Summary The search for a root economics spectrum (RES) has been sparked by recent interest in trait‐based plant ecology. By analogy with the one‐dimensional leaf economics spectrum (LES), fine‐root traits are hypothesised to match leaf traits which are coordinated along one axis from resource acquisitive to conservative traits. However, our literature review and meta‐level analysis reveal no consistent evidence of an RES mirroring an LES. Instead the RES appears to be multidimensional. We discuss three fundamental differences contributing to the discrepancy between these spectra. First, root traits are simultaneously constrained by various environmental drivers not necessarily related to resource uptake. Second, above‐ and belowground traits cannot be considered analogues, because they function differently and might not be related to resource uptake in a similar manner. Third, mycorrhizal interactions may offset selection for an RES. Understanding and explaining the belowground mechanisms and trade‐offs that drive variation in root traits, resource acquisition and plant performance across species, thus requires a fundamentally different approach than applied aboveground. We therefore call for studies that can functionally incorporate the root traits involved in resource uptake, the complex soil environment and the various soil resource uptake mechanisms – particularly the mycorrhizal pathway – in a multidimensional root trait framework.

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 literature in plant physiology, ecophysiology, ecology, agronomy and soil science, we review 24 aspects of plant and ecosystem functioning and their relationships with a number of traits of root systems, including aspects of architecture, physiology, morphology, anatomy, chemistry, biomechanics and biotic interactions. Based on this assessment, we critically evaluate the current strengths and gaps in our knowledge, and identify future research challenges in the field of root ecology. Most importantly, we found that below-ground traits with widest importance in plant and ecosystem Accepted Article This article is protected by copyright. All rights reserved functioning are not those most commonly measured. Also, the fair 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 strongest influence on the functions, and to link genotypes to plant and ecosystem functioning.

I.Introduction: continuing to face up to root ecology's challenges 975 II.Semantics: defining concepts for better understanding and communication 977III. Species-level vs ecosystem-level measurements 978

Dead wood provides a huge terrestrial carbon stock and a habitat to wide-ranging organisms during its decay. Our brief review highlights that, in order to understand environmental change impacts on these functions, we need to quantify the contributions of different interacting biotic and abiotic drivers to wood decomposition. LOG-LIFE is a new long-term 'common-garden' experiment to disentangle the effects of species' wood traits and siterelated environmental drivers on wood decomposition dynamics and its associated diversity of microbial and invertebrate communities. This experiment is firmly rooted in pioneering experiments under the directorship of Terry Callaghan at Abisko Research Station, Sweden. LOGLIFE features two contrasting forest sites in the Netherlands, each hosting a similar set of coarse logs and branches of 10 tree species. LOGLIFE welcomes other researchers to test further questions concerning coarse wood decay that will also help to optimise forest management in view of carbon sequestration and biodiversity conservation.

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Patterns in intraspecific variation in root traits are species-specific along an elevation gradient Abstract 1. Intraspecific trait variation is an important driver of plant performance in different environments. Although roots acquire essential resources that vary with the environment, most studies have focused on intraspecific variation in leaf traits, and research on roots is often restricted to a few species. It remains largely unclear how and to what extent root traits vary with the environment and whether general intraspecific patterns exist across species. 2. We compared intraspecific variation in specific root length (SRL), root diameter, root tissue density (RTD) and root branching density of 11 species along a 1000 m elevation gradient in the French Alps. We tested 1) the extent of intra-versus interspecific root trait variation along the gradient, 2) whether intraspecific trait patterns with elevation were consistent among species and 3) whether environmental variables better explained intraspecific variation in root traits than elevation. Specifically, we hypothesised that within a species, root trait values would adjust to enhance resource acquisition (either through an increase in SRL or root diameter, and/or branching density) and/or conservation (increased RTD) at higher elevations. 3. Species identity explained most of the overall variation in root traits. Elevation explained only a minor proportion of intraspecific root trait variation, which varied more strongly within than between elevations. Also, trait relationships with elevation rarely agreed with our hypotheses, Accepted Article This article is protected by copyright. All rights reserved varied strongly across species, and were often differently related to environmental variation. Generally, climate, soil and vegetation properties better explained intraspecific root variation than elevation, but these relationships were highly species-dependent. 4. Along complex environmental gradients where multiple properties simultaneously change, roots of different species vary in different ways, leading to species-specific patterns in intraspecific root trait variation. The lack of support for our hypotheses may be caused by the multiple interactions between environmental properties, small-scale soil heterogeneity, species phylogeny, and changing plant-plant interactions. Our findings suggest that, to enhance our understanding of the effects of environmental change on plant performance, we need to better integrate the multiple dimensions of plant responses to change and measure a broader set of root traits and environmental variables.

The root economics spectrum (RES) hypothesis predicts that fast‐growing tree species have short‐lived roots with high specific root length (SRL) to allow rapid resource uptake, and opposite trait expressions for slow‐growing species. Yet, the mixed support for this hypothesis suggests that trees can adopt alternative strategies to increase resource uptake, besides an increase in SRL. We combined a novel mechanistic whole‐tree model and empirical fine‐root data of 10 tree species to test the effects of one of these alternative strategies, notably increasing fine‐root mass, on the tree's net C gain (used here as a proxy for tree performance), and to assess how fine‐root life span influences the relative importance of SRL and fine‐root mass for the C balance of trees. Our results indicate that accounting for the short life span of high‐SRL roots has important implications for explaining tree performance and the role of roots herein. Without considering their faster turnover, high‐SRL roots and low fine‐root mass resulted in the highest performance as predicted from the RES. Yet, when their higher turnover rates were accounted for, a high fine‐root mass and low SRL lead to the highest performance. Both our model outcomes and field data further show a negative relationship between SRL and fine‐root mass through which species aim to realize a similar root length density. This trade‐off further indicates how high a SRL and low fine‐root mass as well as opposite trait values can both lead to a positive C balance in a similar environment. Our study may explain why high‐SRL roots do not necessarily lead to the fastest tree growth as often hypothesized and demonstrates the importance of fine‐root mass in combination with fine‐root life span for explaining interspecific differences in tree performance. More generally, our work demonstrates the value of identifying and investigating different below‐ground strategies across species from a whole‐plant modelling perspective, and identifies the relationship between SRL, fine‐root biomass and life span as an important functional dimension to variation in species’ performance. A free Plain Language Summary can be found within the Supporting Information of this article.

Aim The fine roots of trees may show plastic responses to their resource environment. Several, contrasting hypotheses exist on this plasticity, but empirical evidence for these hypotheses is scattered. This study aims to enhance our understanding of tree root plasticity by examining intra-specific variation in fine-root mass and morphology, fine-root growth and decomposition, and associated mycorrhizal interactions in beech (Fagus sylvatica L.) and spruce (Picea abies (L.) Karst.) forests on soils that differ in resource availability. MethodsWe measured the mass and morphological traits of fine roots (i.e. ≤ 2 mm diameter) sampled to 50 cm depth. Fine-root growth was measured with ingrowth cores, and fine-root decomposition with litter bags. Mycorrhizal fungal biomass was determined using ingrowth mesh bags. Results Both tree species showed more than three times higher fine-root mass, and a ten-fold higher fine-root growth rate on sand than on clay, but no or marginal differences in overall fine-root morphology. Within the fine-root category however, beech stands had relatively more root length of their finest roots on clay than on sand. In the spruce stands, ectomycorrhizal mycelium biomass was larger on sand than on clay. Conclusions In temperate beech and spruce forests, fine-root mass and mycorrhizal fungal biomass, rather than fine-root morphology, are changed to ensure uptake under different soil resource conditions. Yet enhancing our mechanistic understanding of fine-root trait plasticity and how it affects tree growth requires more attention to fine-root dynamics, the functional diversity within the fine-roots, and mycorrhizal symbiosis as an important belowground uptake strategy.