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The promise of “trait-based” plant ecology is one of generalized prediction across organizational and spatial scales, independent of taxonomy. This promise is a major reason for the increased popularity of this approach. Here, we argue that some important foundational assumptions of trait-based ecology have not received sufficient empirical evaluation. We identify three such assumptions and, where possible, suggest methods of improvement: (i) traits are functional to the degree that they determine individual fitness, (ii) intraspecific variation in functional traits can be largely ignored, and (iii) functional traits show general predictive relationships to measurable environmental gradients.

Since proposed two decades ago, the stress-gradient hypothesis (SGH), suggesting that species interactions shift from competition to facilitation with stress, has been widely examined. Despite broad support across species and ecosystems, ecologists debate whether the SGH applies to extreme environments, arguing that species interactions switch to competition or collapse under extreme stress. We show that facilitation often expands distributions on species borders. SGH exceptions occur when weak stress gradients or stresses outside of species' niches are examined, multiple stresses co-occur canceling out their effects, temporally dependent effects are involved, or results are improperly analyzed. We suggest that ecologists resolve debates by standardizing key SGH terms, such as fundamental and realized niche, stress gradients vs. environmental gradients, by quantitatively defining extreme stress, and by critically evaluating the functionality of stress gradients. We also suggest that new research examine the breadth and relevance of the SGH. More rigor needs to be applied to SGH tests to identify actual exceptions rather than those due to failures to meet its underlying assumptions, so that the general principles of the SGH and its exceptions can be incorporated into ecological theory, conservation strategies, and environmental change predictions.

Alien grass invasions in arid and semi-arid ecosystems are resulting in grass–fire cycles and ecosystem-level transformations that severely diminish ecosystem services. Our capacity to address the rapid and complex changes occurring in these ecosystems can be enhanced by developing an understanding of the environmental factors and ecosystem attributes that determine resilience of native ecosystems to stress and disturbance, and resistance to invasion. Cold desert shrublands occur over strong environmental gradients and exhibit significant differences in resilience and resistance. They provide an excellent opportunity to increase our understanding of these concepts. Herein, we examine a series of linked questions about (a) ecosystem attributes that determine resilience and resistance along environmental gradients, (b) effects of disturbances like livestock grazing and altered fire regimes and of stressors like rapid climate change, rising CO₂, and N deposition on resilience and resistance, and (c) interacting effects of resilience and resistance on ecosystems with different environmental conditions. We conclude by providing strategies for the use of resilience and resistance concepts in a management context. At ecological site scales, state and transition models are used to illustrate how differences in resilience and resistance influence potential alternative vegetation states, transitions among states, and thresholds. At landscape scales management strategies based on resilience and resistance—protection, prevention, restoration, and monitoring and adaptive management—are used to determine priority management areas and appropriate actions.

Patterns of environmental spatial structure lie at the heart of the most fundamental and familiar patterns of diversity on Earth. Antarctica contains some of the strongest environmental gradients on the planet and therefore provides an ideal study ground to test hypotheses on the relevance of environmental variability for biodiversity. To answer the pivotal question, \"How does spatial variation in physical and biological environmental properties across the Antarctic drive biodiversity?\" we have synthesized current knowledge on environmental variability across terrestrial, freshwater, and marine Antarctic biomes and related this to the observed biotic patterns. The most important physical driver of Antarctic terrestrial communities is the availability of liquid water, itself driven by solar irradiance intensity. Patterns of biota distribution are further strongly influenced by the historical development of any given location or region, and by geographical barriers. In freshwater ecosystems, free water is also crucial, with further important influences from salinity, nutrient availability, oxygenation, and characteristics of ice cover and extent. In the marine biome there does not appear to be one major driving force, with the exception of the oceanographic boundary of the Polar Front. At smaller spatial scales, ice cover, ice scour, and salinity gradients are clearly important determinants of diversity at habitat and community level. Stochastic and extreme events remain an important driving force in all environments, particularly in the context of local extinction and colonization or recolonization, as well as that of temporal environmental variability. Our synthesis demonstrates that the Antarctic continent and surrounding oceans provide an ideal study ground to develop new biogeographical models, including life history and physiological traits, and to address questions regarding biological responses to environmental variability and change.

1. Functional traits are increasingly used to investigate community structure, ecosystem functioning or to classify species into functional groups. These functional traits are expected to be variable between and within species. Intraspecific functional variability is supposed to influence and modulate species responses to environmental changes and their effects on their environment. However, this hypothesis remains poorly tested and species are mostly described by mean trait values without any consideration of variability in individual trait values. 2. In this study, we quantify the extent of intraspecific plant functional trait variability, its spatial structure and its response to environmental factors. Using a sampling design structured along two direct and orthogonal climatic gradients in an alpine valley, we quantified and analysed the intraspecific variability for three functional traits (height, leaf dry matter content and leaf nitrogen content) measured on sixteen plant species with contrasting life histories. 3. Results showed a large variability of traits within species with large discrepancies between functional traits and species. This variability did not appear to be structured within populations. Between populations, the overall variability was partly explained by the selected gradients. Despite the strong effects of temperature and radiation on trait intraspecific variability, the response curves of traits along gradients were partly idiosyncratic. 4.Synthesis. Giving a comprehensive quantification of intraspecific functional variability through the analysis of an original data set, we report new evidence that using a single trait value to describe a given species can hide large functional variation for this species along environmental gradients. These findings suggest that intraspecific functional variability should be a concern for ecologists and its recognition opens new opportunities to better understand and predict ecological patterns in a changing environment. Further analyses are, however, required to compare inter- and intraspecific variability.

1. Once neglected, the role of facilitative interactions in plant communities has received considerable attention in the last two decades, and is now widely recognized. It is timely to consider the progress made by research in this field. 2. We review the development of plant facilitation research, focusing on the history of the field, the relationship between plant-plant interactions and environmental severity gradients, and attempts to integrate facilitation into mainstream ecological theory. We then consider future directions for facilitation research. 3. With respect to our fundamental understanding of plant facilitation, clarification of the relationship between interactions and environmental gradients is central for further progress, and necessitates the design and implementation of experiments that move beyond the clear limitations of previous studies. 4. There is substantial scope for exploring indirect facilitative effects in plant communities, including their impacts on diversity and evolution, and future studies should connect the degree of non-transitivity in plant competitive networks to community diversity and facilitative promotion of species coexistence, and explore how the role of indirect facilitation varies with environmental severity. 5. Certain ecological modelling approaches (e.g. individual-based modelling), although thus far largely neglected, provide highly useful tools for exploring these fundamental processes. 6. Evolutionary responses might result from facilitative interactions, and consideration of facilitation might lead to re-assessment of the evolution of plant growth forms. 7. Improved understanding of facilitation processes has direct relevance for the development of tools for ecosystem restoration, and for improving our understanding of the response of plant species and communities to environmental change drivers. 8. Attempts to apply our developing ecological knowledge would benefit from explicit recognition of the potential role of facilitative plant-plant interactions in the design and interpretation of studies from the fields of restoration and global change ecology. 9. Synthesis: Plant facilitation research provides new insights into classic ecological theory and pressing environmental issues. Awareness and understanding of facilitation should be part of the basic ecological knowledge of all plant ecologists.

Reynolds Functional Groups (RFG) were designed to represent the diversity of phytoplankton assemblages of freshwater ecosystems and are among the most enduring legacies of C. S. Reynolds to freshwater phytoplankton ecologists. The RFG concept summarises a rich base of knowledge, clustering species according to functional criteria. RFG allow researchers to organise information, understand community and ecosystem functioning, and identify future outcomes, also contributing to construct hypotheses and define community assembly. This approach represents the environmental requirements, tolerances and sensitivities of species, organizing them into diagnostic environmental axes representing the habitat template of phytoplankton communities. In this contribution, we highlight the importance of this trait-based approach for phytoplankton ecology, and summarise its history and usage for phytoplankton species classifications and prediction based on environmental gradients. We present some of the applications of the approach, to describe patterns, explain mechanisms and predict new situations. We hope that this review will contribute both to describe the trajectory and practice of RFG and to encourage its use in addressing new ecological questions and in generating new avenues of research.

Stomatal behavior plays a critical role in determining the vegetation water‐carbon cycles under climate change. While research on the response of leaf stomata to environmental stresses has become increasingly prevalent, understanding of the variability of stomatal behavior in climate‐sensitive and ecologically fragile regions remains limited. We selected typical ecosystems of the inland Heihe River Basin in China, compiled data on water‐carbon fluxes, meteorological factors, and soil water content across ecosystems and environmental gradients, quantified thresholds of atmospheric (AS), soil (SS), and compound water stresses (CS) that suppress ecosystem evapotranspiration, then estimated the canopy stomatal conductance (Gc) by eliminating non‐vegetation information from the surface conductance, finally analyzed differences in water‐stressed Gc and quantified the sensitivity of Gc to environmental and biological factors using the machine learning approach. We found that water stress thresholds varied along with environmental conditions, and these thresholds were correlated with stomatal parameters in the Medlyn equation (R2 ≥ 0.30). The mean Gc was minimal under CS, and the mean Gc under AS was lower than that under SS. The sensitivity of Gc to influencing factors under SS was greater than under other water stresses, and the effects of soil water on Gc were uniquely negative in riparian ecosystems. By emphasizing the complexity and variability of canopy stomatal behaviors under water stresses within the basin, our study advances the understanding of plant physiological responses to environmental stresses and informs the enhancement of stomatal conductance parameterizations in Earth system models.

Patterns of species turnover are central to the geography of biodiversity and resulting challenges for conservation, but at broad scales remain relatively little understood. Here, we take a first spatially-explicitly and global perspective to link the spatial turnover of species and environments. We compare how major groups of vertebrate ectotherms (amphibians) and endotherms (birds) respond to spatial environmental gradients. We find that high levels of species turnover occur regardless of environmental turnover rates, but environmental turnover provides a lower bound for species turnover. This lower bound increases more steeply with environmental turnover in tropical realms. While bird and amphibian turnover rates are correlated, the rate of amphibian turnover is four times steeper than bird rates. This is the same factor by which average geographic ranges of birds are larger than those of amphibians. Narrow-ranged birds exhibit rapid rates of species turnover similar to those for amphibians, while wide-ranged birds largely drive the aggregate patterns of avian turnover. We confirm a strong influence of the environment on species turnover that is mediated by range sizes and regional history. In contrast to geographic patterns of species richness, we find that the turnover in one group (amphibians) is a much better predictor for the turnover in another (birds) than is environment. This result confirms the role of amphibian sensitivity to environmental conditions for patterns of turnover and supports their value as a surrogate group. This spatially-explicit analysis of environmental turnover provides understanding for conservation planning in changing environments.

1. Savanna ecosystems span a diverse range of climates, edaphic conditions, and disturbance regimes, the complexity of which has stimulated long-standing interest in the mechanisms that maintain tree-grass coexistence. One hypothesis suggests that tree establishment is strongly limited by one or several demographic bottlenecks at early stages of the tree life cycle. A major impediment to testing this hypothesis is the lack of data on the relative strengths of different bottlenecks across key environmental gradients. 2. To identify demographic bottlenecks that limit early tree establishment (0-18 months), we conducted a series of transplant experiments with two savanna trees species (Acacia robusta and Acacia tortilis) across a natural rainfall and soil fertility gradient in the Serengeti ecosystem, Tanzania. We tested the interactive effects of precipitation, herbivory, seed scarification, grass competition, water limitation, and tree species identity on two key life stages: germination and early seedling survival (0-2 months), and juvenile seedling survival (2-18 months). 3. Germination and early seedling survival increased as a function of rainfall, in the absence of herbivores and when seeds were scarified. Juvenile seedling survival, in contrast, decreased with rainfall but increased in the absence of herbivores. Grass removal had the single strongest (positive) effect on juvenile seedling survival of any treatment. Soil moisture monitoring and grass-addition treatments revealed that grasses negatively affected seedlings in ways that were not necessarily linked to soil moisture. 4. A demographic model combining all effects across early life stages showed that the strength of grass competition on juvenile seedling survival was the key factor limiting early tree establishment. While rainfall had an unexpected opposing effect on the two life stages, the net effect of mean annual precipitation on early tree establishment was positive. 5. Synthesis. Successful tree establishment in Serengeti is maximized by a seemingly unlikely sequence of events: (a) scarification of seeds by browsers, (b) heavy rainfall to promote germination, (c) intensive grazing (but absence of browsers), and (d) dry conditions during juvenile seedling growth (>2 months) to reduce competition with grasses. By considering a wide suite of conditions and their interactions, our experimental results are relevant to ongoing debates about savanna vegetation dynamics and structural shifts in tree:grass ratios.