Abstract The distribution of plants with different photosynthetic pathways is strongly structured by climate, with C 3 plants favoured in cooler temperate regions and C 4 plants in hotter, high-light conditions. The relative abundance of C 3 and C 4 plants across the world has cascading impacts on local food webs, decomposition, productivity and other vital ecosystem processes. Human impacts, including climate change, changes to herbivore assemblages, and increased nutrient availability, are shifting the optimal conditions for important C 3 and C 4 -dominated ecosystems and crops. Using 3,184 plot-level observations from 112 sites across six continents, we reveal how chronic nutrient enrichment disrupts the climate-driven balance between C 3 and C 4 plants in grasslands. We found that, consistent with expectations, the global distribution of C 4 plants was strongly related to climate. However, experimental nutrient addition reduced the relative cover of C 4 species, with the strongest declines found when nitrogen and phosphorus were added together. Herbivore exclusion had no consistent effect on C 4 plants. Our results provide global experimental evidence that elevated nutrients, particularly nitrogen, alter competitive outcomes among plant functional types to suppress C 4 grasses, even in climatically optimal conditions. This has major implications for predicting vegetation responses to global change, with consequences for carbon cycling, primary productivity, herbivore dynamics, and food security.

The diversity–productivity relationship suggests that increasing plant species could increase primary productivity, with this effect being explained in part by the suppression of plant antagonists. We conducted a global synthesis of 609 studies to investigate how plant diversity affects plants and their antagonists. Here we show that increasing plant species consistently promotes plant performance and suppresses antagonist performance in agro-ecosystems, grasslands and forests, for herbaceous and woody plants, across tropical and temperate zones, and for replacement series and additive experimental design studies. Crop diversification (for example, intercropping and cover cropping) indirectly promotes crop production through the suppression of pests. This shows that diversifying planting systems can increase productivity while reducing reliance on synthetic pesticides, offering a sustainable pathway for agriculture from subsistence to large-scale agriculture. Overall, these results suggest that crop diversification has considerable potential to support sustainable agro-ecosystems that benefit productivity while reducing reliance on synthetic pesticides. A global synthesis of >600 studies finds that across agro-ecosystems, grasslands and forests in temperate and tropical zones, increasing plant diversity has a consistently positive effect on plant performance and the suppression of antagonists.

Rangelands are crucial to human well-being, but their ability to provide ecosystem services is threatened. We (1) quantified key ecosystem services provided by rangelands, (2) assessed short- and long-term impacts of fertilization (nutrient addition) and the exclusion of large grazing herbivores with fences (herbivore exclusion) on services, and (3) identified synergies and trade-offs among services. We measured indicators of ecosystem services and plant diversity at 79 sites across six continents in the global Nutrient Network. Short-term herbivore exclusion increased forage quantity and soil fertility, but longer-term herbivore exclusion decreased both along with plant richness and pollination. Nutrient addition improved forage provisioning, soil stability, climate regulation, and control of soil erosion but lowered plant diversity and impeded delivery of related services, especially after prolonged application. We found synergies between plant diversity and pollination, as well as between soil fertility, soil stability, and climate regulation. Trade-offs between forage stability and quality persisted after nutrient addition but disappeared with herbivore exclusion. Our results suggest that alternative management actions may sustain livestock production while maintaining rangeland ecosystem services.

Multitrophic interactions can strongly influence the structure and functioning of ecosystems, but how plant diversity influences the direction and predictability of multitrophic interactions across agricultural and natural ecosystems remains unclear. Using 149 field studies across five continents, we found that, on average, increasing plant diversity tended to exert differential top-down and bottom-up effects in croplands versus grasslands and forests. Organic and nonorganic croplands exhibited 846 and 148% higher invertebrate natural enemy-to-herbivore abundance ratios under increased plant diversity, consistent with top-down control patterns where predator gains cause herbivore declines, enhancing crop outcomes. In grasslands and forests, increasing plant diversity was associated with bottom-up effects where enhanced productivity increased both herbivore and predator populations, with the enemy-to-herbivore ratio increasing 4.73% for grasslands and 21.2% for forests. Our findings suggest that biodiversity effects on productivity are not solely explained by direct plant-plant interactions and the resulting biodiversity-productivity relationship. Rather, they reveal patterns consistent with the framework of top-down and bottom-up effects, the relative balance of which may vary depending on ecosystem and management type. The magnitude of the effects of diversified farming on crop pests suggests that crop diversification may be an important avenue for managing crop pests preventatively and thereby enhancing agricultural sustainability.

Pollinators are declining globally due to anthropogenic pressures, such as intensive grazing and nutrient enrichment. Yet, their combined effects on pollinators and plant-pollinator interactions remain poorly understood, particularly in the Arctic. Here, we experimentally tested how grazing exclusion and nutrient addition shape flower-visiting insect communities and interaction networks in a high-productivity montane and a low-productivity tundra grassland, in northern Fennoscandia. Grazing exclusion emerged as the primary driver across both grasslands, increasing flower-visitor family richness, diversity, and the number of plant-pollinator interactions. These effects were especially strong in the tundra grassland, where resource limitation is more pronounced. In contrast, nutrient addition had weaker and more context-dependent effects, enhancing flower-visitor richness and the number of interactions mainly in the tundra grassland and particularly when combined with grazing exclusion. At both sites, grazing exclusion and nutrient addition altered community composition, with the strongest shifts occurring under their combined effects. Despite large effects on interaction numbers, network structure responded more subtly. Weighted connectance declined with grazing exclusion, especially in the tundra, indicating that interactions became more concentrated on a subset of plant species where floral resources were abundant. In contrast, network specialization (H2') and nestedness showed limited responses to experimental treatments, suggesting relatively stable interaction organization. Overall, our results demonstrate that grazing strongly regulates pollinator communities and interaction networks in Arctic grasslands, while nutrient enrichment plays a secondary, context-dependent role. The pronounced sensitivity of tundra grasslands highlights their vulnerability to environmental change and underscores the importance of managing grazing pressure to sustain pollination processes in Arctic ecosystems.

Aim: Rapid warming across the tundra biome is driving widespread changes in vascular plant community composition. While species turnover is well-documented, the ramifications for tundra functional diversity are unknown. Here, we quantify biome-scale spatial gradients and temporal trends in the functional diversity of tundra vegetation for the first time. Location: A biome-scale synthesis of in situ vegetation surveys and resurveys from 2087 plots across 45 sites throughout the high-latitude tundra. Time Period: 1984-2022 Major Taxa Studied: 352 vascular plant species encompassing shrub, graminoid and forb functional groups Methods: We used tundra species trait data alongside long-term, plot-based sampling of species composition to estimate three functional diversity metrics: functional richness, functional evenness and functional dispersion. We used Bayesian mixed-models to test for latitudinal gradients in functional diversity, temporal trends in functional diversity and major abiotic and biotic correlates of functional diversity over space and time. Results: Mirroring biogeographic gradients in species diversity, functional richness declined at high latitude and colder sites. However, functional richness exhibited no net directional change across the three-decade study period. Plots dominated by single growth forms had reduced functional diversity when compared with plots where individual growth forms had intermediate abundance. Changes in temperature and precipitation were not linked to temporal changes in functional diversity. Where shrubs were increasing in abundance, functional richness and dispersion declined, whereas increases in forbs were accompanied by increases in both aspects of functional diversity. Main Conclusions: The functional diversity of tundra plants is currently lowest in colder and high latitude sites. Despite rapid warming of the tundra biome, we have yet to see broad-scale changes in functional diversity over time. However, where shrubification occurs, we anticipate accompanying reductions in functional diversity. Our results highlight the potential consequences of changes in tundra species composition for ecosystem functioning over the coming decades.

Abstract Understanding how habitat connectivity shapes biodiversity remains a major ecological challenge. In particular, the roles of connectivity and ecological heterogeneity on co-variation in plant species diversity and intraspecific genetic diversity is not understood. We combined species distribution modelling, resistance-to-movement mapping, landscape connectivity analysis and population genomics to investigate diversity patterns in three wet meadow herbs, Scorzonera humilis, Oenanthe peucedanifolia and Lychnis flos-cuculi , and their surrounding plant communities. Genetic diversity patterns differed strongly among co-occurring species. Connectivity metrics explained genetic diversity only in O. peucedanifolia , and environmental drivers of genetic diversity were highly species specific. Importantly, genetic diversity changed with the presence of some species in the community, but it was consistently unrelated to indicators of local plant community diversity. Overall, the processes shaping within-species biodiversity may differ fundamentally from those structuring habitat connectivity and plant species communities, with important implications for conservation.

Understanding and predicting future plant biodiversity and productivity is critical for prioritizing global change mitigation, conservation, and restoration efforts. One major challenge is that we know remarkably little of how interspecific interactions may modulate the effects of global change factors on diversity and productivity. Here, we develop and test a synthetic conceptual framework about how different ‘biotic modulators’ (herbivory, plant-plant interactions, pathogens, mycorrhiza) can either amplify or mitigate the effects of global change drivers (nutrient and CO2 enrichment, changes in rainfall and temperature) on plant community biomass and diversity. We report that herbivores mitigated both biomass increment and diversity decline caused by different global change drivers, while plant competition did not significantly alter global change impacts due to mixed effects (both amplification and mitigation). Pathogens tended to function similarly to herbivores, while mycorrhiza both amplified and mitigated community responses. Our conceptual framework further identifies mechanisms by which species interactions can modify global change effects, provides new testable hypotheses, and identifies research gaps and future research directions. We conclude that plant consumers can be important agents stabilizing plant productivity and safeguarding plant biodiversity in the Anthropocene, while more research is urgently needed to understand the role of other biotic modulators. ### Competing Interest Statement The authors have declared no competing interest.

Trophic interactions can strongly influence metacommunity dynamics and patterns of biodiversity in spatially heterogeneous environments. Theory predicts that herbivory facilitates plant species coexistence at small scales by reducing extinctions and promoting colonisations but reduces diversity at larger scales by promoting dominance of herbivore-resistant species. We examined how mammalian herbivory interacts with habitat size and connectivity to affect plant diversity in a unique, naturally fragmented grassland metacommunity system located in Southern Finland. We found that herbivory increased plant diversity across scales of measurement. In addition, herbivory reversed the diversity-area relationship such that there was a positive diversity-area relationship in grazed grasslands, but a negative relationship in ungrazed grasslands. Connectivity exhibited a unimodal relationship with diversity but did not interact with herbivory. Our empirical results demonstrate that herbivores can promote plant coexistence across scales and highlight the interplay between habitat area and trophic interactions in facilitating plant biodiversity in grassland metacommunities.

Nutrient enrichment typically causes local plant diversity declines. A common but untested expectation is that nutrient enrichment also reduces variation in nutrient conditions among localities and selects for a smaller pool of species, causing greater diversity declines at larger than local scales and thus biotic homogenization. Here we apply a framework that links changes in species richness across scales to changes in the numbers of spatially restricted and widespread species for a standardized nutrient addition experiment across 72 grasslands on six continents. Overall, we find proportionally similar species loss at local and larger scales, suggesting similar declines of spatially restricted and widespread species, and no biotic homogenization after 4 years and up to 14 years of treatment. These patterns of diversity changes are generally consistent across species groups. Thus, nutrient enrichment poses threats to plant diversity, including for widespread species that are often critical for ecosystem functions.

Forbs ("wildflowers") are important contributors to grassland biodiversity but are vulnerable to environmental changes. In a factorial experiment at 94 sites on 6 continents, we test the global generality of several broad predictions: (1) Forb cover and richness decline under nutrient enrichment, particularly nitrogen enrichment. (2) Forb cover and richness increase under herbivory by large mammals. (3) Forb richness and cover are less affected by nutrient enrichment and herbivory in more arid climates, because water limitation reduces the impacts of competition with grasses. (4) Forb families will respond differently to nutrient enrichment and mammalian herbivory due to differences in nutrient requirements. We find strong evidence for the first, partial support for the second, no support for the third, and support for the fourth prediction. Our results underscore that anthropogenic nitrogen addition is a major threat to grassland forbs, but grazing under high herbivore intensity can offset these nutrient effects.

Anthropogenic biodiversity decline threatens the functioning of ecosystems and the many benefits they provide to humanity1. As well as causing species losses in directly affected locations, human influence might also reduce biodiversity in relatively unmodified vegetation if far-reaching anthropogenic effects trigger local extinctions and hinder recolonization. Here we show that local plant diversity is globally negatively related to the level of anthropogenic activity in the surrounding region. Impoverishment of natural vegetation was evident only when we considered community completeness: the proportion of all suitable species in the region that are present at a site. To estimate community completeness, we compared the number of recorded species with the dark diversity-ecologically suitable species that are absent from a site but present in the surrounding region2. In the sampled regions with a minimal human footprint index, an average of 35% of suitable plant species were present locally, compared with less than 20% in highly affected regions. Besides having the potential to uncover overlooked threats to biodiversity, dark diversity also provides guidance for nature conservation. Species in the dark diversity remain regionally present, and their local populations might be restored through measures that improve connectivity between natural vegetation fragments and reduce threats to population persistence.

The abundance and composition of flowers within plant communities shape the resources available to flower‐visiting insects. However, it remains unclear how nutrient enrichment and changes in grazing impact plant communities and their floral resources, and whether these effects contribute to ongoing pollinator decline. We investigated how the abundance, diversity and species composition of flowers (excluding graminoids) respond to long‐term nutrient additions and grazing exclusion in two high‐latitude grasslands (tundra and montane). We used two factorial experimental designs: 1) a nutrient experiment with additions of nitrogen (N), phosphorus (P), and potassium (K with micronutrients), and 2) a grazing experiment that included combined NPK addition with and without grazing exclusion. Flower abundance was assessed in 5 × 5 m experimental plots after 6–10 years of treatments. N and P interactively affected flower abundance, with N counteracting the positive effects of P. N reduced flower abundance in the montane grassland, whereas P and K increased it. Grazing exclusion increased flower abundance, with its effect amplified under NPK addition, but this joint treatment reduced flower species richness. Grazing exclusion reduced flower species diversity, with contrasting site‐specific effects depending on NPK. N, P and K additions as well as grazing exclusion and NPK addition changed the composition of flower assemblages. At the montane grassland joint NPK addition and grazing exclusion led to strong dominance of forbs, such as Anthriscus sylvestris and Geranium sylvaticum , and grazing exclusion led to homogenisation of the flower community at the montane grassland. Our study demonstrate the importance of grazing and nutrients as key determinants of floral resources, which are vital for flower‐visiting insects. We suggest that quantifying floral resources could contribute to ecosystem status assessment. Our findings offer insights for managing and conserving pollinator habitats in the face of environmental change.

The soil seed bank is a hidden community below-ground and a crucial component of plant biodiversity. Nitrogen (N) enrichment can reduce plant species diversity both in above-ground plant communities and seed banks. However, the mechanisms by which N enrichment affects soil seed banks are not clear. We investigated the direct and indirect mechanisms of N enrichment in soil seed banks in an N addition experiment in an alpine meadow on the eastern Tibetan Plateau. We combined above-ground plant community and litter data, fungal diversity, seed bank surveys and seed rain monitoring, and conducted a seed burial experiment to explore seed viability. We found that the rate of loss of species diversity was slower in seed banks than in above-ground plant communities. N enrichment directly affected seed banks by decreasing seed viability and enhancing seed germination. Furthermore, small seeds were more prone to loss of viability and increased seed germination under N enrichment than large seeds. At the same time, N enrichment also indirectly decreased seed bank species richness and number of seeds by altering the composition of the plant community and by increasing the amount of litter and fungal diversity. N enrichment also indirectly decreased number of seeds in seed banks through decreasing number of seeds of seed rain, with increasing negative effects due to higher levels of N enrichment. Synthesis. Our results provide novel insights into multiple direct and indirect mechanisms that can lead to loss of plant diversity in seed banks under N enrichment, with important ramifications on the maintenance of plant biodiversity, ecosystem resilience and restoration in N-enriched systems.

Soil nutrients and vertebrate herbivory are key ecological factors with opposite and interactive effects on grassland plant traits and biomass. Partitioning trait changes into species turnover and intraspecific change provides a mechanistic linkage between trait shifts and biomass responses. However, their relative contributions in determining plant responses to nutrients and herbivory remain unclear. Based on a long-term experiment in two grasslands differing in productivity, we examined how nutrient addition and herbivore exclusion influenced plant functional composition and biomass, and quantified contributions of inter- and intraspecific trait change. Nutrient addition shifted leaf economics traits to be faster-growing and increased plant height, while herbivore exclusion boosted height and leaf area, both mainly through intraspecific changes. These effects were habitat-dependent: leaf economics traits dominated in the low-productivity grassland, while size-related traits prevailed in the high-productivity grassland. Nutrient addition and herbivore exclusion had weak effects on plant defense traits (tannins). Biomass responses to nutrient addition and herbivore exclusion were, to a greater extent, associated with intraspecific trait variation than species turnover. This study highlights how partitioning traits into different dimensions helps understand the distinct pathways through which nutrients and herbivores shape plant communities, how these vary across environments, and ultimately influence ecosystem functioning.

Nutrient availability and grazing are known as main drivers of grassland plant diversity, and increased nutrient availability and long‐term cessation of grazing often decrease local‐scale plant diversity. Experimental tests of mechanisms determining plant diversity focus mainly on vascular plants (VP), whereas non‐vascular plants (NVP, here bryophytes) have been ignored. It is therefore not known how the current models based on VPs predict the rates of total (NVP + VP) losses in plant diversity. Here we used plant community data, including VPs and NVPs, from nine sites in Europe and North America and belonging to the Nutrient Network experiment, to test whether neglecting NVPs leads to biased estimates of plant diversity loss rates. The plant communities were subjected to factorial addition of nitrogen (N), phosphorus (P), potassium with micronutrients (K +μ ), as well as a grazing exclusion combined with multi‐nutrient fertilization (NPK +μ ) treatment. We found that nutrient additions reduced both NVP and VP species richness, but the effects on NVP species richness were on average stronger than on VPs: NVP species richness decreased 67%, while VP species richness decreased 28%, causing their combined richness to decrease 38% in response to multi‐nutrient (NPK +μ ) fertilization. Thus, VP diversity alone underestimated total plant diversity loss by 10 percentage points. Although NVP and VP species diversities similarly declined in response to N and NPK +μ fertilizations, the evenness of NVPs increased and that of VPs remained unchanged. NP, NPK +μ fertilization and NPK +μ fertilization combined with grazing exclusion, associated with decreasing light availability at ground level, led to the strongest loss of NVP species or probability of NVP presence. However, grazing did not generally mitigate the fertilization effects. Synthesis . In nine grassland sites in Europe and North America, nutrient addition caused a larger relative decline in non‐vascular plant (NVP) than vascular plant species richness. Hence, not accounting for NVPs can lead to underestimation of losses in plant diversity in response to continued nutrient pollution of grasslands.

Ecosystems are experiencing changing global patterns of mean annual precipitation (MAP) and enrichment with multiple nutrients that potentially colimit plant biomass production. In grasslands, mean aboveground plant biomass is closely related to MAP, but how this relationship changes after enrichment with multiple nutrients remains unclear. We hypothesized the global biomass-MAP relationship becomes steeper with an increasing number of added nutrients, with increases in steepness corresponding to the form of interaction among added nutrients and with increased mediation by changes in plant community diversity. We measured aboveground plant biomass production and species diversity in 71 grasslands on six continents representing the global span of grassland MAP, diversity, management, and soils. We fertilized all sites with nitrogen, phosphorus, and potassium with micronutrients in all combinations to identify which nutrients limited biomass at each site. As hypothesized, fertilizing with one, two, or three nutrients progressively steepened the global biomass-MAP relationship. The magnitude of the increase in steepness corresponded to whether sites were not limited by nitrogen or phosphorus, were limited by either one, or were colimited by both in additive, or synergistic forms. Unexpectedly, we found only weak evidence for mediation of biomass-MAP relationships by plant community diversity because relationships of species richness, evenness, and beta diversity to MAP and to biomass were weak or opposing. Site-level properties including baseline biomass production, soils, and management explained little variation in biomass-MAP relationships. These findings reveal multiple nutrient colimitation as a defining feature of the global grassland biomass-MAP relationship.

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.

Implementing precision fertilization to maximize crop yield while minimizing economic and environmental impacts has become critical for agriculture. Variability in biomass response to fertilization within fields, among regions, and over time creates simultaneous risks of under-yielding and overfertilization. We quantify factors determining fertilization responsiveness (i.e., biomass increases with fertilization) up to 15 years in 61 unfertilized rangelands on six continents. We demonstrate widespread multi-year variability in responsiveness, with fertilization increasing average yield by 43% but failing to improve biomass 26% of the time. All sites were responsive at least once, but only four of 61 responded in all plots and years. Modelled management scenarios highlighted that fertilizer cessation is likely to generate sizable economic savings but always reduces yield because of the difficulty in predicting when and where biomass will be unresponsive. This work reveals substantial scale-dependent variability in fertilization responsiveness globally, while clarifying the prospects and pitfalls of managing more spatially and temporally precise nutrient application.