Summary paragraphRecent studies of plant fine roots have greatly advanced our understanding of their geometric properties and symbiotic relationships, but knowledge of how these roots are spatially distributed across the soil matrix lags far behind. An improved understanding of broad-scale variability in root vertical distribution is critical for understanding plant-soil-atmosphere interactions and their influence on the land carbon sink. Here we analyze a continental-scale dataset of plant roots reaching 2-meters depth, spanning 19 ecoclimatic domains ranging from Alaskan tundra to Puerto Rican neotropical forest. Contrary to the common expectation that fine root abundance decays exponentially with increasing soil depth, we found surprising root bimodality at ~20% of 44 field sites —a secondary peak of fine root biomass far beneath the soil surface. All of the secondary root peaks were observed deeper than 60cm (with 33% below 1m), far deeper than the sampling depth commonly used in ecosystem studies and forestry surveys. We demonstrate that root bimodality is more likely in places with relatively low total fine root biomass, and is more frequently associated with shrubland vegetation but less with grassland. Further statistical analyses revealed that the secondary peak of root biomass coincided with unexpected high soil nitrogen contents at depth. By linking roots and nutrient distributions, we further demonstrate that deep soil nutrients tend to be underexploited by plant rooting systems, yet root bimodality offers a unique mechanism by which fine roots can tap into soil resources in the deep. Our findings suggest that empirical practices have often systematically overlooked root dynamics in deep soils, and as a result the current-generation global climate and vegetation models have relied on overly simplistic assumptions for plant rooting distribution.

Although ectomycorrhizal (ECM) fungi are well recognized symbionts impacting tree health and ecosystem productivity globally, understanding of their timing of proliferation in soils across seasons remains limited. We analyzed intra-annual patterns of ECM fungal abundance and community structure in five monodominant forest plots in the midwestern USA via quantitative PCR and Illumina sequencing from soil cores collected at monthly intervals. We found that dynamics of ECM fungal seasonality differed by host tree leaf habit, fungal exploration type, and fungal genus. ECM fungal total abundances and species richness were more dynamic in deciduous than evergreen plots. Short-delicate exploration types peaked earlier in spring, while short-coarse peaked later in autumn, and medium-smooth and long-distance dipped in mid-year summer. With evergreen hosts, Amphinema peaked in spring, then Tuber and Cortinarius in summer, and Wilcoxina in autumn. With deciduous hosts, Cenococcum peaked slightly earlier in summer and Cortinarius declined in summer. Contrary to expectations, neither soil temperature nor moisture consistently predicted ECM fungal abundances. Our findings suggest that phenology is an important ECM fungal trait best explained by both host and fungal contributions. Future studies should consider sampling across multiple seasons to expand understanding of seasonality drivers of ECM fungal phenology in other biomes. Summary The timing of belowground production is poorly understood for ectomycorrhizal fungal root symbionts. We collected soils monthly from five monodominant forest plots. Host tree leaf habit and ectomycorrhizal exploration strategy explain seasonal variation in fungal community abundance and composition. Future studies should sample across seasons to best characterize communities, and carbon models should consider seasonality in soil fungal production. ### Competing Interest Statement The authors have declared no competing interest. National Science Foundation, https://ror.org/021nxhr62, 2129312 United States Department of Energy, DE-SC0023480

Tree roots form symbioses with soil microbes to acquire nutrients, but the relationships between root nutrient acquisition strategies and microbial community composition remain poorly understood. Here, we measured root traits and root-associated fungal and bacterial guilds in 336 trees of 52 species from a subtropical forest. We found a fungal gradient from ectomycorrhizal to saprotrophic dominance, which corresponded with a shift from organic to mineral nutrient economics. This fungal gradient was aligned with the increase of root nitrogen concentration, suggesting a linkage from simple root trait to fungal-mediated carbon-nutrient cycling. We also found that the functional composition of fungal and bacterial communities was closely correlated with host root-zone pH, which often varied among coexisting trees. Root-zone pH was independent of the common root traits, underpinning a potential new gradient in the root trait space. Our findings integrate microbial functions into the root economics framework, thereby advancing the understanding of diversity of nutrient acquisition strategies across forest trees.

Fine‐root branching, vertical distribution and morphology together with root growth rate are key dimensions that determine root strategies for belowground resource acquisition. However, few studies have addressed these traits together with coordinated measures of root growth rates, limiting generalizations about how these root traits coordinate among species. We conducted a common garden experiment to examine interspecific variation and coordination among architectural and morphological traits together with vertical distribution and growth rate of fine‐roots (≤ 2 mm in diameter) across 11 temperate shrub species. Across all species, root morphological traits showed only moderate differences among the first three branching orders and changed more dramatically in higher orders. We found that thin‐rooted shrub species had greater branching intensity than thick‐rooted species. Live fine‐root density (length and mass) decreased as an exponential pattern with increasing soil depth while the density of dead fine‐roots remained relatively constant. Patterns of fine‐root growth rates were independent of morphological and architectural traits, but were negatively related to rooting depth. The different root traits and relationships observed suggest diverse strategies for soil resource acquisition among shrub species. A deep root system would be associated with a slow growth rate. In contrast, the rooting depth was largely independent of root architecture and morphology.

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Convergent patterns in morphological and genetic traits of mycorrhizas have been well-documented and reflect common selection forces that define mycorrhizas. However, generalizable patterns of mycorrhiza-associated chemical alterations, which are immediately linked to plant and fungal strategies for successful symbiosis, have yet to be emerged. Comparing root metabolomes of phylogenetically-diverse plants inoculated by mycorrhizal fungi across two major lifestyles (arbuscular- vs . ecto-mycorrhizas), our study uncovers metabolite changes unique to each mycorrhizal lifestyle and those common across plant-mycorrhizal combinations irrespective of lifestyles. Arbuscular and ecto- mycorrhizal colonized roots accumulated different sets of carbohydrates, indicating unique carbon partitioning strategies: particularly, arbuscular mycorrhizal roots accumulated cyclic polyols inaccessible for symbionts, suggesting tighter regulation of plants in carbon partitioning. Mycorrhizas also altered specialized metabolism, featuring frequent increases of flavan-3-ols and decreases of flavanols irrespective of mycorrhizal lifestyles, suggesting tactical reconfiguration of specialized metabolites to facilitate/contain symbiosis. Our data show for the first time, to our knowledge, that part of the root metabolite alterations by mycorrhizas were relatively common across plant-mycorrhizal systems, highlighting their potentially critical regulatory and evolutionary role for successful symbiosis. This commonality appears robust to phylogenetic diversity of host plants and thus may be widespread in land plants. Our findings offer future research venues to elucidate the finer roles of these common traits of mycorrhiza-associated metabolite alterations and thus help to eventually develop a comprehensive understanding of this omnipresent plant-fungus partnership.

Rock fragments are widespread in soil profiles. Despite direct effects of rock fragment content (RFC) on vegetation and soil properties, how plants respond to variations in RFC remains poorly understood. In this work, we investigated responses of two contrasting xerophytic species to varying RFC. Root biomass allocation, vertical distribution and above-ground growth were measured in Artemisia vestita and Bauhinia brachycarpa after 2 years of growth in an experiment with four levels of RFC (0, 25, 50 and 75% ν ν−1). The responses of above-ground growth and total biomass of both species showed a unimodal curve with values increasing up to intermediate RFC (25% and 50%) and then declining. Both species increased relative biomass allocation to roots at the highest RFC level (75%). A. vestita had a shallow rooting profile and greater declines in plant growth with high RFC compared with B. brachycarpa which had a deeper rooting profile. We found that intermediate RFCs were beneficial for growth of both species and both species increased root-to-shoot ratios to compensate for high RFC. The higher overall root fraction and deeper rooting profile may make B. brachycarpa more suitable than A. vestita for areas with high RFC, enabling greater extraction of increasingly limited soil resources.

Whether and how warming alters functional traits of absorptive plant roots remains to be answered across the globe. Tackling this question is crucial to better understanding terrestrial responses to climate change as fine-root traits drive many ecosystem processes. We carried out a detailed synthesis of fine-root trait responses to experimental warming by performing a meta-analysis of 964 paired observations from 177 publications. Warming increased fine-root biomass, production, respiration and nitrogen concentration as well as decreased root carbon : nitrogen ratio and nonstructural carbohydrates. Warming effects on fine-root biomass decreased with greater warming magnitude, especially in short-term experiments. Furthermore, the positive effect of warming on fine-root biomass was strongest in deeper soil horizons and in colder and drier regions. Total fine-root length, morphology, mortality, life span and turnover were unresponsive to warming. Our results highlight the significant changes in fine-root traits in response to warming as well as the importance of warming magnitude and duration in understanding fine-root responses. These changes have strong implications for global soil carbon stocks in a warmer world associated with increased root-derived carbon inputs into deeper soil horizons and increases in fine-root respiration.

Harsh environmental conditions affect both leaf structure and root traits. However, shoot growth in high‐latitude systems is predominately under photoperiod control while root growth may occur for as long as thermal conditions are favorable. The different sensitivities of these organs may alter functional relationships above‐ and belowground along environmental gradients. We examined the relationship between absorptive root and foliar traits of Scots pine trees growing in situ along a temperate‐boreal transect and in trees grown in a long‐term common garden at a temperate latitude. We related changes in foliar nitrogen, phosphorus, specific leaf area, needle mass and 13C signatures to geographic trends in absorptive root biomass to better understand patterns of altered tree nutrition and water balance. Increased allocation to absorptive fine roots was associated with greater uptake of soil nutrients and subsequently higher needle nutrient contents in the northern provenances compared with more southern provenances when grown together in a common garden setting. In contrast, the leaf δ13C in northern and southern provenances were similar within the common garden suggesting that higher absorptive root biomass fractions could not adequately increase water supply in warmer climates. These results highlight the importance of allocation within the fine‐root system and its impacts on needle nutrition while also suggesting increasing stomatal limitation of photosynthesis in the context of anticipated climatic changes.

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Root systems show a tremendous diversity both between and within species, suggesting a large variability in plant functioning and effects on ecosystem properties and processes. In recent decades, developments in many areas of root research have brought considerable advances in our understanding of root traits and their contribution to plant and ecosystem functioning. However, despite major progress, a comprehensive overview—bridging research fields—is lacking. Furthermore, considerable uncertainties exist in the identification of root entities, and the selection and standardized measurement of traits. Here, we provide a comprehensive overview on root entities, exemplify recent advances in our understanding of both theoretical and demonstrated relationships between root traits and plant or ecosystem functioning, discuss trait-trait relationships and hierarchies among traits, and critically assess current strengths and gaps in our knowledge.

Key functions of fine roots are often related to their morphological traits, yet little is known about the patterns and controls on fine-root morphological traits in the tropical forest biome. In this study, we consolidated data on key root morphological traits to describe patterns of root trait variation among different tropical regions and examined the relationships among root traits and climate and soil properties. We synthesized root traits (root diameter, specific root length (SRL), specific root area (SRA) and root tissue density (RTD)) from 59 site observations from nine countries in Africa, Asia, and Central and South America to determine the patterns and variation in root traits among different tropical regions. We also examined relationships among fine-root morphological traits, climate (mean annual precipitation, MAP; mean annual temperature, MAT) and soil properties (including available phosphorus (P), base saturation and clay content) using linear mixed-effects modelling. Plant fine-root morphological traits showed systematic variation among tropical regions-Africa, Asia and the Neotropics. Across the tropical biome, SRL was positively related to MAT, suggesting that in warmer tropical sites, plants tended to produce thinner roots with high SRL. Specific root length and SRA were positively related to base saturation, while soil available P explained some of the variation in SRL. This study demonstrates that soil properties, and to a lesser extent MAT, partially explain variation in key fine-root morphological traits across tropical forests. Importantly, we also identified wide variation in fine-root morphological trait values in the tropical biome that encompasses much of the variation seen worldwide. Consequently, attempts to predict tropical forest ecosystem functioning could improve significantly if regional differences in root traits are incorporated into process-based models.

Plant economics run on carbon and nutrients instead of money. Leaf strategies aboveground span an economic spectrum from "live fast and die young" to "slow and steady," but the economy defined by root strategies belowground remains unclear. Here, we take a holistic view of the belowground economy and show that root-mycorrhizal collaboration can short circuit a one-dimensional economic spectrum, providing an entire space of economic possibilities. Root trait data from 1810 species across the globe confirm a classical fast-slow "conservation" gradient but show that most variation is explained by an orthogonal "collaboration" gradient, ranging from "do-it-yourself" resource uptake to "outsourcing" of resource uptake to mycorrhizal fungi. This broadened "root economics space" provides a solid foundation for predictive understanding of belowground responses to changing environmental conditions.

Fine roots mediate below-ground resource acquisition, yet understanding of how fine-root functional traits vary along environmental gradients, within branching orders and across phylogenetic scales remains limited. Morphological and architectural fine-root traits were measured on individual root orders of 20 oak species (genus Quercus) from divergent climates of origin that were harvested after three growing seasons in a glasshouse. These were then compared with similar measurements obtained from a phylogenetically diverse dataset of woody species from the Fine-Root Ecology Database (FRED). For the oaks, only precipitation seasonality and growing season moisture availability were correlated to aspects of root diameter and branching. Strong correlations among root diameters and architecture of different branch orders were common, while correlations between diameter and length were weakly negative. By contrast, the FRED dataset showed strong positive correlations between diameter and length and fewer correlations between root diameter and architectural traits. Our findings suggest that seasonal patterns of water availability are more important drivers of root adaptation in oaks than annual averages in precipitation and temperature. Furthermore, contrasting patterns of trait relationships between the oak and FRED datasets suggest that branching patterns are differentially constrained at narrow vs broad phylogenetic scales.

Fine root decomposition constitutes a critical yet poorly understood flux of carbon and nutrients in terrestrial ecosystems. Here, we present the first large-scale synthesis of species trait effects on the early stages of fine root decomposition at both global and local scales. Based on decomposition rates for 279 plant species across 105 studies and 176 sites, we found that mycorrhizal association and woodiness are the best categorical traits for predicting rates of fine root decomposition. Consistent positive effects of nitrogen and phosphorus concentrations and negative effects of lignin concentration emerged on decomposition rates within sites. Similar relationships were present across sites, along with positive effects of temperature and moisture. Calcium was not consistently related to decomposition rate at either scale. While the chemical drivers of fine root decomposition parallel those of leaf decomposition, our results indicate that the best plant functional groups for predicting fine root decomposition differ from those predicting leaf decomposition.

Fine‐root traits show remarkable variation with plant community structure and environmental shifts, but there is limited understanding of how trait covariation that exists among fine‐root traits shifts among different communities, especially in forests. We explored links among the fine‐root traits of forest communities to determine whether community root traits shift predictably according to an economics framework along environmental gradients. Measurements of root morphology, nitrogen, phosphorus and carbon concentrations, and measures of standing root densities were collected on fine‐roots (diameter ≤ 2 mm) from 129 forest plots in five subalpine forests. This study demonstrates an existence of a community‐level fine‐root economics spectrum (REScom) in the subalpine forests, in which specific root length was strongly and positively related to root nitrogen and phosphorus contents, but negatively related to root diameter. Soil nutrient limitation was a major driver of the REScom as changes in soil [N], [P] and [C] contents were related to changes in SRL, root [N], root [P]. Variables related to standing fine‐root length and mass were independent of the REScom and were primarily related to forest community structure, particularly in tree closure and herbaceous plant mass. Synthesis. These results indicate two distinct functional dimensions of community fine‐root trait variation: resource‐use efficiency via changes in root structure and construction, and separately via changes in the standing root system. Identifying shifts in allocation to and investment in fine‐roots enhances our understanding of a root and whole‐plant economics spectrum and community functioning.

Aim Rising air temperature and changing precipitation patterns already strongly influence forest ecosystems, yet large-scale patterns of belowground root trait variation and their underlying drivers are poorly understood. Here, we investigated general patterns of root tip adjustments within fine-root systems and the potential ecological implications of these patterns. Location Global. Methods We synthesize key fine-root traits related to resource acquisition and determined their responses along climate and edaphic gradients. We specifically identified patterns of root tip abundance (number of root tips per dry biomass of fine roots <= 2 mm in diameter), and root tip density (number of root tips per soil volume) among angiosperm and gymnosperm trees to climate, edaphic gradients and stand properties. Results We found that angiosperm trees, which were more common in warmer, sometimes drier climates with more fertile soil, formed more root tips (higher root tip abundance, root tip density and higher slope of root tip density vs. fine-root biomass) than gymnosperm trees, which lived in cooler, wetter climates with poor soil. Angiosperm and gymnosperm trees exhibited opposing trends in response to gradients in climate as gymnosperm trees tended to decrease root tip abundance and root tip density but alternatively increase mycorrhizal mycelial biomass with increasing MAT/MAP (ratio of mean annual temperature to mean annual precipitation), while angiosperm trees tended to increase root tip abundance and root tip density with increasing MAT/MAP. However, the individual trends of root tip abundance and root tip density for angiosperm and gymnosperm trees to MAT or MAP were more similar and often non-significant. Main conclusions These results suggest disparate carbon or biomass adjustment strategies within gymnosperm and angiosperm tree fine-root systems along climate gradients. Differences in angiosperm and gymnosperm tree adjustments in their fine-root systems to changing environments have implications for how these plant groups are likely to perform in different environments and how their responses to future climate change should be modelled.

Variation in resource acquisition strategies enables plants to adapt to different environments and may partly determine their responses to climate change. However, little is known about how belowground plant traits vary across climate and soil gradients. Focusing on interior Douglas-fir (Pseudotsuga menziesii var. glauca) in western Canada, we tested whether fine-root traits relate to the environment at the intraspecific level. We quantified the variation in commonly measured functional root traits (morphological, chemical, and architectural traits) among the first three fine-root orders (i.e., absorptive fine roots) and across biogeographic gradients in climate and soil factors. Moderate but consistent trait-environment linkages occurred across populations of Douglas-fir, despite high levels of within-site variation. Shifts in morphological traits across regions were decoupled from those in chemical traits. Fine roots in colder/drier climates were characterized by a lower tissue density, higher specific area, larger diameter, and lower carbon-to-nitrogen ratio than those in warmer/wetter climates. Our results showed that Douglas-fir fine roots do not rely on adjustments in architectural traits to adapt rooting strategies in different environments. Intraspecific fine-root adjustments at the regional scale do not fit along a single axis of root economic strategy and are concordant with an increase in root acquisitive potential in colder/drier environments.