Background and aims Plant roots provide mechanical cohesion (cr) to soil on slopes which are prone to shallow landslides. cr 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 fromx (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 cr increment in winter and spring than in summer and autumn. Roots which were ]1, 2] mm in diameter contributed the most to cr 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 cr. We suggest taking into consideration the seasonality of root dynamics when studying root reinforcement. (Résumé d'auteur

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 d'auteur

International audienceRaw geological materials excavated then piled on large surface areas are submitted to pedogenetic factors. Knowledge of the evolution of this freshly exposed material is required to predict the newly developed Technosol and the corresponding ecosystem. The pedogenetic evolution of a Technosol developed on excavated Callovo-Oxfordian clay minerals (COx) is assessed from the analysis of natural soils developed on analogous materials. Work was based on the hypothesis that the evolution of raw geological materials could be deduced from that of natural soils on similar outcrops in similar climatic conditions. A comparison was made on the basis of mineralogical and geochemical criteria between the unweathered clays and soil material collected from the horizons of two reference pedons on a COx outcrop in North-Eastern France, used by forest or agriculture, and a 10-year-old Technosol derived from freshly excavated COx material piled under outdoor climatic conditions. Soil profiles were described and each sampled horizon was characterized for mineralogy, and physical and chemical properties. Soil profiles were classified as Vertic Stagnic Hypereutric Cambisol (Forest) and Vertic Stagnic Calcaric Cambisol (Meadow), and compared to the Calcaric Technosol (no vegetation yet). They clearly showed brunification and decarbonatization as the major processes having governed their evolution. Signs of clay leaching were also visible on the forest soil. A fairly good adequacy was observed between soil material sampled in the soil deep horizons (2.5 m), the Technosol deep horizon (1.7 m), and the freshly excavated material. Slight differences were recorded, however, i.e., salt loss (NaCl), sulfide oxidation, and precipitation of gypsum from sulfate and calcium released by decarbonatization, as earlier observed in leaching experiments. Salt loss is a rapid process almost completed to the same extent in the Technosol after 10 years than in the Calcaric Cambisol after centuries. The soil under forest showed signs of stronger pedogenetic evolution than the soil submitted to agriculture (meadow), probably because the latter has been refreshed by man-induced erosion. COx evolution can be predicted from the analysis of soils developed on similar outcrops. The Technosol developed on these carbonated materials would evolve as a result of extremely rapid leaching of the soluble phases, then surface enrichment of organic matter, surface decarbonatization, and progressive brunification of the whole profile as the major processes, and give rise to a Cambisol. Technosol study is an interesting approach for dating the age of pedogenetic processes

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