Frédéric Danjon

The role of fine and coarse roots in shallow slope stability and soil erosion control with a focus on root system architecture: a review

The contribution of plant root systems to slope stability and soil erosion control has received a lot of attention in recent years. The plant root system is an intricate and adaptive object, and understanding the details of soil–root interaction is a difficult task. Although the morphology of a root system greatly influences its soil-fixing efficiency, limited architectural work has been done in the context of slope stabilization and erosion control, and hence it remains unknown exactly which characteristics are important. Many of the published research methods are tedious and time-consuming. This review deals with the underlying mechanisms of shallow slope stabilization and erosion control by roots, especially as determined by their architectural characteristics. The effect of soil properties as well as the relative importance of different root sizes and of woody versus non-woody species are briefly discussed. Empirically and intuitively, architectural features seem to determine the effect of root systems on erosion phenomena and an effort is therefore made here to link both aspects. Still, the research to underpin this relationship is poorly developed. A variety of methods are available for detailed root system architectural measurement and analysis. Although, generally time-consuming, a full 3D architectural description followed by analysis in software such as AMAPmod offers the possibility to extract relevant information on almost any root system architectural characteristic. Combining several methods of measurement and analysis in a complementary way may be a useful option, especially in a context of modelling.

Characterisation of structural tree root architecture using 3D digitising and AMAPmod software

A low-magnetic-field digitising device combined with AMAPmod, a software designed to analyse plant architecture, provided a very efficient method for measuring and studying the geometry and topology of the structural roots of trees. The digitising device measures co-ordinates in a 3D space. AMAPmod was used to assess several characteristics of the root architecture including spatial position, root lengths and volumes, branching order and branching pattern, and to reconstruct 3D images from the data to check for measurement errors. Structural root systems of three 20 to 28-year-old Quercus petraea and thirty 5-year-old Pinus pinaster were uprooted by using rapid mechanised techniques. Only roots with a diamter larger than 2 mm were measured. A fast and precise spatial localisation in combination with the topological characterisation of all root segments was carried out. Oak showed a stronger more oblique and vertical rooting, more branches, more forks and narrower branching angles than pine. Oak had only few small roots in the north west direction whereas half of pine root volume was located in the 10 cm upper soil volume. The contribution of this new method in the characterisation of structural root systems is discussed and other possible applications of this method in root studies are proposed. Since this method is precise and fairly rapid, it may be used for agronomic testing (i.e. comparing treatments) involving several dozen root systems. Almost all parameters needed for tree root system simulations can be estimated from such data.

Structural root architecture of 5-year-old Pinus pinaster measured by 3D digitising and analysed with AMAPmod

Pinus pinaster (Ait.) is a high yielding forest tree, producing nearly a fourth of French marketed timber essentially from intensively managed stands located in southwestern France, in the Landes Forest. This species has generally a poor stem straightness, especially when it grows in poor sandy podzol of the Landes Forest, affected by summer droughts and winter floods. Above- and below-ground architecture and biomass as well as stem straightness were measured on twenty-nine 5-year-old planted trees uprooted by pulling with a lumbering crane. A very precise numeric representation of the geometry and topology of structural root architecture was gained using a low-magnetic-field digitising device (Danjon et al., 1998; Sinoquet and Rivet, 1997). Data were analysed with AMAPmod, a database software designed to analyse plant topological structures (Godin et al., 1997). Several characteristics of root architecture were extracted by queries including root number, length, diameter, volume, spatial position, ramification order, branching angle and inter-laterals length. Differences between root systems originated from their dimensions, but also from the proportion of deep roots and the taproot size, which represented 8% of the total root volume. The proportion of root volume in the zone of rapid taper was negatively correlated with the proportion of root volume in the taproot indicating a compensation between taproot and main lateral root volume. Among all studied root characteristics the maximal rooting depth, the proportion of deep roots and the root partitioning coefficient were correlated with stem straightness.

Towards developmental modelling of tree root systems

Abstract Knowledge of belowground structures and processes is essential for understanding and predicting ecosystem functioning, and consequently in the development of adaptive strategies to safeguard production from trees and woody plants into the future. In the past, research has mainly been concentrated on growth models for the prediction of agronomic or forest production. Newly emerging scientific challenges, e.g. climate change and sustainable development, call for new integrated predictive methods where root systems development will become a key element for understanding global biological systems. The types of input data available from the various branches of woody root research, including biomass allocation, architecture, biomechanics, water and nutrient supply, are discussed with a view to the possibility of incorporating them into a more generic developmental model. We discuss here the main focus of root system modelling to date, including a description of simple allometric biomass models, and biomechanical stress models, and then build in complexity through static growth models towards architecture models. The next progressive and logical step in developing an inclusive developmental model that integrates these modelling approaches is discussed.

Influence of wind loading on root system development and architecture in oak (Quercus robur L.) seedlings

The effect of wind loading on seedlings of English oak (Quercus robur L.) was investigated. Instead of using a traditional wind tunnel, an innovative ventilation system was designed. This device was set up in the field and composed of a rotating arm supporting an electrical fan, which emitted an air current similar to that of wind loading. Oaks were sown from seed in a circle around the device. A block of control plants was situated nearby, and was not subjected to artificial wind loading. After 7 months, 16 plants from each treatment were excavated, and root architecture and morphological characteristics measured using a 3D digitiser. The resulting geometrical and topological data were then analysed using AMAPmod software. Results showed that total lateral root number and length in wind stressed plants were over two times greater than that in control trees. However, total lateral root volume did not differ significantly between treatments. In comparing lateral root characters between the two populations, it was found that mean root length, diameter and volume were similar between the two treatments. In trees subjected to wind loading, an accentuated asymmetry of root distribution and mean root length was found between the windward and leeward sides of the root system, with windward roots being significantly more numerous and longer than leeward roots. However, no differences were found when the two sectors perpendicular to the wind direction were compared. Mean tap root length was significantly higher in control samples compared to wind stressed plants, whilst mean diameter was greater in the latter. Wind loading appears to result in increased growth of lateral roots at the expense of the tap root. Development of the lateral root system may therefore ensure better anchorage of young trees subjected to wind loading under certain conditions.

Improving models of forest nutrient export with equations that predict the nutrient concentration of tree compartments

Abstract• The objective of this study was to explore the distribution of major nutrients (N, P, K, Ca and Mg) in the aboveground compartments of an intensively managed tree species (Pinus pinaster Ait.). A total of 53 trees were cut down in even-aged stands respectively 8, 16, 26, 32 and 40 years old. The nutrient concentrations of the aboveground compartments were analysed.• Nutrient concentrations of foliage did not vary with any of the variables used, except needle age. Nutrient concentrations of living branches, stem bark, stem sapwood, stem heartwood, stemwood and stem decreased with increasing branch diameter, bark thickness, sapwood thickness and heartwood thickness, respectively. Beyond a certain value of the predictive variable (stem diameter ≈ 15 cm; branch diameter ≈ 2.5 cm), the concentration of all the nutrients stabilised.• A 50 year-old pine stand was used to obtain a validation dataset for nitrogen concentration. For this nutrient, the regression relationships gave satisfactory estimates for most compartments (mean error = 12–25%) and particularly for the stem.• A procedure is proposed to estimate the nutrient exports associated with harvests of Pinus pinaster biomass.Résumé• L’objectif de cette étude est d’explorer la distribution des éléments majeurs (N, P, K, Ca, Mg) dans les compartiments aériens d’une essence gérée de manière intensive. Au total, 53 pins maritimes (Pinus pinaster Ait.) ont été abattus parmi des peuplements équiennes de 8, 16, 26, 32 et 40 ans.• Les concentrations en nutriments du feuillage ne varient pas pour une classe d’âge d’aiguilles donnée. Les concentrations des branches vivantes, de l’écorce, de l’aubier et du duramen décroissent lorsque le diamètre ou l’épaisseur du compartiment considéré augmente. La concentration de l’ensemble des nutriments devient constante lorsque la variable prédictive (diamètre ou épaisseur) atteint une valeur plateau.• Un jeu de données de validation pour les concentrations en azote, provenant d’un peuplement équienne de pins de 50 ans, permet de confirmer les performances satisfaisantes des modèles construits (erreur moyenne = 12–25 %) et en particulier pour le tronc.• Une procédure d’estimation des exportations de nutriments associées aux récoltes de biomasse de Pinus pinaster est présentée.

Soil water stress affects both cuticular wax content and cuticle-related gene expression in young saplings of maritime pine (Pinus pinaster Ait)

BackgroundThe cuticle is a hydrophobic barrier located at the aerial surface of all terrestrial plants. Recent studies performed on model plants, such as Arabidopsis thaliana, have suggested that the cuticle may be involved in drought stress adaptation, preventing non-stomatal water loss. Although forest trees will face more intense drought stresses (in duration and intensity) with global warming, very few studies on the role of the cuticle in drought stress adaptation in these long-lived organisms have been so far reported.ResultsThis aspect was investigated in a conifer, maritime pine (Pinus pinaster Ait.), in a factorial design with two genetic units (two half-sib families with different growth rates) and two treatments (irrigated vs non-irrigated), in field conditions. Saplings were grown in an open-sided greenhouse and half were irrigated three times per week for two growing seasons. Needles were sampled three times per year for cuticular wax (composition and content) and transcriptome (of 11 genes involved in cuticle biosynthesis) analysis. Non-irrigated saplings (i) had a higher cuticular wax content than irrigated saplings and (ii) overexpressed most of the genes studied. Both these trends were more marked in the faster growing family.ConclusionsThe higher cuticular wax content observed in the non-irrigated treatment associated with strong modifications in products from the decarbonylation pathway suggest that cuticular wax may be involved in drought stress adaptation in maritime pine. This study provides also a set of promising candidate genes for future forward genetic studies in conifers.

Descendant root volume varies as a function of root type: estimation of root biomass lost during uprooting in Pinus pinaster

Root systems of woody plants generally display a strong relationship between the cross-sectional area or cross-sectional diameter (CSD) of a root and the dry weight of biomass (DWd) or root volume (Vd) that has grown (i.e., is descendent) from a point. Specification of this relationship allows one to quantify root architectural patterns and estimate the amount of material lost when root systems are extracted from the soil. However, specifications of this relationship generally do not account for the fact that root systems are comprised of multiple types of roots. We assessed whether the relationship between CSD and Vd varies as a function of root type. Additionally, we sought to identify a more accurate and time-efficient method for estimating missing root volume than is currently available. We used a database that described the 3D root architecture of Pinus pinaster root systems (5, 12, or 19 years) from a stand in southwest France. We determined the relationship between CSD and Vd for 10,000 root segments from intact root branches. Models were specified that did and did not account for root type. The relationships were then applied to the diameters of 11,000 broken root ends to estimate the volume of missing roots. CSD was nearly linearly related to the square root of Vd, but the slope of the curve varied greatly as a function of root type. Sinkers and deep roots tapered rapidly, as they were limited by available soil depth. Distal shallow roots tapered gradually, as they were less limited spatially. We estimated that younger trees lost an average of 17% of root volume when excavated, while older trees lost 4%. Missing volumes were smallest in the central parts of root systems and largest in distal shallow roots. The slopes of the curves for each root type are synthetic parameters that account for differentiation due to genetics, soil properties, or mechanical stimuli. Accounting for this differentiation is critical to estimating root loss accurately.

The effect of mechanical stimulation on root and shoot development of young containerised Quercus robur and Robinia pseudoacacia trees

Thigmomorphogenesis is a well-studied process in agricultural crops and coniferous trees. Nevertheless, the effects on both shoot and root characteristics for deciduous woody species received little attention so far. In this study, the objective was to understand the effect of aboveground flexing treatments on the development of structural, mechanical and physiological root and shoot characteristics for two deciduous tree species, Black locust (Robinia pseudoacacia L.) and English oak (Quercus robur L.). Flexing treatments were performed using an electromechanical device with a rotating arm touching and bending the plants at regular intervals. A wide range of stem, shoot as well as root system characteristics was measured. The different flexing treatments altered above- and belowground plant development for both species, with strongest effects on Quercus and most significant differences between the control and the unidirectional flexing treatment. Some responses are in accordance with previous findings, such as stem eccentricity and reduced shoot elongation under unidirectional flexing, but others are renewing, such as the lower stomatal density and larger epidermal cell surface for the Quercus plants under variable flexing direction. Despite some common responses, both species frequently differed in the way they were affected. Belowground, Quercus plants under unidirectional flexing invested relatively more in their first order root and deeper second order roots, whereas Robinia plants allocated relatively more to fine root biomass and horizontal shallow roots. Both strategies potentially increased pull-out as well as overturning resistance in their own way. The presented findings are valid for young trees grown in small containers. Based on practical know-how and shortcomings experienced in the course of this experiment, methodological recommendations are formulated. We finally stress the complex variability in growth responses, especially for root systems, observed in different studies and related to dissimilarity in species, soil conditions, plant history or type of mechanical perturbation.

Biomass and nutrients in tree root systems-sustainable harvesting of an intensively managed Pinus pinaster (Ait.) planted forest.

To develop sources of renewable energy and to reduce greenhouse gas emissions, increasing attention has been given to the extraction of forest biomass, especially in the form of harvest residues. However, increasing the removal of biomass, and hence nutrients, has raised concerns about the sustainability of site fertility and forest productivity. The environmental cost of harvesting belowground biomass is still not fully understood. The objectives of this study were to (i) estimate the stocks of belowground biomass that potentially can be collected; (ii) measure the nutrient (N, P, K, Ca, Mg) concentrations of the different root compartments (stumps, coarse and thin roots); and to (iii) quantify the biomass and nutrient exports under different scenarios, including harvests of above and belowground compartments. The study was carried out on Pinus pinaster stands located in south‐western France. Results showed that roots could be a significant fuelwood resource, particularly at forest clear cutting. Negative relationships between root diameter and root nutrient concentration were observed, independently of root function or tree age. Such relationships can be used to accurately simulate nutrient concentrations in roots as well as nutrient exports. Combining our original results on roots with previously published data on the aboveground compartments showed that nutrient losses were higher in canopy harvest scenarios than in root harvest scenarios. This was mainly due to high nutrient concentrations of needles. We concluded that stump and root harvest could be sustainable in our study context, conversely to foliage harvest. Because thin roots have higher nutrient concentrations than coarse roots and the proportion of thin roots increased with an increase in the distance from the tree, collecting roots only in the close vicinity of the stumps should limit nutrient exports (particularly N) without unnecessarily reducing fuelwood biomass.