Forests can prevent and/or mitigate hydrogeomorphic hazards in mountainous landscapes. Their effect is particularly relevant in the case of shallow landslides phenomena, where plants decrease the water content of the soil and increase its mechanical strength. Although such an effect is well known, its quantification is a relatively new challenge. The present work estimates the effect of some forest species on hillslope stability in terms of additional root cohesion by means of a model based on the classical Wu and Waldron approach (Wu in Alaska Geotech Rpt No 5 Dpt Civ Eng Ohio State Univ Columbus, USA, 1976; Waldron in Soil Sci Soc Am J 41:843-849, 1977). The model is able to account for root distribution with depth and nonsimultaneous root breaking. Samples of European beech (Fagus sylvatica L.), Norway spruce (Picea abies (L.) Karst.), European larch (Larix decidua Mill.), sweet chestnut (Castanea sativa Mill.) and European hop-hornbeam (Ostrya carpinifolia Scop.), were taken from different locations of Lombardy (Northern Italy) to estimate root tensile strength, the Root Area Ratio and the root cohesion distribution in the soil. The results show that, in spite of its dramatic variability within the same species at the same location and among different locations, root cohesion can be coherently interpreted using the proposed method. The values herein obtained are significant for slope stabilisation, are consistent with the results of direct shear tests and back-analysis data, and can be used for the estimation of the stability of forested hillslopes in the Alps.

Forest vegetation is known to increase hillslope stability by reinforcing soil shear resistance and by influencing hydrologic conditions of soil. Although the importance of plant root systems for hillslope stability has received considerable attention in recent years, the quantification of such an effect needs more investigation. In this paper, we present a synthesis of the data gathered in the last 5 years for some species in different locations of the Alps and Prealps of Lombardy (Northern Italy) with the aim to increase our knowledge on root tensile strength and on Root Area Ratio distribution within the soil. Concerning root tensile strength we developed tensile strength-diameter relationships for eight species: green alder (Alnus viridis (Chaix) D.C.), beech (Fagus sylvatica L.), red willow (Salix purpurea L.), goat willow (Salix caprea L.), hazel (Corylus avellana L.), European ash (Fraxinus excelsior L.), Norway spruce (Picea abies (L.) Karst.) and European larch (Larix decidua Mill.). Results show a great variability among the different species and also for the same species. In general, however, root strength (in terms of tension) tends to decrease with diameter according to a power law, as observed by other Authors. Comparing the power law fitting curves for the considered species, it can be observed that they fall in a relatively narrow band, with the exception of hazel, which appears the most resistant. Concerning the evaluation of root distribution within the soil we estimated the Root Area Ratio (the ratio between the area occupied by roots in a unit area of soil) according to its depth for five species (beech, Norway spruce, European larch, mixed hazel and ash) in three locations of Lombardy. Results show that there is a great variability of root density for the same species well as for different points at the same locality. The general behaviour of root density, in any case, is to decrease with depth according to a gamma function for all the studied species. The results presented in this paper contribute to expanding the knowledge on root resistance behaviour and on root density distribution within the soil. The studied location have allowed the implementation of soil-root reinforcement models and the evaluation of the vegetation contribution to soil stability.

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Root reinforcement is a key factor when dealing with slope stability problems and is an important quantitative criterion for the evaluation of the protective function of forests against shallow landslides, as well as for the adoption of appropriate practices in protection forest management.Although many models have been developed to estimate root reinforcement, a reliable quantification that considers both its spatial and temporal variability still remains a challenge. This work aims to extend the understanding of the long term spatial and temporal dynamics of root reinforcement after forest harvest in subalpine spruce forests by supplying new experimental data and applying a state-of-the-art model.We estimated root reinforcement decay 5, 10 and 15 years after timber had been harvested in spruce stands in a small catchment in the Swiss Alps. We collected root distribution data at different distances from the trees and calibrated and validated a root distribution model (RootDis). To

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Studies conducted at the field scale report significant reductions in the irrigation requirements of rice when continuous submergence (CS) is replaced by less water-demanding regimes such as flush-irrigation (FI, i.e. intermittent irrigations of rice growing in non-submerged soils). However, the effects of their extensive application in paddy areas with shallow groundwater is much less studied. We present a scenario analysis investigating the impacts on irrigation requirements induced by a shift from CS to FI in an irrigation district of Northern Italy where rice is the main crop, followed by maize and poplar. The area is characterised by a shallow water Table whose depth fluctuates between two meters (in winter) and less than 1 m (in summer). We applied a three-stage procedure, where we first analysed present state conditions using the SWAP (Soil, Water, Atmosphere, Plant) model to simulate irrigation deliveries and percolation fluxes. Then, we calibrated an empirical relationship between estimated percolation fluxes and measured depths to groundwater. Finally, we applied this relationship, in combination with the SWAP model, to predict the variation of district irrigation requirements due to a widespread shift from CS to FI. Results show that neglecting the feedback between groundwater recharge due to irrigation and groundwater depth led to overestimating the reduction of irrigation requirements of rice, which decreased from around 80% when no feedback was considered to around 60% when it was accounted for. Moreover, increased groundwater depths resulted in higher irrigation requirements for maize with an estimated growth of more than 50% due to the need of shortening the irrigation turn. These results demonstrate the importance of considering the impacts on the hydrological processes at larger scales when planning the conversion of CS into more efficient field irrigation methods.

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