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The concept of functional redundancy is at the core of theory relating changes in ecosystem functioning to species loss. However, few empirical studies have investigated the strength and form of the relationship between species and functional diversity (i.e., the presence of functional redundancy in ecological communities) in this context. In particular, we know little about how local extinctions in real communities might impact functional diversity. Here, we examined the relationship between species and functional diversity in plant communities along a grazing gradient across Mongolian rangeland ecosystems. We applied a recently described measure of functional diversity that incorporates species' dissimilarities defined from plant functional traits and tested several hypothesized forms of the relationship between species and functional diversity using linear and nonlinear modeling techniques. We found a significant sigmoid logistic relationship between species richness and functional diversity in relatively benign environmental conditions. This indicates high functional redundancy at low levels of species richness followed by a rapid increase at intermediate levels, until functional diversity reaches an asymptote at high levels (i.e., two-phase functional redundancy). In contrast, we generally observed a positive linear relationship between these parameters in relatively harsh environmental conditions, indicating low functional redundancy. Observed functional redundancy probably resulted from two factors, intrinsic redundancy in species' functional traits and extrinsic redundancy caused by nonrandom compositional change that is nonrandom with respect to functional traits. Lack of either intrinsic or extrinsic redundancy may result in low functional redundancy. Two-phase functional redundancy suggests that functional traits are abruptly lost from a community below a certain level of species richness, and a community then shifts into a contrasting state that has a few limited functional groups characterized by disturbance-resistant traits, as a consequence of disturbances such as livestock grazing. This study represents a major step forward in predicting the consequences of livestock grazing on the functioning of Mongolian rangeland ecosystems.

The plant is an important component of the riparian ecosystem, which could reflect both the environmental and functional characteristics of the riparian zone. Studies on species composition, diversity, community structure, distribution pattern, and adaptation strategies of plant communities in the riparian zone of the Three Gorges Reservoir (TGR) will help to explore the maintaining mechanism of the plant communities’ ecological function under severe water-level fluctuation. The paper reviewed the plant community characteristics, functional traits as well as their eco-physiological responses and environmental adaptations in this special ecological zone. Based on this, future research orientations in this field were also prospected, which may focus on the maintenance mechanism of the plant community, suitable plants selection and their adaptation mechanism, the relationship between plant functional traits and ecosystem functions, plant niche in the riparian zone, and the connectivity of riparian zone to the surrounding environment. The results can promote the correlational research on plant communities in the riparian zone and deepen the understanding of ecosystem services the riparian ecotone provides.

Comparative functional ecology seeks to understand why and how ecological systems and their components operate differently across environments. Although traditionally used in (semi)-natural situations, its concepts and methods could certainly apply to address key issues in the large variety of agricultural systems encountered across the world. In this review, we present major advances in comparative plant functional ecology that were made possible over the last two decades by the rapid development of a trait-based approach to plant functioning and prospects to apply it in agricultural situations. The strength of this approach is that it enables us to assess the interactions between organisms and their environment simultaneously on a large number of species, a prerequisite to address questions relative to species distribution, community assembly and ecosystem functioning. The trait concept will be first defined, before presenting a conceptual framework to understand the effects of environmental factors on plant community structure and ecosystem properties via plant traits. We will then argue that leading dimensions of variation among species can be captured by some selected traits and show that a combination of three easily measured traits—specific leaf area (the ratio of leaf area to leaf dry mass), plant height and seed mass—enables us to assess how different species use their resources, interact with neighbours and disperse in time and space. The use of traits to address central questions in community ecology will be reviewed next. It will be shown that traits allow us to (1) understand how plant species are sorted according to the nature of environmental gradients, (2) evaluate the relative importance of habitat filtering and limiting similarity in the process of community assembly and (3) quantify two main components of community functional structure, namely, community-weighted means of traits and community functional divergence. The relative impacts of these two components on ecosystem properties will then be discussed in the case of several components of primary productivity, litter decomposition, soil water content and carbon sequestration. There is strong support for the biomass ratio hypothesis, which states that the extent to which the traits of a species affect those ecosystem properties depends on the abundance of this species in the community. Assessing the role of functional divergence among species on ecosystem properties will require major methodological breakthroughs, both in terms of metrics and statistical procedures to be used. In agricultural situations, we show that trait-based approaches have been successfully developed to assess the impacts of management practices on (1) the agronomic value of grasslands and (2) the functional composition and structure of crop weed communities and how these could affect the functioning of the crop. Applications in forestry are still poorly developed, especially in temperate regions where the number of species in managed forest remains relatively low. The last decades of research have led to the constitution of large data sets of plant traits, which remain poorly compatible and accessible. Recent advances in the field of ecoinformatics suggest that major progress could be achieved in this area by using improved metadata standards and advancing trait domain ontologies. Finally, concluding remarks, unanswered questions and directions for research using the functional approach to biodiversity made possible by the use of traits will be discussed in the contexts of ecological and agronomical systems. The latter indeed cover a wide range of environmental conditions and biological diversity, and the prospect for reducing environmental impacts in highly productive, low-diversity systems will certainly imply improving our skills for the management of more diverse systems prone to a trait-based approach as reviewed here.

Fine-root traits play key roles in ecosystem processes, but the drivers of fine-root trait diversity remain poorly understood. The plant economic spectrum (PES) hypothesis predicts that leaf and root traits evolved in coordination. Mycorrhizal association type, plant growth form and climate may also affect root traits. However, the extent to which these controls are confounded with phylogenetic structuring remains unclear. Here we compiled information about root and leaf traits for > 600 species. Using phylogenetic relatedness, climatic ranges, growth form and mycorrhizal associations, we quantified the importance of these factors in the global distribution of fine-root traits. Phylogenetic structuring accounts for most of the variation for all traits excepting root tissue density, with root diameter and nitrogen concentration showing the strongest phylogenetic signal and specific root length showing intermediate values. Climate was the second most important factor, whereas mycorrhizal type had little effect. Substantial trait coordination occurred between leaves and roots, but the strength varied between growth forms and clades. Our analyses provide evidence that the integration of roots and leaves in the PES requires better accounting of the variation in traits across phylogenetic clades. Inclusion of phylogenetic information provides a powerful framework for predictions of belowground functional traits at global scales.

1.Ecosystem functioning relies heavily on belowground processes, which are largely regulated by plant fine-roots and their functional traits. However, our knowledge of fine-root trait distribution relies to date on local- and regional-scale studies with limited numbers of species, growth forms and environmental variation. 2.We compiled a worldwide fine-root trait dataset, featuring 1115 species from contrasting climatic areas, phylogeny and growth forms to test a series of hypotheses pertaining to the influence of plant functional types, soil and climate variables, and the degree of manipulation of plant growing conditions on species fine-root trait variation. Most particularly, we tested the competing hypotheses that fine-root traits typical of faster return on investment would be most strongly associated with conditions of limiting versus favourable soil resource availability. We accounted for both data source and species phylogenetic relatedness. 3.We demonstrate that (1) Climate conditions promoting soil fertility relate negatively to fine-root traits favouring fast soil resource acquisition, with a particularly strong positive effect of temperature on fine-root diameter and negative effect on specific root length (SRL), and a negative effect of rainfall on root nitrogen concentration; (2) Soil bulk density strongly influences species fine-root morphology, by favouring thicker, denser fine-roots; (3) Fine-roots from herbaceous species are on average finer and have higher SRL than those of woody species, and N2-fixing capacity positively relates to root nitrogen; (4) Plants growing in pots have higher SRL than those grown in the field. 4.Synthesis. This study reveals both the large variation in fine-root traits encountered globally and the relevance of several key plant functional types and soil and climate variables for explaining a substantial part of this variation. Climate, particularly temperature, and plant functional types were the two strongest predictors of fine-root trait variation. High trait variation occurred at local scales, suggesting that wide-ranging belowground resource economics strategies are viable within most climatic areas and soil conditions.

Aim Plant trait databases often contain traits that are correlated, but for whom direct (undirected statistical dependency) and indirect (mediated by other traits) connections may be confounded. The confounding of correlation and connection hinders our understanding of plant strategies, and how these vary among growth forms and climate zones. We identified the direct and indirect connections across plant traits relevant to competition, resource acquisition and reproductive strategies using a global database and explored whether connections within and between traits from different tissue types vary across climates and growth forms. Location Global. Major taxa studied Plants. Time period Present. Methods We used probabilistic graphical models and a database of 10 plant traits (leaf area, specific leaf area, mass‐ and area‐based leaf nitrogen and phosphorous content, leaf life span, plant height, stem specific density and seed mass) with 16,281 records to describe direct and indirect connections across woody and non‐woody plants across tropical, temperate, arid, cold and polar regions. Results Trait networks based on direct connections are sparser than those based on correlations. Land plants had high connectivity across traits within and between tissue types; leaf life span and stem specific density shared direct connections with all other traits. For both growth forms, two groups of traits form modules of more highly connected traits; one related to resource acquisition, the other to plant architecture and reproduction. Woody species had higher trait network modularity in polar compared to temperate and tropical climates, while non‐woody species did not show significant differences in modularity across climate regions. Main conclusions Plant traits are highly connected both within and across tissue types, yet traits segregate into persistent modules of traits. Variation in the modularity of trait networks suggests that trait connectivity is shaped by prevailing environmental conditions and demonstrates that plants of different growth forms use alternative strategies to cope with local conditions.

The insurance hypothesis, stating that biodiversity can increase ecosystem stability, has received wide research and political attention. Recent experiments suggest that climate change can impact how plant diversity influences ecosystem stability, but most evidence of the biodiversity-stability relationship obtained to date comes from local studies performed under a limited set of climatic conditions. Here, we investigate how climate mediates the relationships between plant (taxonomical and functional) diversity and ecosystem stability across the globe. To do so, we coupled 14 years of temporal remote sensing measurements of plant biomass with field surveys of diversity in 123 dryland ecosystems from all continents except Antarctica. Across a wide range of climatic and soil conditions, plant species pools, and locations, we were able to explain 73% of variation in ecosystem stability, measured as the ratio of the temporal mean biomass to the SD. The positive role of plant diversity on ecosystem stability was as important as that of climatic and soil factors. However, we also found a strong climate dependency of the biodiversity-ecosystem stability relationship across our global aridity gradient. Our findings suggest that the diversity of leaf traits may drive ecosystem stability at low aridity levels, whereas species richness may have a greater stabilizing role under the most arid conditions evaluated. Our study highlights that to minimize variations in the temporal delivery of ecosystem services related to plant biomass, functional and taxonomic plant diversity should be particularly promoted under low and high aridity conditions, respectively.

Anthropogenic climate change (ACC) will influence all aspects of plant biology over coming decades. Many changes in wild species have already been well-documented as a result of increased atmospheric CO2 concentrations, warming climate and changing precipitation regimes. A wealth of available data has allowed the use of meta-analyses to examine plant-climate interactions on more sophisticated levels than before. These analyses have revealed major differences in plant response among groups, e.g. with respect to functional traits, taxonomy, life-history and provenance. Interestingly, these meta-analyses have also exposed unexpected mismatches between theory, experimental, and observational studies. We reviewed the literature on species' responses to ACC, finding ∼42 % of 4000 species studied globally are plants (primarily terrestrial). We review impacts on phenology, distributions, ecophysiology, regeneration biology, plant-plant and plant-herbivore interactions, and the roles of plasticity and evolution. We focused on apparent deviations from expectation, and highlighted cases where more sophisticated analyses revealed that unexpected changes were, in fact, responses to ACC. We found that conventionally expected responses are generally well-understood, and that it is the aberrant responses that are now yielding greater insight into current and possible future impacts of ACC. We argue that inconclusive, unexpected, or counter-intuitive results should be embraced in order to understand apparent disconnects between theory, prediction, and observation. We highlight prime examples from the collection of papers in this Special Issue, as well as general literature. We found use of plant functional groupings/traits had mixed success, but that some underutilized approaches, such as Grime's C/S/R strategies, when incorporated, have improved understanding of observed responses. Despite inherent difficulties, we highlight the need for ecologists to conduct community-level experiments in systems that replicate multiple aspects of ACC. Specifically, we call for development of coordinating experiments across networks of field sites, both natural and man-made.

1. Plant traits can provide unique insights into plant performance at the community scale. Functional composition, defined by both functional diversity and community-weighted trait means (CWMs), can affect the stability of above-ground net primary production (ANPP) in response to climate extremes. Further complexity arises, however, when functional composition itself responds to environmental change. The duration of climate extremes, such as drought, is expected to increase with rising global temperatures; thus, understanding the impacts of long-term drought on functional composition and the corresponding effect that has on ecosystem function could improve predictions of ecosystem sensitivity to climate change. 2. We experimentally reduced growing season precipitation by 66% across six temperate grasslands for 4 years and measured changes in three indices of functional diversity (functional dispersion, richness and evenness), community-weighted trait means and phylogenetic diversity (PD). Specific leaf area (SLA), leaf nitrogen content (LNC) and (at most sites) leaf turgor loss point (πTLP) were measured for species cumulatively representing ~90% plant cover at each site. 3. Long-term drought led to increased community functional dispersion in three sites, with negligible effects on the remaining sites. Species re-ordering following the mortality/senescence of dominant species was the main driver of increased functional dispersion. The response of functional diversity was not consistently matched by changes in phylogenetic diversity. Community-level drought strategies (assessed as CWMs) largely shifted from drought tolerance to drought avoidance and/or escape strategies, as evidenced by higher community-weighted , πTLP, SLA and LNC. Lastly, ecosystem drought sensitivity (i.e. relative reduction in ANPP in drought plots) was positively correlated with community-weighted SLA and negatively correlated with functional diversity. 4. Synthesis. Increased functional diversity following long-term drought may stabilize ecosystem functioning in response to future drought. However, shifts in community-scale drought strategies may increase ecosystem drought sensitivity, depending on the nature and timing of drought. Thus, our results highlight the importance of considering both functional diversity and abundance-weighted traits means of plant communities as their collective effect may either stabilize or enhance ecosystem sensitivity to drought.

1. Understanding the importance of biotic interactions in driving the distribution and abundance of species is a central goal of plant ecology. Early vascular plants likely colonized land occupied by biocrusts — photoautotrophic, surface-dwelling soil communities comprised of cyanobacteria, bryophytes, lichens and fungi — suggesting biotic interactions between biocrusts and plants have been at play for some 2,000 million years. Today, biocrusts coexist with plants in dryland ecosystems worldwide, and have been shown to both facilitate or inhibit plant species performance depending on ecological context. Yet, the factors that drive the direction and magnitude of these effects remain largely unknown. 2. We conducted a meta-analysis of plant responses to biocrusts using a global data-set encompassing 1,004 studies from six continents. 3. Meta-analysis revealed there is no simple positive or negative effect of biocrusts on plants. Rather, plant responses differ by biocrust composition and plant species traits and vary across plant ontogeny. Moss-dominated biocrusts facilitated, while lichen-dominated biocrusts inhibited overall plant performance. Plant responses also varied among plant functional groups: C₄ grasses received greater benefits from biocrusts compared to C₃ grasses, and plants without N-fixing symbionts responded more positively to biocrusts than plants with N-fixing symbionts. Biocrusts decreased germination but facilitated growth of non-native plant species. 4. Synthesis. Results suggest that interspecific variation in plant responses to biocrusts, contingent on biocrust type, plant traits, and ontogeny can have strong impacts on plant species performance. These findings have important implications for understanding biocrust contributions to plant productivity and community assembly processes in ecosystems worldwide.