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.

Global change is associated with variable shifts in the annual production of aboveground plant biomass, suggesting localized sensitivities with unclear causal origins. Combining remotely sensed normalized difference vegetation index data since the 1980s with contemporary field data from 84 grasslands on 6 continents, we show a widening divergence in site-level biomass ranging from +51% to -34% globally. Biomass generally increased in warmer, wetter and species-rich sites with longer growing seasons and declined in species-poor arid areas. Phenological changes were widespread, revealing substantive transitions in grassland seasonal cycling. Grazing, nitrogen deposition and plant invasion were prevalent in some regions but did not predict overall trends. Grasslands are undergoing sizable changes in production, with implications for food security, biodiversity and carbon storage especially in arid regions where declines are accelerating. Remotely sensed NDVI data and contemporary field data from 84 grasslands on 6 continents show increasing divergence in aboveground plant biomass between sites in different bioclimatic regions.

Abiotic resource limitation presents organisms with a dilemma about how to use resources when they become available. Characterizing how resource allocation affects investment in growth or defensive traits that affect organismal survival strategies allows us to understand the environmental contexts in which species interact. Our goal was to measure how macronutrient availability drives nitrogen and phosphorus allocation towards functional growth and defensive chemical traits of the Neotropical passion vine, Passiflora biflora. We investigated this question with a paired field study in La Selva, Costa Rica and a full factorial greenhouse experiment to determine whether the concentration of a key secondary chemical defence is driven by nitrogen availability. We correlated defensive chemical concentration with soil nitrogen availability in naturally occurring plants, measured the effects of nitrogen and phosphorus availability on growth and secondary chemical defence traits in the greenhouse and characterized the P. biflora leaf metabolome to assess how nutrient availability affected shifts in metabolism related to plant fitness. We found that nutrient allocation increased the magnitude of both growth and defence traits. Increased nitrogen availability resulted in higher concentrations of toxic leaf secondary chemicals, longer vines, greater biomass and more leaves with a superior ability to capture sunlight. In addition, plants from high nitrogen environments had metabolomes with significantly greater secondary metabolite richness and biochemical pathway diversity, as well as increases in the number of metabolites from several chemical classes related to basic cell function and defence. Nutrient availability had no significant effect on the richness and diversity of primary metabolites involved in basic cell functions. A direct comparison of the relative strength of quantitative growth and secondary defence traits indicated that P. biflora favours nutrient allocation to growth at low nitrogen levels but invests in both strategies more evenly as nitrogen availability increases. These findings lead us to predict that passion vines are better prepared to tolerate and resist herbivory when nutrients are plentiful and experience a trade-off between growth and chemical defence against natural enemies when they are scarce. These findings are consistent with both the 'escape' and 'defend' syndromes that are often used to describe tropical plant survival strategies. This is one of the only studies to measure nutrient allocation in vines, a group comprising a significant percentage of global plant diversity. Moreover, this work demonstrated the power of leveraging untargeted metabolomics to characterize how nutrient addition affects plant growth and defence, highlighting its potential for understanding functional trait variation.Read the free Plain Language Summary for this article on the Journal blog. La limitacion de recursos abioticos presenta a los organismos con un dilema sobre como utilizar los recursos cuando estan disponibles. Caracterizar como la asignacion de recursos afecta la inversion en rasgos de crecimiento o defensa que influyen en las estrategias de supervivencia de los organismos nos permite entender los contextos ambientales en los que interactuan las especies. Nuestro objetivo fue medir como la disponibilidad de macronutrientes afecta la asignacion de nitrogeno y fosforo hacia caracteristicas de crecimiento funcional y quimicos defensivos de la enredadera neotropical Passiflora biflora. Investigamos esta pregunta con un estudio de campo en La Reserva Biologica La Selva, Costa Rica, y un experimento de invernadero con factorial completo para determinar si la concentracion de una defensa quimica secundaria esta afectada por la disponibilidad de nitrogeno. Correlacionamos la concentracion de productos quimicos defensivos con la disponibilidad de nitrogeno en el suelo en plantas que ocurren naturalmente, medimos los efectos de la disponibilidad de nitrogeno y fosforo en el crecimiento y las caracteristicas de defensa quimica secundaria en el invernadero, y caracterizamos el metaboloma (perfil quimico total) de las hojas de P. biflora para observar como la disponibilidad de nutrientes afecto los cambios en el metabolismo relacionados con la defensa de la planta. Encontramos que la asignacion de nutrientes aumento la magnitud tanto de los caracteristicas de crecimiento como de defensa. Una mayor disponibilidad de nitrogeno resulto en concentraciones mas altas de productos quimicos secundarios toxicos en las hojas, en endredederas mas largas, mayor masa y hojas con una capacidad superior para capturar la luz solar. Ademas, las plantas crecidas con alto contenido de nitrogeno tenian metabolomas con una riqueza de metabolitos secundarios significativamente mayor y una mayor diversidad de vias bioquimicas, asi como un aumento en el numero de metabolitos de varias clases quimicas relacionadas con la funcion celular basica y la defensa. La disponibilidad de nutrientes no tuvo un efecto significativo en la riqueza y diversidad de metabolitos primarios involucrados en funciones celulares basicas. Una comparacion directa de la fuerza relativa de los caracteristicas de crecimiento y defensa secundaria indico que P. biflora prefiere la asignacion de nutrientes al crecimiento en niveles bajos de nitrogeno, pero invierte en ambas estrategias de manera mas igual a medida que aumenta la disponibilidad de nitrogeno. Estos resultados nos llevan a predecir que las Passiflora estan mejor preparadas para tolerar y resistir el herbivorismo cuando los nutrientes son abundantes, y tienen un intercambio entre el crecimiento y la defensa quimica contra los enemigos naturales cuando los nutrients son pobres. Estos resultados son consistentes tanto con los sindromes de 'escape' y 'defence' que muchos utilizan para describir las estrategias de supervivencia de las plantas tropicales. Este es uno de los pocos estudios que miden la asignacion de nutrientes en enredaderas, un grupo que tiene un porcentaje grande de la diversidad vegetal global. Ademas, este trabajo demostro el poder de aprovechar la metabolomica para caracterizar como la adicion de nutrientes afecta el crecimiento y la defensa de las plantas, mostrando su potencial para entender la variacion en los caracteristicas funcionales. Read the free Plain Language Summary for this article on the Journal blog.image

L'écologie des écosystèmes, la sous-discipline de l'écologie qui adopte une approche écosystémique pour étudier la biosphère, cherche à comprendre comment l'énergie et les ressources circulent dans un système intégré, des composants abiotiques aux composants biotiques et inversement. Les écosystèmes sont façonnés non seulement par des facteurs abiotiques mais aussi par les espèces présentes dans et autour d'eux.

Several recent regional studies have cast doubt on the widespread assumption that nitrogen-fixing plants (N-fixers) act as facilitators of neighbouring plant communities. We conducted a meta-analysis to synthesize the effects of N-fixers on plant communities and to understand how ecological context moderates these effects. We analysed studies that assessed paired effects of N-fixers and non-fixers on soil N, neighbouring-plant (non-fixer) biomass and plant community diversity; ecological moderators included climate, soil texture and N-fixer growth form and invasive status. N-fixers led to higher soil N and neighbouring-plant biomass, but lower community diversity compared to non-fixers. The effect of N-fixers on neighbouring-plant biomass was strongly mediated by soil texture; N-fixer invasive status and growth form were also significant mediators of the facilitative effects of N-fixers. Synthesis: N-fixer effects lie on a continuum between facilitation and suppression that is moderated by intrinsic and extrinsic processes, and this analysis provides insight into how these factors moderate the effects of N-fixers. Overall, N-fixers facilitate neighbour biomass but suppress diversity, though high variation in these effects can be explained in part by ecological context. N-fixer effects lie on a continuum between facilitation and suppression that is moderated by intrinsic and extrinsic processes, and this analysis provides insight into how these factors moderate the effects of N-fixers. Overall, N-fixers facilitate neighbour biomass but suppress diversity, though high variation in these effects can be explained in part by ecological context.image

AbstractPlants adjust their allocation to different organs based on nutrient supply. In some plant species, symbioses with nitrogen‐fixing bacteria that live in root nodules provide an alternate pathway for nitrogen acquisition. Does access to nitrogen‐fixing bacteria modify plants' biomass allocation? We hypothesized that access to nitrogen‐fixing bacteria would have the same effect on allocation to aboveground versus belowground tissues as access to plentiful soil nitrogen. To test this hypothesis and related hypotheses about allocation to stems versus leaves and roots versus nodules, we conducted experiments with 15 species of nitrogen‐fixing plants in two separate greenhouses. In each, we grew seedlings with and without access to symbiotic bacteria across a wide gradient of soil nitrogen supply. As is common, uninoculated plants allocated relatively less biomass belowground when they had more soil nitrogen. As we hypothesized, nitrogen fixation had a similar effect as the highest level of fertilization on allocation aboveground versus belowground. Both nitrogen fixation and high fertilization led to ~10% less biomass allocated belowground (~10% more aboveground) than the uninoculated, lowest fertilization treatment. Fertilization reduced allocation to nodules relative to roots. The responses for allocation of aboveground tissues to leaves versus stems were not as consistent across greenhouses or species as the other allocation trends, though more nitrogen fixation consistently led to relatively more allocation to leaves when soil nitrogen supply was low. Synthesis: Our results suggest that symbiotic nitrogen fixation causes seedlings to allocate relatively less biomass belowground, with potential implications for competition and carbon storage in early forest development.

Climate change is increasing the frequency and severity of short-term (~1 y) drought events-the most common duration of drought-globally. Yet the impact of this intensification of drought on ecosystem functioning remains poorly resolved. This is due in part to the widely disparate approaches ecologists have employed to study drought, variation in the severity and duration of drought studied, and differences among ecosystems in vegetation, edaphic and climatic attributes that can mediate drought impacts. To overcome these problems and better identify the factors that modulate drought responses, we used a coordinated distributed experiment to quantify the impact of short-term drought on grassland and shrubland ecosystems. With a standardized approach, we imposed ~a single year of drought at 100 sites on six continents. Here we show that loss of a foundational ecosystem function-aboveground net primary production (ANPP)-was 60% greater at sites that experienced statistically extreme drought (1-in-100-y event) vs. those sites where drought was nominal (historically more common) in magnitude (35% vs. 21%, respectively). This reduction in a key carbon cycle process with a single year of extreme drought greatly exceeds previously reported losses for grasslands and shrublands. Our global experiment also revealed high variability in drought response but that relative reductions in ANPP were greater in drier ecosystems and those with fewer plant species. Overall, our results demonstrate with unprecedented rigor that the global impacts of projected increases in drought severity have been significantly underestimated and that drier and less diverse sites are likely to be most vulnerable to extreme drought.

Allometric equations are often used to estimate plant biomass allocation to different tissue types from easier-to-measure quantities. Biomass allocation, and thus allometric equations, often differs by species and sometimes varies with nutrient availability. We measured biomass components for five nitrogen-fixing tree species ( Robinia pseudoacacia , Gliricidia sepium , Casuarina equisetifolia , Acacia koa , Morella faya ) and three non-fixing tree species ( Betula nigra , Psidium cattleianum , Dodonaea viscosa ) grown in field sites in New York and Hawaii for 4–5 years and subjected to four fertilization treatments. We measured total aboveground, foliar, main stem, secondary stem, and twig biomass in all species, and belowground biomass in Robinia pseudoacacia and Betula nigra , along with basal diameter, height, and canopy dimensions. The individuals spanned a wide size range (<1–16 cm basal diameter; 0.24–8.8 m height). For each biomass component, aboveground biomass, belowground biomass, and total biomass, we determined the following four allometric equations: the most parsimonious (lowest AIC) overall, the most parsimonious without a fertilization effect, the most parsimonious without canopy dimensions, and an equation with basal diameter only. For some species, the most parsimonious overall equation included fertilization effects, but fertilization effects were inconsistent across fertilization treatments. We therefore concluded that fertilization does not clearly affect allometric relationships in these species, size classes, and growth conditions. Our best-fit allometric equations without fertilization effects had the following R 2 values: 0.91–0.99 for aboveground biomass (the range is across species), 0.95 for belowground biomass, 0.80–0.96 for foliar biomass, 0.94–0.99 for main stem biomass, 0.77–0.98 for secondary stem biomass, and 0.88–0.99 for twig biomass. Our equations can be used to estimate overall biomass and biomass of tissue components for these size classes in these species, and our results indicate that soil fertility does not need to be considered when using allometric relationships for these size classes in these species.

Light and soil nitrogen availability can be strong controls of plant nitrogen (N) fixation, but data on how understory N-fixing plants respond to these drivers are limited despite their important role in ecosystem N cycling. Furthermore, ecosystem N cycling can be altered by the introduction of species with nutrient use patterns that differ from natives. We assessed how N fixation of two exotic, understory species responded to varying light and soil N environments. We sampled leaf tissue from Mimosa pudica L., Desmodium triflorum (L.) DC., and a non-fixing reference plant (Axonopus) growing in control and two N fertilization treatments under either N-fixing or non-N-fixing trees, which may alter local soil nutrient cycling, across a range of light conditions. We measured N fixation with 15N isotope dilution, and ensured that N-fixing neighbor trees were in fact fixing N. All understory plants were wild-growing species not native to the study location. Desmodium and Mimosa acquired 82.6% and 71.6% of their nitrogen from fixation (%Ndfa) in the control, compared to 66.8% and 58.1% in the +10 g N m−2 y−1 treatment and 73.1% and 64.7% in the +15 g N m−2 y−1 treatment. These subtle %Ndfa differences across fertilization treatments were more apparent at low light availability and disappeared at high light availability. The amount of N fixed by neighboring trees did not influence %Ndfa in the understory species. Our study shows some differences in N fixation across different nutrient environments at low light for two N-fixing species, though the changes were small, and both species derived most of their N from fixation. These findings imply that introduced N-fixing species could exacerbate ecosystem N enrichment, particularly under high soil N conditions

Nitrogen (N)-fixing trees are thought to break a basic rule of leaf economics: higher leaf N concentrations do not translate into higher rates of carbon assimilation. Understanding how leaf N affects photosynthesis and water use efficiency (WUE) in this ecologically important group is critical.We grew six N-fixing and four non-fixing tree species for 4-5 years at four fertilization treatments in field experiments in temperate and tropical regions to assess how functional type (N fixer vs. non-fixer) and N limitation affected leaf N and how leaf N affected light-saturated photosynthesis (Asat), stomatal conductance (gsw) and WUE (WUEi and & delta;13C).Asat, WUEi and & delta;13C, but not gsw, increased with higher leaf N. Surprisingly, N-fixing and non-fixing trees displayed similar scaling between leaf N and these physiological variables, and this finding was supported by reanalysis of a global dataset. N fixers generally had higher leaf N than non-fixers, even when non-fixers were not N-limited at the leaf level. Leaf-level N limitation did not alter the relationship of Asat, gsw, WUEi and & delta;13C with leaf N, although it did affect the photosynthetic N use efficiency. Higher WUE was associated with higher productivity, whereas higher Asat was not.Synthesis: The ecological success of N-fixing trees depends on the effect of leaf N on carbon gain and water loss. Using a field fertilization experiment and reanalysis of a global dataset, we show that high leaf-level photosynthesis and WUE in N fixers stems from their higher average leaf N, rather than a difference between N fixers and non-fixers in the scaling of photosynthesis and WUE with leaf N. By clarifying the mechanism by which N fixers achieve and benefit from high WUE, our results further the understanding of global N fixer distributions. The ecological success of N-fixing trees depends on the effect of leaf N on carbon gain and water loss. Using a field fertilization experiment and reanalysis of a global dataset, we show that high leaf-level photosynthesis and water use efficiency (WUE) in N fixers stems from their higher average leaf N, rather than a difference between N fixers and non-fixers in the scaling of photosynthesis and WUE with leaf N. By clarifying the mechanism by which N fixers achieve and benefit from high WUE, our results further the understanding of global N fixer distributions.image

Symbiotic nitrogen (N)-fixing plants can enrich ecosystems with N, which can alter the cycling and demand for other nutrients. Researchers have hypothesized that fixed N could be used by plants and soil microbes to produce extracellular phosphatase enzymes, which release P from organic matter. Consistent with this speculation, the presence of N-fixing plants is often associated with high phosphatase activity, either in the soil or on root surfaces, although other studies have not found this association, and the connection between phosphatase and rates of N fixation-the mechanistic part of the argument-is tenuous. Here, we measured soil phosphatase activity under N-fixing trees and non-fixing trees transplanted and grown in tropical and temperate sites in the USA: two sites in Hawaii, and one each in New York and Oregon. This provides a rare example of phosphatase activity measured in a multi-site field experiment with rigorously quantified rates of N fixation. We found no difference in soil phosphatase activity under N-fixing vs. non-fixing trees nor across rates of N fixation, though we note that no sites were P limited and only one was N limited. Our results add to the literature showing no connection between N fixation rates and phosphatase activity.

Whether wild herbivores confer biotic resistance to invasion by exotic plants remains a key question in ecology. There is evidence that wild herbivores can impede invasion by exotic plants, but it is unclear whether and how this generalises across ecosystems with varying wild herbivore diversity and functional groups of plants, particularly over long‐term (decadal) time frames. Using data from three long‐term (13‐ to 26‐year) exclosure experiments in central Kenya, we tested the effects of wild herbivores on the density of exotic invasive cacti, Opuntia stricta and O. ficus‐indica (collectively, Opuntia), which are among the worst invasive species globally. We also examined relationships between wild herbivore richness and elephant occurrence probability with the probability of O. stricta presence at the landscape level (6150 km2). Opuntia densities were 74% to 99% lower in almost all plots accessible to wild herbivores compared to exclosure plots. Opuntia densities also increased more rapidly across time in plots excluding wild herbivores. These effects were largely driven by megaherbivores (≥1000 kg), particularly elephants. At the landscape level, modelled Opuntia stricta occurrence probability was negatively correlated with estimated species richness of wild herbivores and elephant occurrence probability. On average, O. stricta occurrence probability fell from ~0.56 to ~0.45 as wild herbivore richness increased from 6 to 10 species and fell from ~0.57 to ~0.40 as elephant occurrence probability increased from ~0.41 to ~0.84. These multi‐scale results suggest that any facilitative effects of Opuntia by wild herbivores (e.g. seed/vegetative dispersal) are overridden by suppression (e.g. consumption, uprooting, trampling). Synthesis. Our experimental and observational findings that wild herbivores confer resistance to invasion by exotic cacti add to evidence that conserving and restoring native herbivore assemblages (particularly megaherbivores) can increase community resistance to plant invasions.

Symbiotic nitrogen fixation (SNF) is a key ecological process whose impact depends on the strategy of SNF regulation-the degree to which rates of SNF change in response to limitation by N versus other resources. SNF that is obligate or exhibits incomplete downregulation can result in excess N fixation, whereas a facultative SNF strategy does not. We hypothesized that tree-based SNF strategies differed by latitude (tropical vs. temperate) and symbiotic type (actinorhizal vs. rhizobial). Specifically, we expected tropical rhizobial symbioses to display strongly facultative SNF as an explanation of their success in low-latitude forests. In this study we used N-15 isotope dilution field experiments in New York, Oregon, and Hawaii to determine SNF strategies in six N-fixing tree symbioses. Nitrogen fertilization with +10 and +15 g N m(-2) year(-1) for 4-5 years alleviated N limitation in all taxa, paving the way to determine SNF strategies. Contrary to our hypothesis, all six of the symbioses we studied sustained SNF even at high N. Robinia pseudoacacia (temperate rhizobial) fixed 91% of its N (%N-dfa) in controls, compared to 64% and 59% in the +10 and +15 g N m(-2) year(-1) treatments. For Alnus rubra (temperate actinorhizal), %N-dfa was 95%, 70%, and 60%. For the tropical species, %N-dfa was 86%, 80%, and 82% for Gliricidia sepium (rhizobial); 79%, 69%, and 67% for Casuarina equisetifolia (actinorhizal); 91%, 42%, and 67% for Acacia koa (rhizobial); and 60%, 51%, and 19% for Morella faya (actinorhizal). Fertilization with phosphorus did not stimulate tree growth or SNF. These results suggest that the latitudinal abundance distribution of N-fixing trees is not caused by a shift in SNF strategy. They also help explain the excess N in many forests where N fixers are common.

Analysing temporal patterns in plant communities is extremely important to quantify the extent and the consequences of ecological changes, especially considering the current biodiversity crisis. Long-term data collected through the regular sampling of permanent plots represent the most accurate resource to study ecological succession, analyse the stability of a community over time and understand the mechanisms driving vegetation change. We hereby present the LOng-Term Vegetation Sampling (LOTVS) initiative, a global collection of vegetation time-series derived from the regular monitoring of vascular plants in permanent plots. With 79 datasets from five continents and 7789 vegetation time-series monitored for at least six years and mostly on an annual basis, LOTVS possibly represents the largest collection of temporally fine-grained vegetation time-series derived from permanent plots and made accessible to the research community. As such, it has an outstanding potential to support innovative research in the fields of vegetation science, plant ecology and temporal ecology.

Ecosystem ecology, the subdiscipline of ecology that takes an ecosystems approach to studying the biosphere, seeks to understand how energy and resources flow through an integrated system, from abiotic to biotic components and back again. Climate is a fundamental determinant of ecosystems and in many ways dictates where and how organisms live. Ecosystems are shaped not only by abiotic factors but also by the species present in and around them. Ecosystems are linked together into meta-ecosystems via movement of organisms, or dispersal, as well as the movement of abiotic components. Ecosystems vary in the degree and efficiency to which they provide ecosystem services. Due to the pervasive and increasing pressures that humans are putting on ecosystems worldwide, some researchers have suggested that human activity rivals state factors such as climate and natural disturbance in its influence on ecosystem extent and functioning.

Excluding large native mammals is an inverse test of rewilding. A 25-year exclosure experiment in an African savanna rangeland offers insight into the potentials and pitfalls of the rewilding endeavor as they relate to the native plant community. A broad theme that has emerged from this research is that entire plant communities, as well as individual plants, adjust to the absence of herbivores in ways that can ill-prepare them for the return of these herbivores. Three lines of evidence suggest that these “naïve” individuals, populations, and communities are likely to initially suffer from herbivore rewilding. First, plots protected from wild herbivores for the past 25 years have developed rich diversity of woody plants that are absent from unfenced plots, and presumably would disappear upon rewilding. Second, individuals of the dominant tree in this system, Acacia drepanolobium , greatly reduce their defences in the absence of browsers, and the sudden arrival of these herbivores (in this case, through a temporary fence break), resulted in far greater elephant damage than for their conspecifics in adjacent plots that had been continually exposed to herbivory. Third, the removal of herbivores favoured the most palatable grass species, and a large number of rarer species, which presumably would be at risk from herbivore re-introduction. In summary, the native communities that we observe in defaunated landscapes may be very different from their pre-defaunation states, and we are likely to see some large changes to these plant communities upon rewilding with large herbivores, including potential reductions in plant diversity. Lastly, our experimental manipulation of cattle represents an additional test of the role of livestock in rewilding. Cattle are in many ways ecologically dissimilar to wildlife (in particular their greater densities), but in other ways they may serve as ecological surrogates for wildlife, which could buffer ecosystems from some of the ecological costs of rewilding. More fundamentally, African savannah ecosystems represent a challenge to traditional Western definitions of “wilderness” as ecosystems free of human impacts. We support the suggestion that as we “rewild” our biodiversity landscapes, we redefine “wildness” in the 21 st Century to be inclusive of (low impact, and sometimes traditional) human practices that are compatible with the sustainability of native (and re-introduced) biodiversity.

Forests are a significant CO2 sink. However, CO2 sequestration in forests is radiatively offset by emissions of nitrous oxide (N2 O), a potent greenhouse gas, from forest soils. Reforestation, an important strategy for mitigating climate change, has focused on maximizing CO2 sequestration in plant biomass without integrating N2 O emissions from soils. Although nitrogen (N)-fixing trees are often recommended for reforestation because of their rapid growth on N-poor soil, they can stimulate significant N2 O emissions from soils. Here, we first used a field experiment to show that a N-fixing tree (Robinia pseudoacacia) initially mitigated climate change more than a non-fixing tree (Betula nigra). We then used our field data to parameterize a theoretical model to investigate these effects over time. Under lower N supply, N-fixers continued to mitigate climate change more than non-fixers by overcoming N limitation of plant growth. However, under higher N supply, N-fixers ultimately mitigated climate change less than non-fixers by enriching soil N and stimulating N2 O emissions from soils. These results have implications for reforestation, suggesting that N-fixing trees are more effective at mitigating climate change at lower N supply, whereas non-fixing trees are more effective at mitigating climate change at higher N supply.

Biodiversity losses are a major driver of global changes in ecosystem functioning. While most studies of the relationship between biodiversity and ecosystem functioning have examined randomized species losses, trait-based filtering associated with species-specific vulnerability to drivers of diversity loss can strongly influence how ecosystem functioning responds to declining biodiversity. Moreover, the responses of ecosystem functioning to diversity loss may be mediated by environmental variability interacting with the suite of traits remaining in depauperate communities. We do not yet understand how communities resulting from realistic diversity losses (filtered by response traits) influence ecosystem functioning (via effect traits of the remaining community), especially under variable environmental conditions. Here, we directly test how realistic and randomized plant diversity losses influence productivity and invasion resistance across multiple years in a California grassland. Compared with communities based on randomized diversity losses, communities resulting from realistic (drought-driven) species losses had higher invasion resistance under climatic conditions that matched the trait-based filtering they experienced. However, productivity declined more with realistic than with randomized species losses across all years, regardless of climatic conditions. Functional response traits aligned with effect traits for productivity but not for invasion resistance. Our findings illustrate that the effects of biodiversity losses depend not only on the identities of lost species but also on how the traits of remaining species interact with varying environmental conditions. Understanding the consequences of biodiversity change requires studies that evaluate trait-mediated effects of species losses and incorporate the increasingly variable climatic conditions that future communities are expected to experience.

Between‐species variation in nutrient resorption is one of the mechanisms explaining the positive relationship between biodiversity and primary productivity. Yet, the role of within‐species variations in nutrient resorption in mediating the relationship between biodiversity and productivity remains unclear. We examined how within‐species nutrient resorption, and ultimately productivity, respond to changes in species richness by using four traits related to nitrogen and phosphorus use in four dominant species from different plant functional groups in a biodiversity removal experiment in the temperate steppe. Nitrogen and phosphorus concentrations in both green and senesced leaves in all species significantly decreased with increasing plant species richness, suggesting that plants used those limiting nutrients more efficiently with increasing biodiversity. Plants in higher diversity communities resorbed more nutrients during senescence, which may facilitate reproduction and vegetative regrowth in the next year. Synthesis. Our results highlight the importance of considering within‐species variation in nutrient resorption as an important underlying mechanism explaining the positive effects of biodiversity on primary productivity and ecosystem carbon accumulation.