Root Strength and Root Area Ratio of Forest Species in Lombardy (Northern Italy)

Abstract

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.

Sensitivity analysis and calibration of a dynamic physically based slope stability model

“…Kim et al, 2013), a linear decrease of c r with depth up to a given rooting depth d r (m) was assumed, accounting for the distribution of roots with depth as observed in other studies (e.g. Bischetti et al, 2005Bischetti et al, , 2009. If the rooting depth exceeds the regolith depth, c r is only considered down to the regolith-bedrock interface (roots are not expected to penetrate the bedrock).…”

Assessment of the protective function of forests against debris flows in a gorge of the Slovenian Alps

“…The portion of conifers should be preserved, in particular fir and larch; spruce is less suitable. Root tensile and root cohesion tests in Italian Alps show that beech roots are significantly more resistant and offer greater reinforcement of soil than spruce roots (Bischetti et al 2005, Vergani et al 2012). In addition, spruce is less suitable for these sites as compared to beech, due to its susceptibility to bark beetles, (three times) lower mechanical resistance to rockfall and weak compartmentalization of trunks after damage (Stokes et al 2005).…”

“…Firstly, small flexible roots mobilize their tensile strength by root-soil friction and thereby increase the compound matrix (soil-fibre) strength. Secondly, large roots intersect the shear surface and act as individual anchors that eventually slip through the soil without braking, thereby mobilizing a soil-root friction force instead of the entire tensile strength (Bischetti et al 2005(Bischetti et al , 2009.…”

Integration of root systems into a GIS-based slip surface model: computational experiments in a generic hillslope environment

“…Moreover, the additional cohesion provided by roots is strongly connected to the distribution of the roots, determining the root system architecture (Sidle et al 1985;Greenway 1987;Sidle and Ochiai 2006;Stokes et al 2008;Ghestem et al 2011). Multiple investigations focused on the distribution and architecture of root systems (Schmidt et al 2001;Schmid and Kazda 2002;Puhe 2003;Bischetti et al 2005;Danjon et al 2008;Bischetti et al 2009) and the differences in root tensile strength and cohesion (Schmidt et al 2001;Preti and Giadrossich 2009). Cohesion forces of forest root biomass that provide additional stability to the soil are often modelled in 2D .…”

“…Dupuy et al 2007;Temgoua et al 2016) or within a infinite slope environment (e.g. Bischetti et al 2005;Danjon et al 2008;Bischetti et al 2009). So far, however, information about forest cover and stand density in general, and mechanical root reinforcement by trees in particular, have been disregarded in GIS-based slip surface models.…”