. 

Abstract Darwin's theory of natural selection provides two seemingly contradictory hypotheses for explaining the success of biological invasions: (1) the pre‐adaptation hypothesis posits that introduced species that are closely related to native species will be more likely to succeed due to shared advantageous characteristics; (2) the limiting similarity hypothesis posits that invaders that are more similar to resident species will be less likely to succeed due to competitive exclusion. Previous studies assessing this conundrum show mixed results, possibly stemming from inconsistent study spatial scales and failure to integrate both functional and phylogenetic information. Here, we address these limitations using a 33‐year grassland successional survey at Cedar Creek Ecosystem Science Reserve (USA). We incorporate functional dissimilarities, phylogenetic distances, environmental covariates, and species origin data for 303 vascular plant taxa (256 native, 47 introduced), collected from 2700 plots. In contrast with other studies, we test both hypotheses at two fine spatial scales—neighborhood (0.5 m²) and site (~40 m²)—to better capture competition and environmental filtering, respectively. Findings related to Darwin's naturalization conundrum depended on spatial scale and dissimilarity metric. Our results agreed with the pre‐adaptation hypothesis at site scale (40 m²)—a much finer resolution than typically used to test the pre‐adaptation hypothesis—highlighting the role of environmental filtering. At the neighborhood scale (0.5 m²), support for the limiting similarity hypothesis emerged when using functional dissimilarity, while phylogenetic distance aligned with the pre‐adaptation hypothesis, demonstrating that different dissimilarity metrics can yield contrasting conclusions. In addition, native and introduced species showed different abundance patterns in relation to functional ranked dissimilarities, with introduced species reaching higher cover when they were taller than co‐occurring species, had higher leaf dry matter content (LDMC) and lower seed mass. Introduced species also reached high cover with higher soil N concentrations and a shorter time after colonization, relative to native species. Our results suggest that inconsistent findings related to Darwin's naturalization conundrum may arise from an overreliance on single dissimilarity metrics and the use of spatial scales failing to capture underlying ecological processes. This highlights the need for more nuanced methodologies when testing the pre‐adaptation and limiting similarity hypotheses. 

ABSTRACT Biodiversity promotes ecosystem productivity and stability, positive impacts that often strengthen over time. But ongoing global changes such as rising atmospheric carbon dioxide (CO₂) levels and anthropogenic nitrogen (N) deposition may modulate the impact of biodiversity on ecosystem productivity and stability over time. Using a quarter‐century grassland biodiversity‐global change experiment we show that diversity increasingly enhanced productivity over time irrespective of global change treatments. In contrast, the positive influence of diversity on ecosystem stability strengthened over time under ambient conditions but weakened to varying degrees under global change treatments, largely driven by a greater reduction in species asynchrony under global changes. Thus, over 25 years, CO₂and N enrichment gradually eroded some of the positive effects of biodiversity on ecosystem stability. As elevated CO₂, N eutrophication, and biodiversity loss increasingly co‐occur in grasslands globally, our results raise concerns about their potential joint detrimental effects on long‐term grassland stability. 

Abstract Global changes such as nitrogen (N) enrichment and elevated carbon dioxide (CO₂) are known to exacerbate biodiversity loss in grassland ecosystems. They do so by modifying processes whose strength may vary at different spatial scales. Yet, whether and how global changes impact plant diversity at different spatial scales remains elusive.We collected data on species presence and cover at a high resolution in the third decade of a long‐term temperate grassland biodiversity—global change experiment. Based on the data, we constructed species—area relationships across three spatial orders of magnitude (from 0.01 to 3.24 m²) and compared them for the different global change treatments.We found that N enrichment, both under ambient and elevated CO₂levels, decreased species richness across almost all spatial scales, with proportional decreases being largest at the smallest spatial scales. Elevated CO₂also reduced richness at both ambient and enriched N supply rates but did so proportionally across all spatial scales. Suppression of diversity was stronger at all scales for diversity indices that include relative abundances than for species richness. Taken together, these results suggest that CO₂and N are re‐organizing this grassland system by increasingly favouring, at fine scales, a small subset of dominant species.Synthesis: Our results highlight the role of spatial scales in influencing biodiversity loss, especially when it is driven by anthropogenic resource changes that might influence species interactions differently across spatial scales. 

Abstract AimGlobal change factors, such as warming, heatwaves, droughts and land‐use changes, are intensifying fire regimes (defined here as increasing frequency or severity of fires) in many ecosystems worldwide. A large body of local‐scale research has shown that such intensified fire regimes can greatly impact on ecosystem structure and function through altering plant communities. Here, we aim to find general patterns of plant responses to intensified fire regimes across climates, habitats and fire regimes at the global scale. LocationWorldwide. Time periodStudies published 1962–2023. Major taxa studiedWoody plants, herbs and bryophytes. MethodsWe carried out a global systematic review and meta‐analysis of the response of plant abundance, diversity and fitness to increased fire frequency or severity. To assess the context dependency of those responses, we tested the effect of the following variables: fire regime component (fire frequency or severity), time since the last fire, fire type (wildfire or prescribed fire), historical fire regime type (surface or crown fire), plant life form (woody plant, herb or bryophyte), habitat type and climate. ResultsIntensified fire regimes reduced overall plant abundance (Hedges'd = −0.24), diversity (d = −0.27), and fitness (d = −0.69). Generally, adverse effects of intensified fire regimes on plants were stronger due to increased severity than frequency, in wildfires compared to prescribed fires, and at shorter times since fire. Adverse effects were also stronger for woody plants than for herbs, and in conifer and mixed forests than in open ecosystems (e.g. grasslands and shrublands). Main conclusionsIntensified fire regimes can substantially alter plant communities in many ecosystems worldwide. Plant responses are influenced by the specific fire regime component that is changing and by the biotic and abiotic conditions.