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The mycorrhizal type affects the structure and functions of tree roots. Therefore, the mechanical traits of the roots of tree species with different types of mycorrhizal fungi may be linked to different root functional traits. Fine roots, in particular, are closely related to the root structure and are also important for slope protection. However, the relationship among the mycorrhizal types of trees, root mechanical traits, and root structure remains unclear. This study aims to investigate the link between fine root tensile strength (Tr) and root morphological and/or structural traits in temperate trees with different mycorrhizal types. We investigated the seedlings of 15 dominant tree species in the cool temperate forests of northern Japan. For each species, fine root Tr and other five common root morphological and structural traits were measured. There was a significant positive correlation between total fine root biomass and fine root Tr consistently, even in the analysis of mycorrhizal types (arbuscular mycorrhizal (AM), and ectomycorrhizae (EM)). Our findings indicate that the root structural trait is an important driver of fine root Tr, especially for AM and EM species, and suggest that including a plant-mycorrhizal framework in future work offers great potential to improve our understanding of forest restoration.

Summary Roots vary in anatomy, morphology and physiology, both spatially (different parts of the same root system) and temporally (plastic changes, root ageing), suggesting that root trait measurements are strongly affected by root sampling categories. In this context, it is urgent to clarify the functional significance of current root sampling categories (e.g. fine roots of the first order, the first three orders, ≤1 mm or ≤2 mm), establish guidelines for choosing between sampling methods and revise root ontology to account for functional differences between traits measured on distinct root categories. Here, we used a worldwide database of fine‐root traits to test the hypothesis that distinct fine‐root trait values – with link to fine‐root functions – were generally affected by different root sampling categories. We observed indeed a clear functional break between first‐order roots and roots of all three other sampling categories, and a smaller but substantial break between roots of the three first orders and the ≤2 mm category, demonstrating globally that different sampling methodologies capture different functional parts of roots. Our synthesis suggests that all current root sampling categories present both advantages and pitfalls and that no single method can appropriately tackle the main current challenge of root functional ecology: i.e. linking fine roots to plant and ecosystem functions in a truly comparable way across all plants. We argue instead that a small set of complementary standardized sampling methods is necessary to capture the linkages between root forms and functions. To assist experimenters selecting adequate sampling we developed a decision table following three logical questions: (i) what plant or ecosystem function must be addressed; (ii) what root categories are involved in this function and (iii) what traits should be measured on these root categories. Challenging, strengthening and expending such common reference framework would be a substantial step towards wider comparability of future functional trait datasets. A lay summary is available for this article. Lay Summary

• Root exudation stimulates microbial decomposition and enhances nutrient availability to plants. It remains difficult to measure and predict this carbon flux in natural conditions, especially for mature woody plants. Based on a known conceptual framework of root functional traits coordination, we proposed that root functional traits may predict root exudation. • We measured root exudation and other seven root morphological/chemical/physiological traits for 18 coexisting woody species in a deciduous-evergreen mixed forest in subtropical China. • Root exudation, respiration, diameter and nitrogen (N) concentration all exhibited significant phylogenetic signals. We found that root exudation positively correlated with competitive traits (root respiration, N concentration) and negatively with a conservative trait (root tissue density). Furthermore, these relationships were independent of phylogenetic signals. A principal component analysis showed that root exudation and morphological traits loaded on two perpendicular axes. • Root exudation is a competitive trait in a multidimensional fine-root functional coordination. The metabolic dimension on which root exudation loaded was relatively independent of the morphological dimension, indicating that increasing nutrient availability by root exudation might be a complementary strategy for plant nutrient acquisition. The positive relationship between root exudation and root respiration and N concentration is a promising approach for the future prediction of root exudation.

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.

• Plant roots exhibit diverse root functional traits to enable soil phosphorus (P) acquisition, including changes in root morphology, root exudation and mycorrhizal symbioses. Yet, whether these traits are differently coordinated among crop species to enhance P acquisition is unclear. • Here, eight root functional traits for P acquisition were characterized in 16 major herbaceous crop species grown in a glasshouse under limiting and adequate soil P availability. • We found substantial interspecific variation in root functional traits among species. Those with thinner roots showed more root branching and less first-order root length, and had consistently lower colonization by arbuscular mycorrhizal fungi (AMF), fewer rhizosheath carboxylates and reduced acid phosphatase activity. In response to limiting soil P, species with thinner roots showed a stronger response in root branching, first-order root length and specific root length of the whole root system, Conversely, species with thicker roots exhibited higher colonization by AMF and/or more P-mobilizing exudates in the rhizosheath. • We conclude that, at the species level, tradeoffs occur among the three groups of root functional traits we examined. Root diameter is a good predictor of the relative expression of these traits and how they change when P is limiting.

The concept of a root economics space (RES) is increasingly adopted to explore root trait variation and belowground resource-acquisition strategies. Much progress has been made on interactions of root morphology and mycorrhizal symbioses. However, root exudation, with a significant carbon (C) cost (c. 5–21% of total photosynthetically fixed C) to enhance resource acquisition, remains a missing link in this RES. Here, we argue that incorporating root exudation into the structure of RES is key to a holistic understanding of soil nutrient acquisition. We highlight the different functional roles of root exudates in soil phosphorus (P) and nitrogen (N) acquisition. Thereafter, we synthesize emerging evidence that illustrates how root exudation interacts with root morphology and mycorrhizal symbioses at the level of species and individual plant and argue contrasting patterns in species evolved in P-impoverished vs N-limited environments. Finally, we propose a new conceptual framework, integrating three groups of root functional traits to better capture the complexity of belowground resource-acquisition strategies. Such a deeper understanding of the integrated and dynamic interactions of root morphology, root exudation, and mycorrhizal symbioses will provide valuable insights into the mechanisms underlying species coexistence and how to explore belowground interactions for sustainable managed systems.

Background Disentangling nutrient acquisition strategies between trees and crops is central to understanding positive nutrient interactions in agroforestry systems for improved low-input agriculture. However, as plants are responsive to a complex soil matrix at multiple scales, generalizable diagnostics across diverse agroforests remains challenging. Scope We synthesize research at various scales of the tree-crop interface that are cumulatively hypothesized to underpin nutrient acquisition strategies in agroforestry systems. These scales span the whole root system to fine-scale sites of acquisition actively engaged in biological and chemical interactions with soil. We target vertical and horizontal dimensions of acquisition patterns; localized root-soil dynamics including biological associations; root-scale plasticity for higher acquisition; and nutrient additions via biological nitrogen fixation and deep soil nutrient uplift. We consolidate methodological advances and the effects of environmental change on well-established nutrient interactions. Conclusions Root distribution patterns remain one of the most universal indicators of nutrient acquisition strategies in a range of agroforestry systems, while root functional traits are emerging as an effective root-scale indicator of nutrient acquisition strategy. We validate that in agroforestry systems crop root functional traits reveal bivariate trade-offs similar to, but weaker than, crops in monoculture, with mechanistic links to nutrient acquisition strategies. While interspecific root overlap may be associated with nutrient competition, clear cases of enhanced chemically and microbially meditated processes result in species- and management-specific nutrient facilitation. We argue for agroforestry science to use distinct and standardized nutrient acquisition indicators and processes at multiple scales to generate more nuanced, while also generalizable, diagnostics of tree-crop interactions. And extensive research is needed on how agroforestry practices stabilize key nutrient acquisition patterns in the face of environmental change.

• We investigated the variation in tree fine root traits and their functional diversity along a local topographic gradient in a Neotropical montane forest to test if fine root trait variation along the gradient is consistent with the predictions of the root economics spectrum on a shift from acquisitive to conservative traits with decreasing resource supply. • We measured five fine root functional traits in 179 randomly selected tree individuals of 100 species and analysed the variation of single traits (using Bayesian phylogenetic multilevel models) and of functional trait diversity with small-scale topography. • Fine roots exhibited more conservative traits (thicker diameters, lower specific root length and nitrogen concentration) at upper slope compared with lower slope positions, but the largest proportion of variation (40–80%) was explained by species identity and phylogeny. Fine root functional diversity decreased towards the upper slopes. • Our results suggest that local topography and the related soil fertility and moisture gradients cause considerable small-scale variation in fine root traits and functional diversity along tropical mountain slopes, with conservative root traits and greater trait convergence being associated with less favourable soil conditions due to environmental filtering. We provide evidence of a high degree of phylogenetic conservation in fine root traits.

Dimensions of tree root systems in savannas are poorly understood, despite being essential in resource acquisition and post-disturbance recovery. We studied tree rooting patterns in Southern African savannas to ask: how tree rooting strategies affected species responses to severe drought; and how potential rooting depths varied across gradients in soil texture and rainfall. First, detailed excavations of eight species in Kruger National Park suggest that the ratio of deep to shallow taproot diameters provides a reasonable proxy for potential rooting depth, facilitating extensive interspecific comparison. Detailed excavations also suggest that allocation to deep roots traded off with shallow lateral root investment, and that drought-sensitive species rooted more shallowly than drought-resistant ones. More broadly across 57 species in Southern Africa, potential rooting depths were phylogenetically constrained, with investment to deep roots evident among miombo Detarioids, consistent with results suggesting they green up before onset of seasonal rains. Soil substrate explained variation, with deeper roots on sandy, nutrient-poor soils relative to clayey, nutrient-rich ones. Although potential rooting depth decreased with increasing wet season length, mean annual rainfall had no systematic effect on rooting depth. Overall, our results suggest that rooting depth systematically structures the ecology of savanna trees. Further work examining other anatomical and physiological root traits should be a priority for understanding savanna responses to changing climate and disturbances.

Plant roots and the associated mycorrhizal fungal hyphae often selectively proliferate into patchily distributed soil nutrient hotspots, but interactions between these two components of a mycorrhizal root system are usually ignored or experimentally isolated in nutrient foraging studies. From studies in which both roots and mycorrhizal hyphae had access to nutrient hotspots, we compiled data on root foraging precision (increase in roots in nutrient hotspots relative to outside hotspots) of plant species from different ecosystems, ranging from temperate grasslands to subtropical forests. We found that root foraging precision across the wide range of plant species was strongly influenced by root morphology and mycorrhizal type. The precision of root nutrient foraging, as a plant functional trait, may coordinate with other root traits that are related to the economics of nutrient acquisition. High foraging precision is expected to associate with the strategy of fast return on the investment in roots, such as low construction cost, high metabolic rate and rapid turnover. Nutrient foraging by mycorrhizal fungi alone may be influenced by functional traits such as hyphal exploration distance, hyphal turnover, and hyphal uptake capacity and efficiency, but such data are limited to a small portion of mycorrhizal fungal species. We propose a conceptual framework in which to simulate nitrogen and phosphorus acquisition from both nutrient hotspots and outside hotspots in mixed‐species plant communities. Simulation outputs suggest that plant species with varying root morphology and mycorrhizal type can be adaptive to a range of nutrient heterogeneity. Although there are still knowledge gaps related to nutrient foraging, as well as many unexplored plant and fungal species, we suggest that scaling nutrient foraging from individual plants to communities would advance understanding of plant species interactions and below‐ground ecosystem function. A plain language summary is available for this article. Plain Language Summary