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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.

Few studies describe root distributions at the species level in diverse forests, although belowground species interactions and traits are often assumed to affect fine-root biomass (FRB). We used molecular barcoding to study how FRB of trees relates to soil characteristics, species identity, root diversity, and root traits, and how these relationships are affected by proximity to ecotones in a temperate forest landscape. We found that soil patch root biomass increased in response to soil resources across all species, and there was little belowground vertical or horizontal spatial segregation among species. Root traits and species relative abundance did not explain significant variation in FRB after correcting for soil fertility. A positive relationship between phylogenetic diversity and FRB indicated significant belowground overyielding attributable to local root diversity. Finally, variation in FRB explained by soil fertility and diversity was reduced near ecotones, but only because of a reduction in biomass in periodically anoxic areas. These results suggest that symmetric responses to soil properties are coupled with complementary species traits and interactions to explain variation in FRB among soil patches. In addition, landscape-level dispersal among habitats and across ecotones helps explain variation in the strength of these relationships in complex landscapes.

Competition or complementarity between associated plants due to belowground interactions has been observed in alley-cropping systems (ACs), but the initialization of these processes remains poorly investigated. Here, we used the core-break and soil coring method to quantify the vertical tree and wheat fine root abundance and biomass down to 120 cm in a 4-year-old temperate AC. Fine roots were measured at 2 m from a reference tree (hornbeam, wild cherry or willow) in tree–wheat AC, pure-forest associated with ryegrass (FC) and wheat sole-crop (CC) plots at the Ramecourt experimental site. The mean wheat fine root abundance (WFRA) was twice as high in the CC plot (874 ± 152 m−2) as in the AC plot (437 ± 47 m−2). It was significantly higher for wheat associated with hornbeam than for willow, particularly at the 10 cm depth. Tree fine root abundance (TFRA) was linearly correlated with tree fine root biomass (TFRB) for hornbeam (R2 = 0.79***), willow (R2 = 0.77***) and wild cherry (R2 = 0.54***). Using TFRA, the van Noordwijk's equation gave a better prediction of the TFRB for willow and wild cherry than for hornbeam. The mean value of the TFRA was seven times higher in the FC plot (1116 ± 97 m−2) than in the AC plot (146 ± 24 m−2) for all soil depths and all tree species due to the lack of nutrients from the absence of fertilization. At 4 years old, willow and hornbeam fine roots cohabited with wheat in the upper soil layer, whereas wild cherry had already developed deep fine roots under the crop rooting zone.

BACKGROUND AND AIMS: The influences of succession and species diversity on fine root production are not well known in forests. This study aimed to investigate: (i) whether fine root biomass and production increased with successional stage and increasing tree species diversity; (ii) how forest type affected seasonal variation and regrowth of fine roots. METHODS: Sequential coring and ingrowth core methods were used to measure fine root production in four Chinese subtropical forests differing in successional stages and species diversity. RESULTS: Fine root biomass increased from 262 g·m⁻² to 626 g·m⁻² with increasing successional stage and species diversity. A similar trend was also found for fine root production, which increased from 86 to 114 g·m⁻² yr ⁻¹ for Cunninghamia lanceolata plantation to 211–240 g·m⁻² yr ⁻¹ for Choerospondias axillaries forest when estimated with sequential coring data. Fine root production calculated using the ingrowth core data ranged from 186 g·m⁻² yr ⁻¹ for C. lanceolata plantation to 513 g·m⁻² yr ⁻¹ for Lithocarpus glaber – Cyclobalanopsis glauca forest. CONCLUSIONS: Fine root biomass and production increased along a successional gradient and increasing tree species diversity in subtropical forests. Fine roots in forests with higher species diversity exhibited higher seasonal variation and regrowth rate.

The objectives of this study were to detect coarse tree root and to estimate root biomass in the field by using an advanced 3D Ground Penetrating Radar (3D GPR) system. This study obtained full-resolution 3D imaging results of tree root system using 500 MHz and 800 MHz bow-tie antennas, respectively. The measurement site included two larch trees, and one of them was excavated after GPR measurements. In this paper, a searching algorithm, based on the continuity of pixel intensity along the root in 3D space, is proposed, and two coarse roots whose diameters are more than 5 cm were detected and delineated correctly. Based on the detection results and the measured root biomass, a linear regression model is proposed to estimate the total root biomass in different depth ranges, and the total error was less than 10%. Additionally, based on the detected root samples, a new index named “magnitude width” is proposed to estimate the root diameter that has good correlation with root diameter compared with other common GPR indexes. This index also provides direct measurement of the root diameter with 13%–16% error, providing reasonable and practical root diameter estimation especially in the field.

BACKGROUND AND AIMS: The quantification of root dynamics remains a major challenge in ecological research because root sampling is laborious and prone to error due to unavoidable disturbance of the delicate soil-root interface. The objective of the present study was to quantify the distribution of the biomass and turnover of roots of poplars (Populus) and associated understory vegetation during the second growing season of a high-density short rotation coppice culture. METHODS: Roots were manually picked from soil samples collected with a soil core from narrow (75 cm apart) and wide rows (150 cm apart) of the double-row planting system from two genetically contrasting poplar genotypes. Several methods of estimating root production and turnover were compared. RESULTS: Poplar fine root biomass was higher in the narrow rows than in the wide rows. In spite of genetic differences in above-ground biomass, annual fine root productivity was similar for both genotypes (ca. 44 g DM m⁻² year⁻¹). Weed root biomass was equally distributed over the ground surface, and root productivity was more than two times higher compared to poplar fine roots (ca. 109 g DM m⁻² year⁻¹). CONCLUSIONS: Early in SRC plantation development, weeds result in significant root competition to the crop tree poplars, but may confer certain ecosystem services such as carbon input to soil and retention of available soil N until the trees fully occupy the site.

Background and aims The GPR indices used for predicting root biomass are measures of root radar reflectance. However, root radar reflectance is highly correlated with root water content. The objectives of this study are to assess the impact of root water content on GPR-based root biomass estimation and to develop more reliable approaches to quantify root biomass using GPR. Methods Four hundred nine roots of five plant species in a sandy area of northern China were examined to determine the general water content range of roots in sandy soils. Two sets of GPR simulation scenarios (including 492 synthesized radargrams in total) were then conducted to compare the changes of root radar signal and the accuracies of root biomass estimation by GPR at different root gravimetric water content levels. In the field, GPR transects were scanned for Ulmus puntila roots buried in sandy soils with three antenna center frequencies (0.5, 0.9, and 2.0 GHz). The performance of two new GPR-based root biomass quantification approaches (one using time interval GPR index and the other using a non-linear regression model) was then tested. Results All studied roots exhibited a broad range of gravimetric water content (>125 %), with the water contents of most roots ranging from 90 % to 150 %. Both field experiments and forward simulations indicated that 1) waveforms of root radar reflection, radar-reflectance related GPR indices, and root biomass estimation accuracy were all affected by root water content; and 2) using time interval index and establishing a nonlinear regression model of root biomass on GPR indices improved the accuracy of root biomass estimation, decreasing the prediction error (RMSE) by 4 to 30 % under field conditions. Conclusions The magnitude of GPR indices depends on both root biomass and root water content, and root water content affects root biomass estimation using GPR indices. Using a linear regression model of root biomass on radar-reflectance related GPR index for root biomass estimation would only be feasible for roots with a relative narrow range of water content (e.g., when gravimetric water contents of studied roots vary within 20 %). Appropriate GPR index and regression models should be selected based on the water content range of roots. The new protocol of root biomass quantification by GPR presented in this study improves the accuracy of root biomass estimation.

We investigated the effects of seasonal changes in soil moisture on the morphological and growth traits of fine roots (<2 mm in diameter) in a mature Turkey-oak stand ( Quercus cerris L.) in the Southern Apennines of Italy. Root samples (diameter: <0.5, 0.5–1.0, 1.0–1.5, and 1.5–2.0 mm) were collected with the Auger method. Mean annual fine-root mass and length on site was 443 g m −2 (oak fine roots 321 g m −2 ; other species 122 g m −2 ) and 3.18 km m −2 (oak fine roots 1.14 km m −2 ; other species 2.04 km m −2 ), respectively. Mean specific root length was 8.3 m g −1 . All fine-root traits displayed a complex pattern that was significantly related to season. In the four diameter classes, both fine-root biomass and length peaked in summer when soil water content was the lowest and air temperature the highest of the season. Moreover, both fine-root biomass and length were inversely related with soil moisture ( p  < 0.001). The finest roots (<0.5 mm in diameter) constituted an important fraction of total fine-root length (79 %), but only 21 % of biomass. Only in this root class, consequent to change in mean diameter, specific root length peaked when soil water content was lowest showing an inverse relationship ( p  < 0.001). Furthermore, fine-root production and turnover decreased with increasing root diameter. These results suggest that changes in root length per unit mass, and pulses in root growth to exploit transient periods of low soil water content may enable trees to increase nutrient and water uptake under seasonal drought conditions.

BACKGROUND AND AIMS: Results from many biodiversity experiments have established evidence for positive effects of diversity on aboveground plant productivity. However, less is known about the relationships between plant diversity and belowground plant community characteristics and their consistency at altered environmental conditions. METHODS: Monocultures, two- and four-species mixtures of two independent pools of four perennial temperate grassland species, each representing two functional groups (grasses, forbs) and two growth statures (tall, small), were grown in a field experiment at crossed levels of light and fertilization. Standing root biomass and root morphological traits were studied in the second year of treatment applications. RESULTS: Increased species richness or fertilization did not influence belowground characteristics. Shading decreased standing root biomass and affected root morphological characteristics. The vertical distribution of standing root biomass and root length density over the depth profile and root morphological traits differed in communities of varying functional composition irrespective of resource availability, and differences were partly increased when shading was combined with fertilization. CONCLUSIONS: Independent of resource availability, plant species richness does not increase vertical root segregation, but the potential for complementary use of belowground resources increases when species with different rooting patterns and root morphological traits are combined in mixtures.

Growth and distribution of coarse roots in time and space represent a gap in our understanding of belowground ecology. Large roots may play a critical role in carbon sequestration belowground. Using ground-penetrating radar (GPR), we quantified coarseroot biomass from an open-top chamber experiment in a scrub-oak ecosystem at Kennedy Space Center, Florida, USA. GPR propagates electromagnetic waves directly into the soil and reflects a portion of the energy when a buried object is contacted. In our study, we utilized a 1500 MHz antenna to establish correlations between GPR signals and root biomass. A significant relationship was found between GPR signal reflectance and biomass (R2 1/4 0.68). This correlation was applied to multiple GPR scans taken from each open-top chamber (elevated and ambient CO2). Our results showed that plots receiving elevated CO2 had significantly (P 1/4 0.049) greater coarse-root biomass compared to ambient plots, suggesting that coarse roots may play a large role in carbon sequestration in scrub-oak ecosystems. This nondestructive method holds much promise for rapid and repeatable quantification of coarse roots, which are currently the most elusive aspect of long-term belowground studies.