Root structure-function relationships in 74 species: evidence of a root economics spectrum related to carbon economy

Biomass, Morphology, and Dynamics of the Fine Root System Across a 3,000-M Elevation Gradient on Mt. Kilimanjaro

Fine roots (≤2 mm) consume a large proportion of photosynthates and thus play a key role in the global carbon cycle, but our knowledge about fine root biomass, production, and turnover across environmental gradients is insufficient, especially in tropical ecosystems. Root system studies along elevation transects can produce valuable insights into root trait-environment relationships and may help to explore the evidence for a root economics spectrum (RES) that should represent a trait syndrome with a trade-off between resource acquisitive and conservative root traits. We studied fine root biomass, necromass, production, and mean fine root lifespan (the inverse of fine root turnover) of woody plants in six natural tropical ecosystems (savanna, four tropical mountain forest types, tropical alpine heathland) on the southern slope of Mt. Kilimanjaro (Tanzania) between 900 and 4,500 m a.s.l. Fine root biomass and necromass showed a unimodal pattern along the slope with a peak in the moist upper montane forest (~2,800 m), while fine root production varied little between savanna and upper montane forest to decrease toward the alpine zone. Root:shoot ratio (fine root biomass and production related to aboveground biomass) in the tropical montane forest increased exponentially with elevation, while it decreased with precipitation and soil nitrogen availability (decreasing soil C:N ratio). Mean fine root lifespan was lowest in the ecosystems with pronounced resource limitation (savanna at low elevation, alpine heathland at high elevation) and higher in the moist and cool forest belt (~1,800-3,700 m). The variation in root traits across the elevation gradient fits better with the concept of a multi-dimensional RES, as root tissue density and specific root length showed variable relations to each other, which does not agree with a simple trade-off between acquisitive and conservative root traits. In conclusion, despite large variation in fine root biomass, production, and morphology among the different plant species and ecosystems, a general belowground shift in carbohydrate partitioning is evident from 900 to 4,500 m a.s.l., suggesting that plant growth is increasingly limited by nutrient (probably N) shortage toward higher elevations.

Distinct fine-root responses to precipitation changes in herbaceous and woody plants: a meta-analysis

Precipitation is one of the most important factors that determine productivity of terrestrial ecosystems. Precipitation across the globe is predicted to change more intensively under future climate change scenarios, but the resulting impact on plant roots remains unclear. Based on 154 observations from experiments in which precipitation was manipulated in the field and root biomass was measured, we investigated responses in fine-root biomass of herbaceous and woody plants to alterations in precipitation. We found that root biomass of herbaceous and woody plants responded differently to precipitation change. In particular, precipitation increase consistently enhanced fine-root biomass of woody plants but had variable effects on herb roots in arid and semi-arid ecosystems. In contrast, precipitation decrease reduced root biomass of herbaceous plants but not woody plants. In addition, with precipitation alteration, the magnitude of root responses was greater in dry areas than in wet areas. Together, these results indicate that herbaceous and woody plants have different rooting strategies to cope with altered precipitation regimes, particularly in water-limited ecosystems. These findings suggest that root responses to precipitation change may critically influence root productivity and soil carbon dynamics under future climate change scenarios.

Contrasting Root System Structure and Belowground Interactions between Black Spruce (Picea mariana (Mill.) B.S.P) and Trembling Aspen (Populus tremuloides Michx) in Boreal Mixedwoods of Eastern Canada

This study explored the underground interactions between black spruce and trembling aspen in pure and mixed stands to understand how their soil resource use help these species coexist in the boreal mixedwoods of Western Quebec. We analyzed species-specific fine root foraging strategies (root biomass and root tissue density) along three soil layers (organic, top 0–15 cm, and bottom 15–30 cm mineral soil), using 180 soil cores. We collected cores in three sites, each containing three 20 × 50 m2 plots of pure spruce, pure aspen, and mixed spruce and aspen stands. Spruce had a shallow rooting, whereas aspen had a deep rooting in both types of stands. Compared to pure spruce stands, spruce had a lower fine root biomass (FRB) and a higher root tissue density (RTD) in the organic layer of mixed stands. Both patterns were indicative of spruce’s more intensive resource use strategy and competitive advantage over aspen in that layer. Aspen FRB in the organic soil did not differ significantly between pure and mixed stands, but increased in the mineral soil of mixed stands. Since we did not observe a significant difference in the nutrient content of the mineral soil layer between pure aspen and mixed stands, we concluded that aspen may experience competitive exclusion in the organic layer by spruce. Aspen exhibited an extensive nutrient uptake strategy in the organic layer of mixed stands: higher FRB and lower RTD than spruce. In mixed stands, the differences in aspen rooting patterns between the organic and mineral layers suggested the use of contrasting nutrient uptake strategies along the soil profile. We speculate that the stronger spatial separation of the roots of spruce and aspen in mixed stands likely contribute to a higher partitioning of their nutrient uptake along the soil profile. These results indicate the competitive exclusion of aspen by spruce in boreal mixedwoods, which likely occurs in the soil organic layer.

A plant economics spectrum of litter decomposition among coexisting fern species in a sub-tropical forest

BACKGROUND AND AIMS

The plant economics spectrum theory provides a useful framework to examine plant strategies by integrating the co-ordination of plant functional traits along a resource acquisition-conservation trade-off axis. Empirical evidence for this theory has been widely observed for seed plants (Spermatophyta). However, whether this theory can be applied to ferns (Pteridophyta), a ubiquitous and ancient group of vascular plants, has rarely been evaluated so far.

METHODS

We measured 11 pairs of plant functional traits on leaves and fine roots (diameter <2 mm) on 12 coexisting fern species in a sub-tropical forest. Litterbags of leaves and roots were placed in situ and exposed for 586 d to measure decomposition rates. The variation of traits across species and the co-ordination among traits within and between plant organs were analysed. Finally, the influence of the traits on decomposition rates were explored.

KEY RESULTS

Most leaf and root traits displayed high cross-species variation, and were aligned along a major resource acquisition-conservation trade-off axis. Many fern traits co-varied between leaves and fine roots, suggesting co-ordinated responses between above- and below-ground organs. Decomposition rates of leaves were significantly higher than those of fine roots, but they were significantly and positively correlated. Finally, our results highlight that the decomposition of both leaves and roots was relatively well predicted by the leaf and root economics spectra.

CONCLUSIONS

Our results support the existence of an acquisition-conservation trade-off axis within ferns and indicate that traits have important 'afterlife' effects on fern litter decomposition. We conclude that the plant economics spectrum theory that is commonly observed across seed plants can be applied to ferns species, thereby extending the generality of this theory to this ancient plant lineage in our study site. Our study further suggests that the evolutionary and ecological basis for the relationships among key economics traits appears to be similar between ferns and seed plants. Future studies involving larger data sets will be required to confirm these findings across different biomes at larger spatial scales.

Maize with fewer nodal roots allocates mass to more lateral and deep roots that improve nitrogen uptake and shoot growth

Simulations indicated that reduced nodal root (NR) number (NRN) was promising for maize breeding, and were partially confirmed by relying on variation in NRN among inbreds. Using maize inbred line B73, experiments were conducted in hydroponics and tall mesocosms containing solid media with treatments involving no NR excision (0% NRE) or excising either 33% or 67% of the NRs as they emerged under high or low levels of nitrogen (N). Reduced NRN was hypothesized to increase elongation of all remaining root classes, N acquisition under low N, and shoot mass. Plants with 67% NRE had 12% and 19% less root mass fraction, 61% and 91% greater lateral to axial root length ratio regardless of N levels, and 61% and 182% greater biomass of embryonic roots under low N, compared with 0% NRE for hydroponics and mesocosms studies, respectively. Under low N in mesocosms, plants with 67% NRE had 52% greater shoot biomass, 450% greater root length at depth, and 232% greater deep-injected 15N content in the shoot relative to 0% NRE. These results reveal the mechanism by which plants with fewer NRs increase N capture and shoot mass by reallocation of biomass to lateral roots, embryonic roots, and first whorl NRs that increases foraging efficiency in solid media.

Is there coordination of leaf and fine root traits at local scales? A test in temperate forest swamps

Examining the coordination of leaf and fine root traits not only aids a better understanding of plant ecological strategies from a whole-plant perspective, but also helps improve the prediction of belowground properties from aboveground traits. The relationships between leaf and fine root traits have been extensively explored at global and regional scales, but remain unclear at local scales. Here, we measured six pairs of analogous leaf and fine root traits related to resource economy and organ size for coexisting dominant and subordinate vascular plants at three successional stages of temperate forest swamps in Lingfeng National Nature Reserve in the Greater Hinggan Mountains, NE China. Leaf and fine root traits related to resource acquisition (e.g., specific leaf area [SLA], leaf N, leaf P, root water content, and root P) decreased with succession. Overall, we found strong linear relationships between leaf dry matter content (LDMC) and root water content, and between leaf and root C, N, and P concentrations, but only weak correlations were observed between leaf area and root diameter, and between SLA and specific root length (SRL). The strong relationships between LDMC and root water content and between leaf and root C, N, and P held at the early and late stages, but disappeared at the middle stage. Besides, C and P of leaves were significantly correlated with those of roots for woody plants, while strong linkages existed between LDMC and root water content and between leaf N and root N for herbaceous species. These results provided evidence for the existence of strong coordination between leaf and root traits at the local scale. Meanwhile, the leaf-root trait relationships could be modulated by successional stage and growth form, indicating the complexity of coordination of aboveground and belowground traits at the local scale.

Tradeoffs among root morphology, exudation and mycorrhizal symbioses for phosphorus-acquisition strategies of 16 crop species

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.

Relationships between plant traits, soil properties and carbon fluxes differ between monocultures and mixed communities in temperate grassland

The use of plant traits to predict ecosystem functions has been gaining growing attention. Above-ground plant traits, such as leaf nitrogen (N) content and specific leaf area (SLA), have been shown to strongly relate to ecosystem productivity, respiration and nutrient cycling. Furthermore, increasing plant functional trait diversity has been suggested as a possible mechanism to increase ecosystem carbon (C) storage. However, it is uncertain whether below-ground plant traits can be predicted by above-ground traits, and if both above- and below-ground traits can be used to predict soil properties and ecosystem-level functions.Here, we used two adjacent field experiments in temperate grassland to investigate if above- and below-ground plant traits are related, and whether relationships between plant traits, soil properties and ecosystem C fluxes (i.e. ecosystem respiration and net ecosystem exchange) measured in potted monocultures could be detected in mixed field communities.We found that certain shoot traits (e.g. shoot N and C, and leaf dry matter content) were related to root traits (e.g. root N, root C:N and root dry matter content) in monocultures, but such relationships were either weak or not detected in mixed communities. Some relationships between plant traits (i.e. shoot N, root N and/or shoot C:N) and soil properties (i.e. inorganic N availability and microbial community structure) were similar in monocultures and mixed communities, but they were more strongly linked to shoot traits in monocultures and root traits in mixed communities. Structural equation modelling showed that above- and below-ground traits and soil properties improved predictions of ecosystem C fluxes in monocultures, but not in mixed communities on the basis of community-weighted mean traits. Synthesis. Our results from a single grassland habitat detected relationships in monocultures between above- and below-ground plant traits, and between plant traits, soil properties and ecosystem C fluxes. However, these relationships were generally weaker or different in mixed communities. Our results demonstrate that while plant traits can be used to predict certain soil properties and ecosystem functions in monocultures, they are less effective for predicting how changes in plant species composition influence ecosystem functions in mixed communities.

Intraspecific Fine-Root Trait-Environment Relationships across Interior Douglas-Fir Forests of Western Canada

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.

Nonlinearity of root trait relationships and the root economics spectrum

The root economics spectrum (RES), a common hypothesis postulating a tradeoff between resource acquisition and conservation traits, is being challenged by conflicting relationships between root diameter, tissue density (RTD) and root nitrogen concentration (RN). Here, we analyze a global trait dataset of absorptive roots for over 800 plant species. For woody species (but not for non-woody species), we find nonlinear relationships between root diameter and RTD and RN, which stem from the allometric relationship between stele and cortical tissues. These nonlinear relationships explain how sampling bias from different ends of the nonlinear curves can result in conflicting trait relationships. Further, the shape of the relationships varies depending on evolutionary context and mycorrhizal affiliation. Importantly, the observed nonlinear trait relationships do not support the RES predictions. Allometry-based nonlinearity of root trait relationships improves our understanding of the ecology, physiology and evolution of absorptive roots.

Kong et al. use a global trait dataset of 800 plant species to examine the root economics spectrum in relation to root diameter, tissue density and root nitrogen concentration. Nonlinear trait relationships were observed, suggesting allometry-based nonlinearity in root trait relationships.

Rhizobacterial Community Assembly Patterns Vary Between Crop Species

Currently our limited understanding of crop rhizosphere community assembly hinders attempts to manipulate it beneficially. Variation in root communities has been attributed to plant host effects, soil type, and plant condition, but it is hard to disentangle the relative importance of soil and host without experimental manipulation. To examine the effects of soil origin and host plant on root associated bacterial communities we experimentally manipulated four crop species in split-plot mesocosms and surveyed variation in bacterial diversity by Illumina amplicon sequencing. Overall, plant species had a greater impact than soil type on community composition. While plant species associated with different Operational Taxonomic Units (OTUs) in different soils, plants tended to recruit bacteria from similar, higher order, taxonomic groups in different soils. However, the effect of soil on root-associated communities varied between crop species: Onion had a relatively invariant bacterial community while other species (maize and pea) had a more variable community structure. Dynamic communities could result from environment specific recruitment, differential bacterial colonization or reflect broader symbiont host range; while invariant community assembly implies tighter evolutionary or ecological interactions between plants and root-associated bacteria. Irrespective of mechanism, it appears both communities and community assembly rules vary between crop species.

Leaf litter decay rates differ between mycorrhizal groups in temperate, but not tropical, forests

Whereas the primary controls on litter decomposition are well established, we lack a framework for predicting interspecific differences in litter decay within and across ecosystems. Given previous research linking tree mycorrhizal association with carbon and nutrient dynamics, we hypothesized that the two dominant mycorrhizal groups in forests - arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi - differ in litter decomposition rates. We compiled leaf litter chemistry and decay data for AM- and ECM-associating angiosperms and gymnosperms (> 200 species) from temperate and tropical/subtropical, and investigated relationships among decay rates, mycorrhizal association, phylogeny and climate. In temperate forests, AM litters decayed faster than ECM litters, with litter nitrogen and phylogeny best explaining variation in litter decay. In sub/tropical forests, we found no significant difference in litter decay rate between mycorrhizal groups, and variation in decay rates was best explained by litter phosphorus. Our results suggest that knowledge of tree mycorrhizal association may improve predictions of species effects on ecosystem processes, particularly in temperate forests where AM and ECM species commonly co-occur, providing a predictive framework for linking litter quality, organic matter dynamics and nutrient acquisition in forests.

Fine-root traits are linked to species dynamics in a successional plant community

Despite the importance of fine roots for the acquisition of soil resources such as nitrogen and water, the study of linkages between traits and both population and community dynamics remains focused on aboveground traits. We address this gap by investigating associations between belowground traits and metrics of species dynamics. Our analysis included 85 species from a long-term data set on the transition from old field to forest in eastern North America (the Buell-Small Succession Study) and the new Fine-Root Ecology Database. Given the prominent roles of life form (woody vs. non-woody) and species origin (native vs. exotic) in defining functional relationships, we also assessed whether traits or their relationships with species dynamics differed for these groups. Species that reached their peak abundance early in succession had fine-root traits corresponding to resource acquisitive strategies (i.e., they were thinner, less dense, and had higher nitrogen concentrations) while species that peaked progressively later had increasingly conservative strategies. In addition to having more acquisitive root traits than native species, exotics diverged from the above successional trend, having consistently thinner fine roots regardless of the community context. Species with more acquisitive fine-root morphologies typically had faster rates of abundance increase and achieved their maximal rates in fewer years. Decreasing soil nutrient availability and increasing belowground competition may become increasingly strong filters in successional communities, acting on root traits to promote a transition from acquisitive to conservative foraging. However, disturbances that increase light and soil resource availability at local scales may allow acquisitive species, especially invasive exotics, to continue colonizing late into the community transition to forest.

Screening for Sugarcane Root Phenes Reveals That Reducing Tillering Does Not Lead to an Increased Root Mass Fraction

Sugarcane root systems are poorly studied and understood due to the perennial nature, tall stature, and the long cropping cycle. Whilst some field studies gave insights into sugarcane root traits, there is no detailed description of root and root system traits available. The objectives of our work were to establish a baseline of sugarcane root trait values that will serve for future studies, and to characterize the degree of root system resilience when restricting tiller number. We first conducted an initial screening for root trait diversity on a collection of twenty cultivars representative of sugarcane breeding from 1930 to now. Then we investigated the effect of reduced tillering, via manual de-tillering, on the plant root and root system traits of five varieties grown under optimal conditions in a glasshouse for 1700°Cd. In addition to establishing baseline means and variation for sugarcane root trait values that could serve as a reference for crop models, we demonstrated that the sugarcane root mass fraction was extremely resilient to drastic reduction in tiller number. Restricted plants were effectively maintaining their root system configuration (opening angle) by dramatically increasing the number of nodal roots produced per tiller as well as maximizing total root length by increasing the specific root length. Using this knowledge of sugarcane root traits in combination with the specific agronomic constraints for sugarcane will now underpin the development of a root system ideotype for sugarcane to enable targeted root trait selection for improving crop productivity.

Physical and Functional Constraints on Viable Belowground Acquisition Strategies

Since their emergence onto land, terrestrial plants have developed diverse strategies to acquire soil resources. However, we lack a framework that adequately captures how these strategies vary among species. Observations from around the world now allow us to quantify the variation observed in commonly-measured fine-root traits but it is unclear how root traits are interrelated and whether they fall along an "economic" spectrum of acquisitive to conservative strategies. We assessed root trait variation and mycorrhizal colonization rates by leveraging the largest global database of fine-root traits (the Fine-Root Ecology Database; FRED). We also developed a heuristic model to explore the role of mycorrhizal fungi in defining belowground exploration efficiency across a gradient of thin- to thick-diameter roots. In support of the expectations of the "root economic spectrum," we found that root diameter was negatively related to specific root length (Pearson's r=-0.76). However, we found an unexpected negative relationship between root diameter and root tissue density (Pearson's r = -0.40), and we further observed that root nitrogen content was largely unrelated to other economic traits. Mycorrhizal colonization was most closely associated with root diameter (Pearson's r = 0.62) and was unrelated to root tissue density and root nitrogen. The heuristic model demonstrated that while thinner roots have inherently greater capacity to encounter soil resources based on higher surface area per unit mass, the potential for increased associations with mycorrhizal fungi in thicker roots, combined with greater hyphal growth, can result in equally acquisitive strategies for both thin- and thick roots. Taken together, our assessments of root trait variation, trade-offs with mycorrhizal fungi, and broader connections to root longevity allowed us to propose a series of fundamental constraints on belowground resource acquisition strategies. Physical tradeoffs based on root construction (i.e., economic traits) and functional limitations related to the capacity of a root to encounter and acquire soil resources combine to limit the two-dimensional belowground trait space. Within this trait space there remains a diversity of additional variation in root traits that facilitates a wide range of belowground resource acquisition strategies.

Mechanical traits of fine roots as a function of topology and anatomy

Background and Aims

Root mechanical traits, including tensile strength (Tr), tensile strain (εr) and modulus of elasticity (Er), are key functional traits that help characterize plant anchorage and the physical contribution of vegetation to landslides and erosion. The variability in these traits is high among tree fine roots and is poorly understood. Here, we explore the variation in root mechanical traits as well as their underlying links with morphological (diameter), architectural (topological order) and anatomical (stele and cortex sizes) traits.

Methods

We investigated the four tropical tree species Pometia tomentosa, Barringtonia fusicarpa, Baccaurea ramiflora and Pittosporopsis kerrii in Xishuangbanna, Yunnan, China. For each species, we excavated intact, fresh, fine roots and measured mechanical and anatomical traits for each branching order.

Key Results

Mechanical traits varied enormously among the four species within a narrow range of diameters (<2 mm): <0.1-65 MPa for Tr, 4-1135 MPa for Er and 0.4-37 % for εr. Across species, Tr and Er were strongly correlated with stele area ratio, which was also better correlated with topological order than with root diameter, especially at interspecific levels.

Conclusions

Root topological order plays an important role in explaining variability in fine-root mechanical traits due to its reflection of root tissue development. Accounting for topological order when measuring fine-root traits therefore leads to greater empirical understanding of plant functions (e.g. anchorage) within and across species.

Differences in the Response to Acute Drought and Phytophthora cinnamomi Rands Infection in Quercus ilex L. Seedlings

The sustainability of “dehesas” is threatened by the Holm oak decline. It is thought that the effects of root rot on plant physiology vary depending on external stress factors. Plant growth and biomass allocation are useful tools to characterize differences in the response to drought and infection. The study of physiological responses together with growth patterns will clarify how and to what extent root rot is able to damage the plant. A fully factorial experiment, including drought and Phytophtora cinnamomi Rands infection as factors, was carried out with Quercus ilex L. seedlings. Photosynthesis, biomass allocation and root traits were assessed. Photosynthetic variables responded differently to drought and infection over time. The root mass fraction showed a significant reduction due to infection. P. cinnamomi root rot altered the growth patterns. Plants could not recover from the physiological effects of infection only when the root rot coincided with water stress. Without additional stressors, the strategy of our seedlings in the face of root rot was to reduce the biomass increment and reallocate resources. Underlying mechanisms involved in plant-pathogen interactions should be considered in the study of holm oak decline, beyond the consideration of water stress as the primary cause of tree mortality.

Contrasting dynamics and trait controls in first-order root compared with leaf litter decomposition

Decomposition is a key component of the global carbon (C) cycle, yet current ecosystem C models do not adequately represent the contributions of plant roots and their mycorrhizae to this process. The understanding of decomposition dynamics and their control by traits is particularly limited for the most distal first-order roots. Here we followed decomposition of first-order roots and leaf litter from 35 woody plant species differing in mycorrhizal type over 6 years in a Chinese temperate forest. First-order roots decomposed more slowly (k = 0.11 ± 0.01 years^-1) than did leaf litter (0.35 ± 0.02 years^-1), losing only 35% of initial mass on average after 6 years of exposure in the field. In contrast to leaf litter, nonlignin root C chemistry (nonstructural carbohydrates, polyphenols) accounted for 82% of the large interspecific variation in first-order root decomposition. Leaf litter from ectomycorrhizal (EM) species decomposed more slowly than that from arbuscular mycorrhizal (AM) species, whereas first-order roots of EM species switched, after 2 years, from having slower to faster decomposition compared with those from AM species. The fundamentally different dynamics and control mechanisms of first-order root decomposition compared with those of leaf litter challenge current ecosystem C models, the recently suggested dichotomy between EM and AM plants, and the idea that common traits can predict decomposition across roots and leaves. Aspects of C chemistry unrelated to lignin or nitrogen, and not presently considered in decomposition models, controlled first-order root decomposition; thus, current paradigms of ecosystem C dynamics and model parameterization require revision.

Soil multifunctionality and drought resistance are determined by plant structural traits in restoring grassland

It is increasingly recognized that belowground responses to vegetation change are closely linked to plant functional traits. However, our understanding is limited concerning the relative importance of different plant traits for soil functions and of the mechanisms by which traits influence soil properties in the real world. Here we test the hypothesis that taller species, or those with complex rooting structures, are associated with high rates of nutrient and carbon (C) cycling in grassland. We further hypothesized that communities dominated by species with deeper roots may be more resilient to drought. These hypotheses were tested in a 3‐yr grassland restoration experiment on degraded ex‐arable land in southern England. We sowed three trait‐based plant functional groups, assembled using database derived values of plant traits, and their combinations into bare soil. This formed a range of plant trait syndromes onto which we superimposed a simulated drought 2 yr after initial establishment. We found strong evidence that community weighted mean (CWM) of plant height is negatively associated with soil nitrogen cycling and availability and soil multifunctionality. We propose that this was due to an exploitative resource capture strategy that was inappropriate in shallow chalk soils. Further, complexity of root architecture was positively related to soil multifunctionality throughout the season, with fine fibrous roots being associated with greater rates of nutrient cycling. Drought resistance of soil functions including ecosystem respiration, mineralization, and nitrification were positively related to functional divergence of rooting depth, indicating that, in shallow chalk soils, a range of water capture strategies is necessary to maintain functions. Finally, after 3 yr of the experiment, we did not detect any links between the plant traits and microbial communities, supporting the finding that traits based on plant structure and resource foraging capacity are the main variables driving soil function in the early years of grassland conversion. We suggest that screening recently restored grassland communities for potential soil multifunctionality and drought resilience may be possible based on rooting architecture and plant height. These results indicate that informed assembly of plant communities based on plant traits could aid in the restoration of functioning in degraded soil.

Root exudation rate as functional trait involved in plant nutrient-use strategy classification

Plants adopt a variety of life history strategies to succeed in the Earth's diverse environments. Using functional traits which are defined as "morphological, biochemical, physiological, or phonological" characteristics measurable at the individual level, plants are classified according to their species' adaptative strategies, more than their taxonomy, from fast growing plant species to slower-growing conservative species. These different strategies probably influence the input and output of carbon (C)-resources, from the assimilation of carbon by photosynthesis to its release in the rhizosphere soil via root exudation. However, while root exudation was known to mediate plant-microbe interactions in the rhizosphere, it was not used as functional trait until recently. Here, we assess whether root exudate levels are useful plant functional traits in the classification of plant nutrient-use strategies and classical trait syndromes? For this purpose, we conducted an experiment with six grass species representing along a gradient of plant resource-use strategies, from conservative species, characterized by low biomass nitrogen (N) concentrations and a long lifespans, to exploitative species, characterized by high rates of photosynthesis and rapid rates of N acquisition. Leaf and root traits were measured for each grass and root exudate rate for each planted soil sample. Classical trait syndromes in plant ecology were found for leaf and root traits, with negative relationships observed between specific leaf area and leaf dry matter content or between specific root length and root dry matter content. However, a new root trait syndrome was also found with root exudation levels correlating with plant resource-use strategy patterns, specifically, between root exudation rate and root dry matter content. We therefore propose root exudation rate can be used as a key functional trait in plant ecology studies and plant strategy classification.

Seeking stable traits to characterize the root system architecture. Study on 60 species located at two sites in natura

Background and Aims

In several disciplines, identifying relevant root traits to characterize the root system architecture of species or genotypes is a crucial step. To address this question, we analysed the inter-specific variations of root architectural traits in two contrasting environments.

Methods

We sampled 60 species in natura, at two sites, each presenting homogeneous soil conditions. We estimated for each species and site a set of five traits used for the modelling of the root system architecture: extreme tip diameters (Dmin and Dmax), relative diameter range (Drange), mean inter-branch distance (IBD) and dominance slope between the diameters of parent and lateral roots (DlDm).

Key Results

The five traits presented a highly significant species effect, explaining between 77 and 98 % of the total variation. Dmin, Dmax and Drange were particularly determined by the species, while DlDm and IBD exhibited a higher percentage of environmental variations. These traits make it possible to confirm two main axes of variation: 'fineness-density' (defined by Dmin and IBD) and 'dominance-heterorhizy' (DlDm and Drange), that together accounted for 84 % of the variations observed.

Conclusions

We confirmed the interest of these traits in the characterization of the root system architecture in ecology and genetics, and suggest using them to enrich the 'root economic spectrum'.

Whole-Plant Metabolic Allocation Under Water Stress

Trade-offs between plant growth and defense depend on environmental resource availability. Plants are predicted to prioritize growth when environmental resources are abundant and defense when environmental resources are scarce. Nevertheless, such predictions lack a whole-plant perspective-they do not account for potential differences in plant allocation above- and belowground. Such accounting is important because leaves and roots, though both critical to plant survival and fitness, differ in their resource-uptake roles and, often, in their vulnerability to herbivores. Here we aimed to determine how water availability affects plant allocation to multiple metabolic components of growth and defense in both leaves and roots. To do this, we conducted a meta-analysis of data from experimental studies in the literature. We assessed plant metabolic responses to experimentally reduced water availability, including changes in growth, nutrients, physical defenses, primary metabolites, hormones, and other secondary metabolites. Both above- and belowground, reduced water availability reduced plant biomass but increased the concentrations of primary metabolites and hormones. Importantly, however, reduced water had opposite effects in different organs on the concentrations of other secondary metabolites: reduced water increased carbon-based secondary metabolites in leaves but reduced them in roots. In addition, plants suffering from co-occurring drought and herbivory stresses exhibited dampened metabolic responses, suggesting a metabolic cost of multiple stresses. Our study highlights the needs for additional empirical studies of whole-plant metabolic responses under multiple stresses and for refinement of existing plant growth-defense theory in the context of whole plants.

Delivering the promises of trait-based approaches to the needs of demographic approaches, and vice versa

Few facets of biology vary more than functional traits and life-history traits. To explore this vast variation, functional ecologists and population ecologists have developed independent approaches that identify the mechanisms behind and consequences of trait variation.Collaborative research between researchers using trait-based and demographic approaches remains scarce. We argue that this is a missed opportunity, as the strengths of both approaches could help boost the research agendas of functional ecology and population ecology.This special feature, which spans three journals of the British Ecological Society due to its interdisciplinary nature, showcases state-of-the-art research applying trait-based and demographic approaches to examine relationships between organismal function, life history strategies and population performance across multiple kingdoms. Examples include the exploration of how functional trait × environment interactions affect vital rates and thus explain population trends and species occurrence; the coordination of seed traits and dispersal ability with the pace of life in plants; the incorporation of functional traits in dynamic energy budget models; or the discovery of linkages between microbial functional traits and the fast-slow continuum.Despite their historical isolation, collaborative work between functional ecologists and population ecologists could unlock novel research pathways. We call for an integrative research agenda to evaluate which and when traits are functional, as well as their ability to describe and predict life history strategies and population dynamics. We highlight promising, complementary research avenues to overcome current limitations. These include a more explicit linkage of selection gradients in the context of functional trait-vital rate relationships, and the implementation of standardised protocols to track changes in traits and vital rates over time at the same location and individuals, thus allowing for the explicit incorporation of trade-offs in analyses of covariation of functional traits and life-history traits.

Predicting the structure of soil communities from plant community taxonomy, phylogeny, and traits

There are numerous ways in which plants can influence the composition of soil communities. However, it remains unclear whether information on plant community attributes, including taxonomic, phylogenetic, or trait-based composition, can be used to predict the structure of soil communities. We tested, in both monocultures and field-grown mixed temperate grassland communities, whether plant attributes predict soil communities including taxonomic groups from across the tree of life (fungi, bacteria, protists, and metazoa). The composition of all soil community groups was affected by plant species identity, both in monocultures and in mixed communities. Moreover, plant community composition predicted additional variation in soil community composition beyond what could be predicted from soil abiotic characteristics. In addition, analysis of the field aboveground plant community composition and the composition of plant roots suggests that plant community attributes are better predictors of soil communities than root distributions. However, neither plant phylogeny nor plant traits were strong predictors of soil communities in either experiment. Our results demonstrate that grassland plant species form specific associations with soil community members and that information on plant species distributions can improve predictions of soil community composition. These results indicate that specific associations between plant species and complex soil communities are key determinants of biodiversity patterns in grassland soils.

Spatial heterogeneity in root litter and soil legacies differentially affect legume root traits

BACKGROUND AND AIMS

Plants affect the soil environment via litter inputs and changes in biotic communities, which feed back to subsequent plant growth. Here we investigated the individual contributions of litter and biotic communities to soil feedback effects, and plant ability to respond to spatial heterogeneity in soil legacy.

METHODS

We tested for localised and systemic responses of Trifolium repens to soil biotic and root litter legacy of seven grassland species by exposing half of a root system to control soil and the other half to specific inoculum or root litter.

RESULTS

Soil inoculation triggered a localised reduction in root length while litter locally increased root biomass independent of inoculum or litter species identity. Nodule formation was locally suppressed in response to soil conditioned by another legume (Vicia cracca) and showed a trend towards systemic reduction in response to conspecific soil. V. cracca litter also caused a systemic response with thinner roots produced in the part of the root system not directly exposed to the litter.

CONCLUSIONS

Spatial heterogeneity in root litter distribution and soil communities generate distinct local and systemic responses in root morphology and nodulation. These responses can influence plant-mutualist interactions and nutrient cycling, and should be included in plant co-existence models.

DigR: a generic model and its open source simulation software to mimic three-dimensional root-system architecture diversity

Background and Aims

Many studies exist in the literature dealing with mathematical representations of root systems, categorized, for example, as pure structure description, partial derivative equations or functional-structural plant models. However, in these studies, root architecture modelling has seldom been carried out at the organ level with the inclusion of environmental influences that can be integrated into a whole plant characterization.

Methods

We have conducted a multidisciplinary study on root systems including field observations, architectural analysis, and formal and mathematical modelling. This integrative and coherent approach leads to a generic model (DigR) and its software simulator. Architecture analysis applied to root systems helps at root type classification and architectural unit design for each species. Roots belonging to a particular type share dynamic and morphological characteristics which consist of topological and geometric features. The DigR simulator is integrated into the Xplo environment, with a user interface to input parameter values and make output ready for dynamic 3-D visualization, statistical analysis and saving to standard formats. DigR is simulated in a quasi-parallel computing algorithm and may be used either as a standalone tool or integrated into other simulation platforms. The software is open-source and free to download at http://amapstudio.cirad.fr/soft/xplo/download.

Key Results

DigR is based on three key points: (1) a root-system architectural analysis, (2) root type classification and modelling and (3) a restricted set of 23 root type parameters with flexible values indexed in terms of root position. Genericity and botanical accuracy of the model is demonstrated for growth, branching, mortality and reiteration processes, and for different root architectures. Plugin examples demonstrate the model's versatility at simulating plastic responses to environmental constraints. Outputs of the model include diverse root system structures such as tap-root, fasciculate, tuberous, nodulated and clustered root systems.

Conclusions

DigR is based on plant architecture analysis which leads to specific root type classification and organization that are directly linked to field measurements. The open source simulator of the model has been included within a friendly user environment. DigR accuracy and versatility are demonstrated for growth simulations of complex root systems for both annual and perennial plants.

Evolutionary history resolves global organization of root functional traits

Plant roots have greatly diversified in form and function since the emergence of the first land plants, but the global organization of functional traits in roots remains poorly understood. Here we analyse a global dataset of 10 functionally important root traits in metabolically active first-order roots, collected from 369 species distributed across the natural plant communities of 7 biomes. Our results identify a high degree of organization of root traits across species and biomes, and reveal a pattern that differs from expectations based on previous studies of leaf traits. Root diameter exerts the strongest influence on root trait variation across plant species, growth forms and biomes. Our analysis suggests that plants have evolved thinner roots since they first emerged in land ecosystems, which has enabled them to markedly improve their efficiency of soil exploration per unit of carbon invested and to reduce their dependence on symbiotic mycorrhizal fungi. We also found that diversity in root morphological traits is greatest in the tropics, where plant diversity is highest and many ancestral phylogenetic groups are preserved. Diversity in root morphology declines sharply across the sequence of tropical, temperate and desert biomes, presumably owing to changes in resource supply caused by seasonally inhospitable abiotic conditions. Our results suggest that root traits have evolved along a spectrum bounded by two contrasting strategies of root life: an ancestral 'conservative' strategy in which plants with thick roots depend on symbiosis with mycorrhizal fungi for soil resources and a more-derived 'opportunistic' strategy in which thin roots enable plants to more efficiently leverage photosynthetic carbon for soil exploration. These findings imply that innovations of belowground traits have had an important role in preparing plants to colonize new habitats, and in generating biodiversity within and across biomes.

Intraspecific genetic diversity modulates plant-soil feedback and nutrient cycling

Plant genetic diversity can affect ecosystem functioning by enhancing productivity, litter decomposition and resistance to natural enemies. However, the mechanisms underlying these effects remain poorly understood. We hypothesized that genetic diversity may influence ecosystem processes by eliciting functional plasticity among individuals encountering kin or genetically diverse neighbourhoods. We used soil conditioned by groups of closely related (siblings) and diverse genotypes of Deschampsia cespitosa - a species known to exhibit kin recognition via root exudation - to investigate the consequences of kin interactions for root litter decomposition and negative feedback between plants and soil biota. Genetically diverse groups produced root litter that had higher nitrogen (N) content, decomposed faster and resulted in greater N uptake by the next generation of seedlings compared with litter produced by sibling groups. However, a similar degree of negative soil feedback on plant productivity was observed in soil conditioned by siblings and genetically diverse groups. This suggests that characteristics of roots produced by sibling groups slow down N cycling but moderate the expected negative impact of soil pathogens in low-diversity stands. These findings highlight interactions between neighbouring genotypes as an overlooked mechanism by which genetic diversity can affect biotic soil feedback and nutrient cycling.

A worldview of root traits: the influence of ancestry, growth form, climate and mycorrhizal association on the functional trait variation of fine-root tissues in seed plants

Fine-root traits play key roles in ecosystem processes, but the drivers of fine-root trait diversity remain poorly understood. The plant economic spectrum (PES) hypothesis predicts that leaf and root traits evolved in coordination. Mycorrhizal association type, plant growth form and climate may also affect root traits. However, the extent to which these controls are confounded with phylogenetic structuring remains unclear. Here we compiled information about root and leaf traits for > 600 species. Using phylogenetic relatedness, climatic ranges, growth form and mycorrhizal associations, we quantified the importance of these factors in the global distribution of fine-root traits. Phylogenetic structuring accounts for most of the variation for all traits excepting root tissue density, with root diameter and nitrogen concentration showing the strongest phylogenetic signal and specific root length showing intermediate values. Climate was the second most important factor, whereas mycorrhizal type had little effect. Substantial trait coordination occurred between leaves and roots, but the strength varied between growth forms and clades. Our analyses provide evidence that the integration of roots and leaves in the PES requires better accounting of the variation in traits across phylogenetic clades. Inclusion of phylogenetic information provides a powerful framework for predictions of belowground functional traits at global scales.

Intraspecific Trait Variation and Coordination: Root and Leaf Economics Spectra in Coffee across Environmental Gradients

Hypotheses on the existence of a universal "Root Economics Spectrum" (RES) have received arguably the least attention of all trait spectra, despite the key role root trait variation plays in resource acquisition potential. There is growing interest in quantifying intraspecific trait variation (ITV) in plants, but there are few studies evaluating (i) the existence of an intraspecific RES within a plant species, or (ii) how a RES may be coordinated with other trait spectra within species, such as a leaf economics spectrum (LES). Using Coffea arabica (Rubiaceae) as a model species, we measured seven morphological and chemical traits of intact lateral roots, which were paired with information on four key LES traits. Field collections were completed across four nested levels of biological organization. The intraspecific trait coefficient of variation (cv) ranged from 25 to 87% with root diameter and specific root tip density showing the lowest and highest cv, respectively. Between 27 and 68% of root ITV was explained by site identity alone for five of the seven traits measured. A single principal component explained 56.2% of root trait covariation, with plants falling along a RES from resource acquiring to conserving traits. Multiple factor analysis revealed significant orthogonal relationships between root and leaf spectra. RES traits were strongly orthogonal with respect to LES traits, suggesting these traits vary independently from one another in response to environmental cues. This study provides among the first evidence that plants from the same species differentiate from one another along an intraspecific RES. We find that in one of the world's most widely cultivated crops, an intraspecific RES is orthogonal to an intraspecific LES, indicating that above and belowground responses of plants to managed (or natural) environmental gradients are likely to occur independently from one another.

A global Fine-Root Ecology Database to address below-ground challenges in plant ecology

Variation and tradeoffs within and among plant traits are increasingly being harnessed by empiricists and modelers to understand and predict ecosystem processes under changing environmental conditions. While fine roots play an important role in ecosystem functioning, fine-root traits are underrepresented in global trait databases. This has hindered efforts to analyze fine-root trait variation and link it with plant function and environmental conditions at a global scale. This Viewpoint addresses the need for a centralized fine-root trait database, and introduces the Fine-Root Ecology Database (FRED, http://roots.ornl.gov) which so far includes > 70 000 observations encompassing a broad range of root traits and also includes associated environmental data. FRED represents a critical step toward improving our understanding of below-ground plant ecology. For example, FRED facilitates the quantification of variation in fine-root traits across root orders, species, biomes, and environmental gradients while also providing a platform for assessments of covariation among root, leaf, and wood traits, the role of fine roots in ecosystem functioning, and the representation of fine roots in terrestrial biosphere models. Continued input of observations into FRED to fill gaps in trait coverage will improve our understanding of changes in fine-root traits across space and time.

Building a better foundation: improving root-trait measurements to understand and model plant and ecosystem processes

Trait-based approaches provide a useful framework to investigate plant strategies for resource acquisition, growth, and competition, as well as plant impacts on ecosystem processes. Despite significant progress capturing trait variation within and among stems and leaves, identification of trait syndromes within fine-root systems and between fine roots and other plant organs is limited. Here we discuss three underappreciated areas where focused measurements of fine-root traits can make significant contributions to ecosystem science. These include assessment of spatiotemporal variation in fine-root traits, integration of mycorrhizal fungi into fine-root-trait frameworks, and the need for improved scaling of traits measured on individual roots to ecosystem-level processes. Progress in each of these areas is providing opportunities to revisit how below-ground processes are represented in terrestrial biosphere models. Targeted measurements of fine-root traits with clear linkages to ecosystem processes and plant responses to environmental change are strongly needed to reduce empirical and model uncertainties. Further identifying how and when suites of root and whole-plant traits are coordinated or decoupled will ultimately provide a powerful tool for modeling plant form and function at local and global scales.