Geneticists and breeders are positioned to breed plants with root traits that improve productivity under drought. However, a better understanding of root functional traits and how traits are related to whole plant strategies to increase crop productivity under different drought conditions is needed. Root traits associated with maintaining plant productivity under drought include small fine root diameters, long specific root length (SRL), and considerable root length density, especially at depths in soil with available water. In environments with late season water deficits, small xylem diameters in targeted seminal roots save soil water deep in the soil profile for use during crop maturation and result in improved yields. Capacity for deep root growth and large xylem diameters in deep roots may also improve root acquisition of water when ample water at depth is available. Xylem pit anatomy that makes xylem less ‘leaky’ and prone to cavitation warrants further exploration holding promise that such traits may improve plant productivity in water-limited environments without negatively impacting yield under adequate water conditions. Rapid resumption of root growth following soil rewetting may improve plant productivity under episodic drought. Genetic control of many of these traits through breeding appears feasible. Several recent reviews have covered methods for screening root traits but an appreciation for the complexity of root systems (e.g. functional differences between fine and coarse roots) needs to be paired with these methods to successfully identify relevant traits for crop improvement. Screening of root traits at early stages in plant development can proxy traits at mature stages but verification is needed on a case by case basis that traits are linked to increased crop productivity under drought. Examples in lesquerella (Physaria) and rice (Oryza) show approaches to phenotyping of root traits and current understanding of root trait genetics for breeding.

Summary: The effects of plants on the biosphere, atmosphere and geosphere are key determinants of terrestrial ecosystem functioning. However, despite substantial progress made regarding plant belowground components, we are still only beginning to explore the complex relationships between root traits and functions. Drawing on the literature in plant physiology, ecophysiology, ecology, agronomy and soil science, we reviewed 24 aspects of plant and ecosystem functioning and their relationships with a number of root system traits, including aspects of architecture, physiology, morphology, anatomy, chemistry, biomechanics and biotic interactions. Based on this assessment, we critically evaluated the current strengths and gaps in our knowledge, and identify future research challenges in the field of root ecology. Most importantly, we found that belowground traits with the broadest importance in plant and ecosystem functioning are not those most commonly measured. Also, the estimation of trait relative importance for functioning requires us to consider a more comprehensive range of functionally relevant traits from a diverse range of species, across environments and over time series. We also advocate that establishing causal hierarchical links among root traits will provide a hypothesis‐based framework to identify the most parsimonious sets of traits with the strongest links on functions, and to link genotypes to plant and ecosystem functioning.

Summary: In the context of a recent massive increase in research on plant root functions and their impact on the environment, root ecologists currently face many important challenges to keep on generating cutting‐edge, meaningful and integrated knowledge. Consideration of the below‐ground components in plant and ecosystem studies has been consistently called for in recent decades, but methodology is disparate and sometimes inappropriate. This handbook, based on the collective effort of a large team of experts, will improve trait comparisons across studies and integration of information across databases by providing standardised methods and controlled vocabularies. It is meant to be used not only as starting point by students and scientists who desire working on below‐ground ecosystems, but also by experts for consolidating and broadening their views on multiple aspects of root ecology. Beyond the classical compilation of measurement protocols, we have synthesised recommendations from the literature to provide key background knowledge useful for: (1) defining below‐ground plant entities and giving keys for their meaningful dissection, classification and naming beyond the classical fine‐root vs coarse‐root approach; (2) considering the specificity of root research to produce sound laboratory and field data; (3) describing typical, but overlooked steps for studying roots (e.g. root handling, cleaning and storage); and (4) gathering metadata necessary for the interpretation of results and their reuse. Most importantly, all root traits have been introduced with some degree of ecological context that will be a foundation for understanding their ecological meaning, their typical use and uncertainties, and some methodological and conceptual perspectives for future research. Considering all of this, we urge readers not to solely extract protocol recommendations for trait measurements from this work, but to take a moment to read and reflect on the extensive information contained in this broader guide to root ecology, including sections I–VII and the many introductions to each section and root trait description. Finally, it is critical to understand that a major aim of this guide is to help break down barriers between the many subdisciplines of root ecology and ecophysiology, broaden researchers’ views on the multiple aspects of root study and create favourable conditions for the inception of comprehensive experiments on the role of roots in plant and ecosystem functioning.

Summary: Ecosystem functioning relies heavily on below‐ground 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. We compiled a world‐wide 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. We demonstrate that: (i) 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; (ii) Soil bulk density strongly influences species fine‐root morphology, by favouring thicker, denser fine‐roots; (iii) 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; and (iv) Plants growing in pots have higher SRL than those grown in the field. 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 below‐ground resource economics strategies are viable within most climatic areas and soil conditions.

Summary: There is limited understanding of patterns of variation that exist among root traits of different species, especially under field conditions. We contrasted 11 fast‐ and slow‐growing species paired within five evolutionary lineages to investigate whether root traits associated with soil resource acquisition were related to species’ potential growth rate. Measurements of root morphology, architecture, nitrogen and phenolic concentration, respiration and phosphorus uptake were taken on fine, non‐woody roots sampled from forest stands in central Pennsylvania, USA. Across all five contrasts, roots of fast‐growing species generally had higher specific root length, smaller diameters, greater degree of branching, and lower phenolic concentrations than those of slow‐growing species. This suggests differences in potential soil exploration and root defences among species differing in potential growth rate. There were no significant differences between fast‐ and slow‐growing species in root tissue density, respiration or P uptake. Lack of root physiological differences between species differing in growth rate contrasted with previous research on chamber‐grown seedlings. These results imply that, while roots of fast‐growing species may be constructed for more rapid soil exploration and shorter life span than those of slow‐growing species, root physiology is either more closely tied to overall plant physiology, which is more similar among mature trees, or masked by variation in soil microsites, root age or interactions with mycorrhizal fungi.

Defining root death in studies of root dynamics is problematic because cell death occurs gradually and the resulting effects on root function are not well understood. In this study, metabolic activity of grape roots of different ages was assessed by excised root respiration and tetrazolium chloride reduction. We investigated changes in metabolic activity and patterns of cell death occurring with root age and changes in root pigmentation. Tetrazolium chloride reduction of roots of different ages was strongly correlated to respiration (R² = 0.786). As roots aged, respiration and tetrazolium chloride reduction declined similarly, with minimum metabolic activity reached at six weeks. Tetrazolium chloride reduction indicated that the onset of root browning corresponded to a 77% reduction in metabolic activity (P < 0.001). Anatomical examination of roots at each pigmentation stage showed that even though some cells in brown roots were still alive, these roots were functionally dead. The effect of using different definitions of root death in relation to root survivorship was determined in a study of ‘Concord’ grapes with two pruning treatments, using three criteria for root death: browning, blackening or shriveling, and disappearance. There was no effect of vine pruning on root life span when life span was defined as the time from first appearance to the onset of browning. However, if death was judged as the point when roots either became black or shriveled or disappeared, vine pruning decreased root life span by 34% and 40%, respectively (P < 0.001), and also increased the decay constant for root decomposition by about 45% (P < 0.001). We conclude that the discrepancy among determinations of root life span assessed with different definitions of death might be partly caused by the latter evaluations of root life span incorporating a portion of root decomposition in definitions of root death.