Fine root morphological and functional traits in Fagus sylvatica and Fraxinus excelsior saplings as dependent on species, root order and competition

Aims

The mechanisms of belowground competition are not well understood. Addressing literature reports on competition-induced changes in tree fine root morphology, we conducted a growth experiment with tree saplings to investigate competition effects on important root morphological and functional traits in a root order-focused analysis.

Methods

European beech and European ash saplings were grown for 34 months in containers under greenhouse conditions in monoculture (2 conspecific plants), in mixture (1 beech and 1 ash) or as single plants. The root system was fractionated according to root orders and eight morphological and functional properties were determined.

Results

Root order was the most influential factor affecting the fine root traits (except for root diameter and δ¹³C); a significant species identity effect was found for root diameter, tissue density, N concentration and δ¹³C. Ash fine roots were thicker, but had lower tissue densities, contained more N and had systematically higher δ¹³C values than beech roots. The competition treatments had no significant effect on morphological root traits but altered δ¹³C in the 2nd root order.

Conclusion

Neither intra- nor interspecific root competition affected fine root morphology significantly suggesting that competition-induced root modification may not be a universal phenomenon in temperate trees.     

Drought changes the structure and elemental composition of very fine roots in seedlings of ten woody tree species. Implications for a drier climate

Background and aims

Water availability is often one of the most limiting factors for plants. Climate change predictions for many areas suggest an intensification of water limitation. The ability of a plant to modify its root characteristics can be an important mechanism for preventing drought stress.

Methods

We studied the drought response of seedlings of 10 woody species and compared the biomass allocation, vertical root distribution across different root diameters, and the key traits of very fine roots (root diameter <0.5 mm) under two water regimes (no water limitation and severe drought).

Results

Under drought conditions, the very fine roots had a higher specific root length (SRL, root length: biomass ratio), smaller root diameter and higher root tissue mass density, as well as a lower nitrogen concentration. A higher value of the mean root plasticity index was related to higher drought resistance. A quantitative literature review showed that there was a wide variation in the effect of the drought on SRL, thus there was not a clear effect of drought on SRL.

Conclusions

Certain species have the necessary root traits and plasticity to survive drought. We have identified plasticity in root characteristics as a whole-plant trait which plays a significant role in separating out species into those which are vulnerable and those which are resistant to drought.     

Branching patterns of root systems: quantitative analysis of the diversity among dicotyledonous species

Background and Aims

Root branching, and in particular acropetal branching, is a common and important developmental process for increasing the number of growing tips and defining the distribution of their meristem size. This study presents a new method for characterizing the results of this process in natura from scanned images of young, branched parts of excavated roots. The method involves the direct measurement or calculation of seven different traits.

Methods

Young plants of 45 species of dicots were sampled from fields and gardens with uniform soils. Roots were separated, scanned and then measured using ImageJ software to determine seven traits related to root diameter and interbranch distance.

Results

The traits exhibited large interspecific variations, and covariations reflecting trade-offs. For example, at the interspecies level, the spacing of lateral roots (interbranch distance along the parent root) was strongly correlated to the diameter of the finest roots found in the species, and showed a continuum between two opposite strategies: making dense and fine lateral roots, or thick and well-spaced laterals.

Conclusions

A simple method is presented for classification of branching patterns in roots that allows relatively quick sampling and measurements to be undertaken. The feasibilty of the method is demonstrated for dicotyledonous species and it has the potential to be developed more broadly for other species and a wider range of enivironmental conditions.     

Insights into root growth, function, and mycorrhizal abundance from chemical and isotopic data across root orders

Background and aims

Detailed analyses of root chemistry by branching order may provide insights into root function, root lifespan and the abundance of root-associated mycorrhizal fungi in forest ecosystems.

Methods

We examined the nitrogen and carbon stable isotopes (δ¹⁵N and δ¹³C) and concentration (%N and %C) in the fine roots of an arbuscular mycorrhizal tree, Fraxinus mandshurica, and an ectomycorrhizal tree, Larix gmelinii, over depth, time, and across five root branching orders.

Results and conclusions

Larix δ¹⁵N increased by 2.3?‰ from 4th order to 1st order roots, reflecting the increased presence of ¹⁵N-enriched ECM fungi on the lower root orders. In contrast, arbuscular mycorrhizal Fraxinus only increased by 0.7?‰ from 4th order to 1st order roots, reflecting the smaller ¹⁵N enrichment and lower fungal mass on arbuscular mycorrhizal fine roots. Isotopic and anatomical mass balance calculations indicate that first, second, and third order roots in ectomycorrhizal Larix averaged 36 %, 23 %, and 8 % fungal tissue by mass, respectively. Using literature values of root production by root branching order, we estimate that about 25 % of fine root production in ECM species like Larix is actually of fungal sheaths. In contrast to %N, %C, and δ¹⁵N, δ¹³C changed minimally across depth, time, and branching order. The homogeneity of δ¹³C suggests root tissues are constructed from a large well-mixed reservoir of carbon, although compound specific δ¹³C data is needed to fully interpret these patterns. The measurements developed here are an important step towards explicitly including mycorrhizal production in forest ecosystem carbon budgets.     

Grassland root demography responses to multiple climate change drivers depend on root morphology

Aims

We examine how root system demography and morphology are affected by air warming and multiple, simultaneous climate change drivers.

Methods

Using minirhizotrons, we studied root growth, morphology, median longevity, risk of mortality and standing root pool in the upper soil horizon of a temperate grassland ecosystem for 3 years. Grassland monoliths were subjected to four climate treatments in a replicated additive design: control (C); elevated temperature (T); combined T and summer precipitation reduction (TD); combined TD and elevated atmospheric CO₂ (TDCO₂).

Results

Air warming (C vs T) and the combined climate change treatment (C vs TDCO₂) had a positive effect on root growth rate and standing root pool. However, root responses to climate treatment varied depending on diameter size class. For fine roots (≤ 0.1 mm), new root length and mortality increased under warming but decreased in response to elevated CO₂ (TD vs TDCO₂); for coarse roots (> 0.2 mm), length and mortality increased under both elevated CO₂ and combined climate change drivers.

Conclusions

Our data suggest that the standing roots pool in our grassland system may increase under future climatic conditions. Contrasted behaviour of fine and coarse roots may correspond to differential root activity of these extreme diameter classes in future climate.     

QTL mapping and phenotypic variation for root architectural traits in maize (Zea mays L.)

Key message

QTL were identified for root architectural traits in maize.

Abstract

Root architectural traits, including the number, length, orientation, and branching of the principal root classes, influence plant function by determining the spatial and temporal domains of soil exploration. To characterize phenotypic patterns and their genetic control, three recombinant inbred populations of maize were grown for 28 days in solid media in a greenhouse and evaluated for 21 root architectural traits, including length, number, diameter, and branching of seminal, primary and nodal roots, dry weight of embryonic and nodal systems, and diameter of the nodal root system. Significant phenotypic variation was observed for all traits. Strong correlations were observed among traits in the same root class, particularly for the length of the main root axis and the length of lateral roots. In a principal component analysis, relationships among traits differed slightly for the three families, though vectors grouped together for traits within a given root class, indicating opportunities for more efficient phenotyping. Allometric analysis showed that trajectories of growth for specific traits differ in the three populations. In total, 15 quantitative trait loci (QTL) were identified. QTL are reported for length in multiple root classes, diameter and number of seminal roots, and dry weight of the embryonic and nodal root systems. Phenotypic variation explained by individual QTL ranged from 0.44 % (number of seminal roots, NyH population) to 13.5 % (shoot dry weight, OhW population). Identification of QTL for root architectural traits may be useful for developing genotypes that are better suited to specific soil environments.     

Estimating the absorptive root area in Norway spruce by using the common direct and indirect earth impedance methods

Aims

Estimates of root absorption magnitude are needed for the balanced management of forest ecosystems, but no methods able to work on the whole tree and stand level were available. Modified earth impedance method was developed recently and here it was tested, by comparing the results with those obtained by combination of several classical methods.

Methods

We used direct (soil cores, scanning and microscopy) and indirect (sap flow patterns and modified earth impedance) methods in an attempt to estimate the absorptive root area indexes (RAI) at two sites of about 25 and 40-years-old Norway spruce. We considered the geometric surfaces of all scanned fine roots to be equal to the fine root absorptive area (RAI _scan ). To estimate the potentially physically permeable area of fine roots, we microscopically evaluated the point of secondary xylem appearance and calculated the geometric area of root portions with primary structure (RAI _micro ). We termed the area of electrically conductive root surface as the active (ion) absorptive area (RAI _mei ) and measured its extent by the modified earth impedance (MEI) method.

Results

The highest values for absorptive root areas at the two experimental sites we obtained with the scanning method (RAI _scan was considered to be 100%), followed by the RAI _micro (51%) and RAI _mei (32%). RAImei reached about 2/3 of RAImicro. The surface area of the ectomycorrhizal hyphae was an order of magnitude larger than that of all fine roots, but the MEI did not measure such increase.

Conclusions

We showed that the absorptive root area, indirectly estimated by the MEI, provides consistent results that approach the values obtained for fine roots with a primary structure estimated by traditional direct methods. The similar range of the values for the absorptive root surface area obtained by microscopy and by the MEI method indicates that this method is feasible and that it could be used to determine the extent of active absorptive root surface areas in forests.     

Maintaining connectivity: understanding the role of root order and mycelial networks in fine root decomposition of woody plants

Background

The predictive power of climate models is limited by an incomplete understanding of the controls on fine root decomposition and thus belowground carbon cycling. To more accurately model rates of decay, fine root heterogeneity needs to be addressed in fine root decomposition studies. Branching order integrates both structural and chemical properties that are important in indicating litter quality and decay rate.

Scope

We discuss current views on the controls and patterns of fine root decomposition in combination with recent findings related to the effects of branching order and mycorrhizal decomposition. We examine the counterintuitive finding that nitrogen rich, lower order roots decompose more slowly than woody, higher order roots in temperate and sub-tropical forests.

Conclusions

We posit that slower decomposition of first and second compared to higher order roots might be caused by the poor carbon quality associated with higher concentrations of phenols in lower order roots or by inhibition of saprophytes by the mycorrhizal fungi that often preferentially inhabit these roots. Alternatively, apparent recalcitrance of lower order roots could be an experimental artifact caused by severing pre-mortem mycelial connections during sample processing, or exclusion of animals that graze fungal structures by the small mesh sizes characteristic of litterbags. To better predict the residence time of the carbon contained in the entire fine root pool, existing methods should be applied to individual root orders when practical. New methods for characterizing decomposition of undisturbed roots that have senesced naturally are greatly needed.

     

Effects of night warming on spruce root around non-growing season vary with branch order and month

Background and aims

The Root is an important plant organ and has high heterogeneity; how it responds to global warming is yet to be answered. This study examined the growth and physiological responses of fine roots to warming around the non-growing season.

Methods

Plants from 4-year-old Picea asperata were grown under experimental warming conditions. A detailed investigation was conducted by measuring biomass, triphenyltetrazolium chloride (TTC) reducing capacity, carbon (C) and nitrogen (N) concentration, non-structural carbohydrate (NSC) of the primal five branch order roots in early (April) and late (September) growing seasons as well as in the non-growing season (December).

Results

Warming promoted fine root growth in April and fine root turnover was mostly in the first four orders. It decreased root C, N concentration in the early and late growing seasons but increased N concentration in the non-growing season. Moreover, it increased NSC concentration (especially soluble sugar) in April but decreased its concentration (soluble sugar and starch) in December. TTC reducing capacity in April was higher than in the other 2 months.

Conclusions

The effect of warming on tree roots varied with its branch order and month. The lower order (first three or four order, in general) roots were sensitive to warming, especially in April (early part of growing season) and December (non-growing season). Warming accelerated the carbon input from root to soil. It is indicated that any changes in winter temperatures could alter the sink strength of terrestrial ecosystems considerably. Moreover, TTC reducing capacity could reflect more information about root, but it was more sensitive than N concentration.     

Fine root dynamics and soil carbon accretion under thinned and un-thinned Cupressus lusitanica stands in, Southern Ethiopia

Background and aims

Forest management activities influences stand nutrient budgets, belowground carbon allocation and storage in the soil. A field experiment was carried out in Southern Ethiopia to investigate the effect of thinning on fine root dynamics and associated soil carbon accretion of 6-year old C. lusitanica stands.

Methods

Fine roots (≤2 mm in diameter) were sampled seasonally to a depth of 40 cm using sequential root coring method. Fine root biomass and necromass, vertical distribution, seasonal dynamics, annual turnover and soil carbon accretion were quantified.

Results

Fine root biomass and necromass showed vertical and temporal variations. More than 70 % of the fine root mass was concentrated in the top 20 cm soil depth. Fine root biomass showed significant seasonal variation with peaks at the end of the major rainy season and short rainy season. Thinning significantly increased fine root necromass, annual fine root production and turnover. Mean annual soil carbon accretion, through fine root necromass, in the thinned stand was 63 % higher than that in the un-thinned stand.

Conclusions

The temporal dynamics in fine roots is driven by the seasonality in precipitation. Thinning of C. lusitanica plantation would increase soil C accretion considerably through increased fine root necromass and turnover.     

Measurement of fine root tissue density: a comparison of three methods reveals the potential of root dry matter content

Aims

Root tissue density (RTD, the ratio of root dry mass to root volume) is a fundamental trait in comparative root ecology, being increasingly used as an indicator of plant species’ resource use strategy. However, the lack of standardized method to measure this trait makes comparisons tricky. This study aims to compare three methods commonly used for determining fine RTD and to test whether root dry matter content (RDMC, the ratio between root dry mass and root fresh mass) could be used as a surrogate of fine root tissue density.

Methods

RTD of 163 fine root samples was determined using (i) Archimedes’ method, (ii) image analysis (WinRHIZO software), and (iii) using the root dry matter content as a proxy. Root samples belonged to different herbaceous species grown in different conditions.

Results

RTD measured with Archimedes’ method was positively correlated with RTD estimated with image analysis and with RDMC. However we demonstrated that RTD measured with Archimedes’ method was better predicted by RDMC (R²?=?0.90) than by RTD measured with image analysis (R²?=?0.56). The performance and limitations of each method were discussed.

Conclusion

RDMC is a quick, cheap and relatively easy measurable root attribute; we thus recommended its measurement as a proxy of fine root tissue density.     

Genetic architecture of aerial and root traits in field-grown grafted grapevines is largely independent

Key message

QTLs were identified for traits assessed on field-grown grafted grapevines. Root number and section had the largest phenotypic variance explained. Genetic control of root and aerial traits was independent.

Abstract

Breeding new rootstocks for perennial crops remains challenging, mainly because of the number of desirable traits which have to be combined, these traits include good rooting ability and root development. Consequently, the present study analyzes the genetic architecture of root traits in grapevine. A segregating progeny of 138 F1 genotypes issued from an inter-specific cross between Vitis vinifera cv. Cabernet-Sauvignon × V. riparia cv. Gloire de Montpellier, used as rootstock, was phenotyped in grafted plants grown for 2 years in the field. Seven traits, related to aerial and root development, were quantified. Heritability ranged between 0.44 for aerial biomass to 0.7 for root number. Total root number was related to the number of fine roots, while root biomass was related to the number of coarse roots. Significant quantitative trait loci (QTLs) were identified for all the traits studied with some of them explaining approximately 20% of phenotypic variance. Only a single QTL co-localized for root and aerial biomass. Identified QTLs for aerial-to-root biomass ratio suggest that aerial and root traits are controlled independently. Genes known to be involved in auxin signaling pathways and phosphorus nutrition, whose orthologues were previously shown to regulate root development in Arabidopsis, were located in the confidence intervals of several QTLs. This study opens new perspectives for breeding rootstocks with improved root development capacities.

     

Wheat root diversity and root functional characterization

Background and Aims

Under limited moisture conditions, roots can play an outstanding role with respect to yield stability by effective absorption of water from soil. A targeted integration of root traits into plant breeding programs requires knowledge on the existing root diversity and access to easy and cost-effective methods. This study aimed to assess wheat root diversity, root properties in relation to water regime, and the efficiency of root capacitance for in situ screening.

Methods

Root morphological, anatomical properties and root capacitance of wheat species from different ploidy levels were studied under field conditions in 2 years contrasting in water regime. Soil water content was weekly measured.

Results

Significant genotypic differences were observed for most root traits. The investigated genotypes exploited different strategies to maximize soil water depletion, e.g. high topsoil root length density, low tissue mass density, high specific root length, deep rooting and looser xylem vessels. Multivariate statistics of root traits revealed an acceptable genotypic differentiation according to regional origin, genetics and capacity to extract soil water.

Conclusions

Under supply-driven environments, dehydration avoidance via water uptake maximization can be achieved through high topsoil rooting density. In this regard, root capacitance can be useful for in situ screening.     

Root trait responses of six temperate grassland species to intensive mowing and NPK fertilisation: a field study in a temperate grassland

Background and aims

Plant traits may characterize functional ecosystem properties and help to predict community responses to environmental change. Since most traits used relate to aboveground plant organs we aim to explore the indicative value of root traits.

Methods

We examined the response of root traits (specific root length [SRL], specific root surface area [SRA], root diameter [RD], root tissue mass density [TMD], root N concentration) in six grassland species (3 grasses, 3 herbs) to four management regimes (low vs. high mowing frequency; no fertilization vs. high NPK fertilization). The replicated experiment in temperate grassland with long continuity simulated the increase in grassland management intensity in the past 50 years in Central Europe.

Results

Increasing mowing frequency (one vs. three cuts per year) led to no significant root trait changes. NPK fertilization resulted in considerable trait shifts with all species responding in the same direction (higher SRL, SRA and N concentration, lower TMD) but at different magnitude. Fertilization-driven increases in SRA were mainly caused by lowered tissue density while root diameter reduction was the main driver of SRL increases.

Conclusion

We conclude that root morphological traits may be used as valuable indicators of environmental change and increasing fertilization in grasslands.     

Root foraging capacity depends on root system architecture and ontogeny in seedlings of three Andean Chenopodium species

Aims

Morphological and ontogenetic variation in root system architecture holds ecological significance, particularly in low-resource habitats where soil rooting is critical for both seedling establishment and water and nutrient uptake. To assess this variation under contrasted agroecological backgrounds, root architecture and rooting patterns were compared in Andean populations of Chenopodium hircinum, Chenopodium pallidicaule and two ecotypes (wet- and dry-habitat) of Chenopodium quinoa.

Methods

Seedlings were grown in rhizotrons under controlled water and nutrient availability. Root branching and elongation dynamics were characterized during 6 weeks after germination, while leaf area, above and below-ground biomass, and specific root length were determined at the end of the experiment.

Results

Despite large differences in aboveground biomass, all populations showed similar herringbone root systems. The dry-habitat C. quinoa had generally the highest root trait values, with fast taproot elongation, thick roots and long root segments resulting in high total root length and deep root proliferation.

Conclusion

Irrespective of their contrasting agroecological background, the studied chenopods displayed a similar root system topology. However, from very early development stages, they showed differential root foraging patterns with two extremes: fast and vigourous rooting at depth in the dry-habitat C. quinoa, and shallow and thin root system in C. pallidicaule adapted to shallow-soil and high-altitude habitats.     

Relation of fine root distribution to soil C in a Cunninghamia lanceolata plantation in subtropical China

Background and aims

Growth and distribution of fine roots closely depend on soil resource availability and affect soil C distribution in return. Understanding of relationships between fine root distribution and soil C can help to predict the contribution of fine root turnover to soil C accumulation.

Methods

A study was conducted in a subtropical Cunninghamia lanceolata plantation to assess the fine root mass density (FRMD), fine root C density (FRCD) of different fine root groups as well as their relations with soil C.

Results

The FRMD and FRCD of short-lived roots, dead roots and herb roots peaked in the 0–10 cm soil layer and decreased with soil depth, while FRMD, FRCD of long-lived roots peaked in the 10–20 cm soil layer. Soil C was positively related to FRMD and FRCD of total fine roots (across all three soil layers), dead roots (0–10 cm) and herb roots (10–20 cm) as well as FRCD of short-lived roots (20–40 cm) (P <0.05).

Conclusions

Soil C was mainly affected by herb roots in upper soil layers and by woody plant roots in deeper soil layers.     

Fine root and litterfall dynamics of three Korean pine (Pinus koraiensis) forests along an altitudinal gradient

Background and aims

Fine root and aboveground litterfall, two large fluxes of nutrients and carbon in the forest ecosystems, are key processes to be considered in efforts of measuring, modeling and predicting soil carbon sequestration.

Methods

We used sequential coring and litter trap to measure seasonal dynamics of fine root and litterfall in three Korean pine dominated forests along an altitudinal gradient in the Changbai Mountain during the 2012 growing season.

Results

Fine root biomass decreased significantly while necromass increased remarkably with altitude. Patterns and amounts of fine root production and mortality varied among forest types. Litterfall decreased significantly with altitude, whereas forest floor mass increased. Carbon inputs through fine root mortality and litterfall decreased significantly with altitude while carbon storage of fine root mass did not differ among forest types and carbon storage of forest floor mass was significantly larger in higher altitudinal forests due to lower turnover rates.

Conclusions

This study provided an insight into the variations of fine root and litterfall dynamics among three Korean pine forests which were associated with different vegetation traits and environmental conditions, and also the quantification of carbon fluxes through fine root mortality and litterfall for estimating carbon budget of temperate forest.     

Short-term effects of litter from 21 woody species on plant growth and root development

Background and aims

Plant litter has an important role in terrestrial ecosystems (Lambers et al. 2008). Our aim was to assess the short-term effect of litter from 21 woody species (deciduous and evergreens) on plant growth and root development.

Methods

We conducted a short-term experiment (10 weeks) under controlled conditions adding litter from 21 woody species to pots with Dactylis glomerata (target species). We determined plant biomass and root development and related these variables to decomposition rate and litter quality.

Results

Litter from two species enhanced plant growth whereas litter of five species inhibited it. Considering all species in the data set, plant growth was associated to litter with high decomposition rate and high litter quality: high Ca and N concentration and low polyphenols concentration. However, excluding from the analyses the two species that increased growth, litter inhibition effect on plant growth was related to the litter-polyphenols concentration. Plants growing with nutrient-richer litter had a lower proportion of fine roots which could be related to a litter mediated increase in soil nutrient.

Conclusions

Enhanced plant growth or, on the contrary, plant growth inhibition could be the result of a positive or, in turn, negative balance between nutrient and polyphenols concentration in litter.     

Leaf litter thickness,but not plant species,can affect root detection by ground penetrating radar

Aim

Ground penetrating radar (GPR), a nondestructive tool that can detect coarse tree roots, has not yet become a mature technology for use in forests. In this study, we asked two questions concerning this technology: (i) Does the leaf litter layer influence root detection and major indices based on the time interval between zero crossings (T) and the amplitude area (A)? (ii) Can GPR images discriminate roots of different plant species?

Methods

Roots buried in a sandy bed, which was covered with different thicknesses of leaf litter, were scanned using a 900 MHz GPR antenna. Roots of four plant species in the bed were also scanned.

Results

Leaf litter decreased root reflections without distorting the shape of the hyperbolas in the radar profile. A values decreased with increasing litter thickness, whereas T was independent of litter thickness. For all species combined, GPR indices were significantly correlated with root diameter.

Conclusions

Leaf litter dramatically decreased root detection, but the influence of the litter could be ignored when the sum of T for all reflection waveforms (ΣT) is adopted to estimate root diameter. To use A values to detect roots, litter should be removed or equalized in thickness. Radar profiles could not reliably differentiate among roots belonging to plants of different species.

     

Spatial distribution and phenotypic variation in root cortical aerenchyma of maize (Zea mays L.)

Background and aims

Previous research has suggested that root cortical aerenchyma (RCA) can enhance soil exploration and crop performance by decreasing root respiration. This trait is a potential breeding target for adaptation to abiotic stresses such as drought and low nutrient availability. However, little is known of phenotypic variation in aerenchyma or its distribution among root classes.

Methods

The spatial distribution of RCA was evaluated in freehand sections from 13 sites in the root systems of 11 recombinant inbred and commercial lines of maize (Zea mays). RCA variation was evaluated in 583 recombinant inbred lines of maize at one sampling position.

Results

RCA varied significantly among root classes and axial positions. Genotypic differences were observed for the amount of RCA at corresponding sampling locations and for the mean amount of RCA across all sampling locations, but genotypes did not differ in the proportional distribution of RCA within the whole root system. The amount of RCA in a cross-section was independent of several other anatomical traits.

Conclusions

There is substantial genetic variation for RCA, and this variation is independent of other anatomical traits. RCA can be phenotyped in greenhouse-grown plants by sampling the middle parts of second- or third-whorl crown roots.