Résumé :

Background and Aims Characterising the spatial distribution of tree fine roots (diameter ≤ 2 mm) is fundamental for a better understanding of belowground functioning when tree are grown with associated crops in agroforestry systems. Our aim was to compare fine root distributions and orientations in trees grown in an alley cropping agroforestry stand with those in a tree monoculture. Methods Fieldwork was conducted in two adjacent 17 year old hybrid walnut (Juglans regia × nigra L.) stands in southern France: the agroforestry stand was intercropped with durum wheat (Triticum turgidum L. subsp. durum) whereas the tree monoculture had a natural understorey. Root intercepts were mapped to a depth of 150 cm on trench walls in both stands, and to a depth of 400 cm in the agroforestry stand in order to characterise tree root distribution below the crop's maximum rooting depth. Soil cubes were then extracted to assess three dimensional root orientation and to establish a predictive model of root length densities (RLD) derived from root intersection densities (RID). Results In the tree monoculture, root mapping demonstrated a very high tree RID in the top 50 cm and a slight decrease in RID with increasing soil depth. However, in the agroforestry stand, RID was significantly lower at 50 cm, tree roots colonized deeper soil layers and were more vertically oriented. In the agroforestry stand, RID and RLD were greater within the tree row than in the inter-row. Conclusions Fine roots of intercropped walnut trees grew significantly deeper, indicating a strong plasticity in root distribution. This plasticity reduced direct root competition from the crop, enabling trees to access deeper water tables not available.

Résumé :

La stabilité structurale et la résistance au cisaillement du sol (cohésion et angle de frottement interne) qualifient respectivement la sensibilité des sols à l’érosion hydrique de surface et l’érosion en masse (glissements de terrains peu profonds). Cette sensibilité dépend des propriétés intrinsèques des sols et du contexte environnant climatique, écologique, topographique et anthropique. Des mesures de ces deux indicateurs ont été effectuées en conditions in situ et parallèlement : i) sur trois type de sols aux propriétés physico-chimiques différentes : un alluviosol (France), un andosol (Costa Rica) et un ferralitisol (Costa Rica) ; ii) en comparant différents agrosystèmes: Monoculture/Agroforesterie ; iii) en prenant en compte l’effet de profondeur entre l’horizon A et B. L’étude a permis de décrire au mieux le caractère érodible intrinsèque des sols, de relever les influences des propriétés physico-chimiques des sols sur la stabilité structurale et la cohésion, et de mieux comprendre les interactions entre ces différents paramètres. Les résultats obtenus montrent l’importance des formes du fer (Fe), de l’aluminium (Al), du carbone organique (SOC), de l’exsudation racinaire et du taux d’argiles sur la stabilité structurale. Sur la cohésion effective, sont mis en avant l’importance de la densité, de l’effet des racines fines (≤0.5mm de diamètre) et du taux d’argiles.

Résumé :

Background Plants alter their environment in a number of ways. With correct management, plant communities can positively impact soil degradation processes such as surface erosion and shallow landslides. However, there are major gaps in our understanding of physical and ecological processes on hillslopes, and the application of research to restoration and engineering projects.

Résumé :

1. There is a fundamental trade-off between leaf traits associated with either resource acquisition or resource conservation. This gradient of trait variation, called the economics spectrum, also applies to fine roots, but whether it is consistent for coarse roots or at the plant community level remains untested. 2. We measured a set of morphological and chemical root traits at a community level (functional parameters; FP) in 20 plant communities located along land-use intensity gradients and across three climatic zones (tropical, mediterranean and montane). We hypothesized (i) the existence of a root economics spectrum in plant communities consistent within root types (fine, < 2 mm; coarse, 2–5 mm), (ii) that variations in root FP occur with soil depths (top 20 cm of soil and 100–150 cm deep) and (iii) along land-use gradients. 3. Root FP covaried, in line with the resource acquisition–conservation trade-off, from communities with root FP associated with resource acquisition (e.g. high specific root length, SRL; thin diameters and low root dry matter contents, RDMC) to root FP associated with resource conservation (e.g. low SRL, thick diameters and high RDMC). This pattern was consistent for both fine and coarse roots indicating a strong consistency of a trade-off between resource acquisition and conservation for plant roots. 4. Roots had different suites of traits at different depths, suggesting a disparity in root function and exploitation capacities. Shallow, fine roots were thinner, richer in nitrogen and with lower lignin con- centrations associated with greater exploitation capacities compared to deep, fine roots. Shallow, coarse roots were richer in nitrogen, carbon and soluble concentrations than deep, coarse roots. 5. Fine root parameters of highly disturbed, herbaceous-dominated plant communities in poorer soils were associated with foraging strategies, that is greater SRL and lower RDMC and lignin concentra- tion than those from less disturbed communities. Coarse roots, however, were less sensitive to the land-use gradient. 6. Synthesis. This study demonstrates the existence of a general trade-off in root construction at a community level, which operates within all root types, suggesting that all plant tissues are controlled by the trade-off between resource acquisition and conservation.

Résumé :

Quantifying the dynamics of root growth is vital when characterising the role of vegetation in carbon cycling. Aims: We examined the temporal dynamics of root growth and responses to spatial (altitude, forest patchiness and soil depth) and biological factors (root diameter and root topology) in mid-montane and upper montane coniferous forest ecosystems. Methods: Using rhizotrons, two indicators were investigated: occurrence, i.e. the proportion of roots which had elongated since the previous measurement of root elongation (%), and daily root elongation speed (mm d−1) once the elongation occurred.

Results :

Spatial factors had a limited effect on root growth. Roots in the same diameter class possessed different elongation speeds and this was related to topological ranking, reflecting a disparity in physiological activity. Temporally, the occurrence of root elongation reached a peak in May–October (up to 90%) and sharply dropped after October 2010. The maximum root elongation speed (mean: 3.0 mm d−1) was measured in July–August. Root growth was the most inactive in February 2011 but some roots still exhibited positive elongation speeds (mean: 0.5 mm d−1). Occurrence and speed of elongation reacted differently with regard to environmental and biological factors.

Conclusions :

span Temporal and biological factors contributed more towards explaining the variability of root growth than spatial factors. In future studies, both occurrence and speed of elongation should be used to characterise root growth.

Résumé :

Over the last 50 years, alterations in land-use coupled with the consequences of climate change have led to severe degradation of mountainous and hilly regions around the world. Once a landslide has occurred or erosion processes are underway, the replacement of soil on a denuded slope may take hundreds or thousands of years through natural processes. In a world where the population is expected to reach 9 billion by 2040, agricultural soil is precious and hillslope stability is now a priority for governments in mountainous countries needing to feed rapidly increasing populations. Equally important are the effects of slope failure and soil erosion on community safety, infrastructures and downstream water quality and habitat. Therefore, the prevention of slope instability, the restoration of degraded slopes and the correct management of steep farmed slopes is of utmost importance. In response to the need for better mitigation strategies, there have been major advances in research and applications for using vegetation to improve slope stability in the last 20 years.

Résumé :

Characterizing the above- and belowground carbon stocks of ecosystems is vital for a better understanding of the role of vegetation in carbon cycling. Yet studies on forest ecosystems at high altitudes remain scarce. We examined above- and belowground carbon partitioning in trees growing in mixed montane/upper montane forest ecosystems in the French Alps. Field work was performed in three forests along a gradient of both altitude (1400 m, 1700 m, and 2000 m) and altitude-induced species composition (from lower altitude Abies alba and Fagus sylvatica to higher altitude Picea abies and Pinus uncinata). We performed forest inventories and root sampling along soil wall profiles, so that the stand basal area (SBA, in m2 ha–1) and root cross-sectional area (RCSA, in m2 ha–1) were estimated at each altitude. To characterize the carbon allocation trend between the above- and belowground compartments, the ratio of RCSA to SBA was then calculated. We found that both SBA and RCSA of coarse roots (diameter > 2 mm) were significantly different among the three altitudes. No significant difference in RCSA of fine roots (diameter 2 mm) was found among altitudes. The ratio of RCSA of fine roots to SBA augmented with increasing elevation, suggesting that forest ecosystems at higher altitudes allocate more carbon from above- to belowground organs. This increased allocation to fine roots would allow trees to scavenge nutrients more efficiently throughout the short growing season. Furthermore, this work highlighted the interest of using easy to measure area-based in- dicators as proxies of root and stem biomass when investigating carbon partitioning in highly heterogeneous montane/upper montane forests.

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Agroforestry systems, i.e., agroecosystems combining trees with farming practices, are of particular interest as they combine the potential to increase biomass and soil carbon (C) storage while maintaining an agricultural production. However, most present knowledge on the impact of agroforestry systems on soil organic carbon (SOC) storage comes from tropical systems. This study was conducted in southern France, in an 18-year-old agroforestry plot, where hybrid walnuts (Juglans regia × nigra L.) are intercropped with durum wheat (Triticum turgidum L. subsp. durum), and in an adjacent agricultural control plot, where durum wheat is the sole crop. We quantified SOC stocks to 2.0 m depth and their spatial variability in relation to the distance to the trees and to the tree rows. The distribu- tion of additional SOC storage in different soil particle-size fractions was also characterized. SOC accumulation rates between the agroforestry and the agricultural plots were 248 ± 31 kg C ha− 1 yr− 1 for an equivalent soil mass (ESM) of 4000 Mg ha−1 (to 26–29 cm depth) and 350 ± 41 kg C ha−1 yr−1 for an ESM of 15,700 Mg ha−1 (to 93–98 cm depth). SOC stocks were higher in the tree rows where herbaceous vegetation grew and where the soil was not tilled, but no effect of the distance to the trees (0 to 10 m) on SOC stocks was observed. Most of the additional SOC storage was found in coarse organic fractions (50–200 and 200–2000 μm), which may be rather labile fractions. All together our study demonstrated the potential of alley cropping agroforestry systems under Mediterranean con- ditions to store SOC, and questioned the stability of this storage

Résumé :

Background: Quantifying the dynamics of root growth is vital when characterising the role of vegetation in carbon cycling. Aims: We examined the temporal dynamics of root growth and responses to spatial (altitude, forest patchiness and soil depth) and biological factors (root diameter and root topology) in mid-montane and upper montane coniferous forest ecosystems. Methods: Using rhizotrons, two indicators were investigated: occurrence, i.e. the proportion of roots which had elongated since the previous measurement of root elongation (%), and daily root elongation speed (mm d?1) once the elongation occurred. Results: Spatial factors had a limited effect on root growth. Roots in the same diameter class possessed different elongation speeds and this was related to topological ranking, reflecting a disparity in physiological activity. Temporally, the occurrence of root elongation reached a peak in May-October (up to 90%) and sharply dropped after October 2010. The maximum root elongation speed (mean: 3.0 mm d?1) was measured in July-August. Root growth was the most inactive in February 2011 but some roots still exhibited positive elongation speeds (mean: 0.5 mm d?1). Occurrence and speed of elongation reacted differently with regard to environmental and biological factors. Conclusions: Temporal and biological factors contributed more towards explaining the variability of root growth than spatial factors. In future studies, both occurrence and speed of elongation should be used to characterise root growth.

Résumé :

Background and aims : Vegetation can be used to stabilise slopes with regard to shallow landslides, but the optimal plant architecture for conferring resistance is not known. This study aims at identifying root morphological traits which confer the most resistance to soil during shearing. Methods : Three species used for slope stabilisation (Ricinus communis L., Jatropha curcas L. and Rhus chinensis Mill.) were grown for 10 months in large shear boxes filled with silty clay similar to that found in Yunnan, China. Direct shear tests were then performed and compared to fallow soil. Root systems were excavated and a large number of traits measured. Results : Shear strength and deformation energy were enhanced by the presence of roots. Regardless of confining pressure, R. communis conferred most resistance due to its taprooted system with many vertical roots. J. curcas possessed oblique and vertical roots which created fragile zones throughout the soil profile. The least efficient root system was R. chinensis which possessed many horizontal lateral roots. Soil mechanical properties were most influenced by (i) density of roots crossing the shear plane, (ii) branching density throughout the soil profile, (iii) total length of coarse roots above the shear plane and (iv) total volume of coarse roots and fine root density below the shear plane. During failure, fine, short and branched roots slipped through soil rather than breaking. Conclusion: Root morphological traits such as density, branching, length, volume, inclination and orientation influence significantly soil mechanical properties.

Résumé :

Vegetation can play an important role in stabilizing soil against shallow landslides. Using a three-dimensional (3D) finite element method, we developed a model to study the impact of different management scenarios on slope stability in mountain forests. Ground truth data were obtained from a mixed forest ecosystem situated at an altitude of 1400 m a.s.l. in the French Alps. Five scenarios representing the forest at different spatial and temporal stages of management were selected: [A] bare soil, [B] tree island (i.e. tree groups growing in clusters) on bare soil, [C] new gap (i.e. canopy free zones with little understorey) in homogeneous forest, [D] old gap (i.e. canopy free zones with abundant understorey) in homogeneous forest and [E] homogeneous forest. For scenarios [B], [C] and [D], the locations of the vegetated patch along the slope (top, centre and toe) were also tested, to determine if vegetation patterns influenced slope stability. As plant roots play a crucial role in reinforcing soil, we altered the 3D spatial distribution of root density in the model using real data. By calculating the factor of safety (FoS), i.e. a measure of the likelihood that the slope will fail, we show that slope morphology, including angle and soil depth, play an essential role in slope stability. Vegetation also exhibited a positive effect on slope stability, but the efficiency of this effect was significantly influenced by slope morphology and root distribution with regard to soil depth. In particular, if a layer of soil beneath the most superficial rooting zone contained few roots, slope integrity was compromised. Compared to bare soil, the FoS increase due to vegetation was only ≤0.2 (i.e. ≤15%), when deeper soil layers contained few or no roots. However, if the soil profile contained roots throughout, the FoS increase was >25% higher. We highlight the importance of taking into account spatial complexity and refining the output, i.e. FoS, during the modelling of slope stability, which can only be achieved through the use of 3D models.

Résumé :

: Background and aims : Plant roots provide mechanical cohesion (c r ) to soil on slopes which are prone to shallow landslides. c r varies in heterogeneous natural forests due to the spatial, inter- and intra-annual dynamics of root demography. Characterizing root initiation density and mortality, as well as how root growth is influenced by abiotic and biotic factors is essential for exploring a root system’s capacity to reinforce soil. Methods : In this study, root demography data were monitored using field rhizotrons during 1.5 years in two naturally regenerated mixed forests in the French Alps. These forests are composed of trees growing in groups (tree islands) with large gaps between the islands. Three categories of driving variables were measured: (i) spatial factors: altitude (1,400 m, 1,700 m), ecological patch (gap, tree island), soil depth (0.0–1.0 m divided into five layers of 0.2 m); (ii) temporal factors: month (12 months from March 2010 to February 2011), winter (winter of 2009–2010 and 2010–2011); (iii) biological factors: root diameter classes (]0, 1] mm, ]1, 2] mm, ]2, 5] mm (according to the international standard ISO 31–11, ]x, y] denotes a left half-open interval from x (excluded) to y (included)). Two types of two-part models, a Hurdle model (H) and a Zero-inflated model (ZI) were used to fit root data with a high zero population, i.e. if root initiation or mortality was zero during a given time period, or if roots were not present at all points throughout a soil profile. Results : Root initiation quantity decreased with increasing soil depth, as well as being lower in tree islands. Both soil depth and ecological patch interacted strongly with altitude. Root dynamics were significantly less active with a lower net production and c r increment in winter and spring than in summer and autumn. Roots which were ]1, 2] mm in diameter contributed the most to c r compared to other diameter classes, as they had a high production but a low mortality. With regard to model selection, both H and ZI demonstrated similar outcomes and underestimated extreme values of root demography data. Conclusion : All factors contributed towards explaining the variability of root demography and c r . We suggest taking into consideration the seasonality of root dynamics when studying root reinforcement.

Résumé :

Agroforestry systems are land use management systems in which trees are grown in combination with crops or pasture in the same field. In silvoarable systems, trees are intercropped with arable crops, and in silvopastoral systems trees are combined with pasture for livestock. These systems may produce forage and timber as well as providing ecosystem services such as climate change mitigation. Carbon (C) is stored in the aboveground and belowground biomass of the trees, and the transfer of organic matter from the trees to the soil can increase soil organic carbon (SOC) stocks. Few studies have assessed the impact of agroforestry systems on carbon storage in soils in temperate climates, as most have been undertaken in tropical regions. This study assessed five silvoarable systems and one silvopastoral system in France. All sites had an agroforestry system with an adjacent, purely agricultural control plot. The land use management in the inter-rows in the agroforestry systems and in the control plots were identical. The age of the study sites ranged from 6 to 41 years after tree planting. Depending on the type of soil, the sampling depth ranged from 20 to 100 cm and SOC stocks were assessed using equivalent soil masses. The aboveground biomass of the trees was also measured at all sites. In the silvoarable systems, the mean organic carbon stock accumulation rate in the soil was 0.24 (0.09–0.46) Mg C ha-1 yr-1 at a depth of 30 cm and 0.65 (0.004–1.85) Mg C ha-1 yr-1 in the tree biomass. Increased SOC stocks were also found in deeper soil layers at two silvoarable sites. Young plantations stored additional SOC but mainly in the soil under the rows of trees, possibly as a result of the herbaceous vegetation growing in the rows. At the silvopastoral site, the SOC stock was significantly greater at a depth of 30–50 cm than in the control. Overall, this study showed the potential of agroforestry systems to store C in both soil and biomass in temperate regions.