Introduction
Vegetation affects slope stability influencing both hydrological processes and mechanical structure
* E-mail: bischetti@unimi.it
of the soil. The magnitude of such effects dep- ends on root system development, which in turn is a function of genetic properties of the species and site characteristics (soil texture and structure, aeration, moisture, temperature and competition with other plants). Due to the variability of such
characteristics, we observe a large spatial vari- ability of root patterns and then a great hetero- geneity in soil reinforcement.
Limiting our attention to the mechanical effects, we can recognise two main actions of roots: the small size flexible roots mobilise their tensile strength by soil–root friction increasing the compound matrix (soil–fibre) strength (Gray and Leiser, 1982), whereas the large size roots that intersect the shear plane act as individual anchors (Coppin and Richards, 1990) and eventually tend to slip through the soil matrix without breaking, mobilising a small portion of their tensile strength
Root tensile strength is affected as much by species as by differences in size (diameter). The generally accepted form for the relationship be- tween root tensile strength (Tr(d)) and diameter (d) is a simple power function (Eq. (3), Gray and Sotir, 1996; Wu, 1995):
TrðdÞ ¼ ad b ; ð3Þ
where a and b are empirical constants depending on species.
To account for the variability in root size Eq. (2) must then be rewritten as:
(Burroughs and Thomas, 1977; O Loughlin and Watson, 1979; Schmidt et al., 2001; Ziemer, 1981). Both the effects can be quantified by modelling
N
tR ¼ X Tr
i¼1
r i
A ð Þ
(see Gray and Leiser, 1982; Wu, 1995) if appropri- ate parameters are provided.
Usually, the only effect considered by most of the Authors is the fibre reinforcement expressed as an additional root cohesion (Abernethy and Rutherfurd, 2001; Bischetti, 2001; Bischetti et al.,
2002; Burroughs and Thomas, 1977; Riestenberg and Sovonick-Dunford, 1983; Schmidt et al.,
2001; Sidle 1992; Sidle et al., 1985; Wu, 1984 a, b; Wu et al., 1979; Wu and Sidle, 1995;) which can be easily incorporated into spatially distributed slope stability models (Chiaradia and Bischetti,
2004; Istanbulluoglu et al., 2004; Pack et al., 1997;
Wu and Sidle, 1995).
The most widespread model for root cohesion
(cr) is the Wu (1976) and Waldron (1977) model:
Cr ¼ K tR; ð1Þ
where tR is the mobilized root tensile strength per soil unit area; K is a factor taking into account that roots are randomly orientated with respect to the failure plane which in most of the cases varies be- tween 1.0 and 1.3 (Waldron, 1977; Wu et al., 1979).
The mobilized root tensile strength per soil unit area (tR) can be written as:
tR ¼ Trar; ð2Þ
where Tr is the average tensile strength per aver- age root cross-sectional area; ar is the Root Area Ratio computed as Ar/A, where Ar is the total cross-sectional area of all roots and A is the area of soil in the sample count.
where i indicates the diameter class and N the
number of classes.
Root Area Ratio (RAR) provides a measure
of root density within the soil and as a conse-
quence it is strongly influenced by local soil and
climate characteristics, land use management and
associated vegetation communities and random-
ness. In general RAR decreases with depth below
the soil surface and with distance from tree trunk
(Abernethy and Rutherfurd, 2001; Greenway,
1987; Nilaweera, 1994; Schmid and Kadza, 2001,
2002; Shields and Gray, 1992; Zhou et al., 1998).
On the basis of the Wu (1976) and Waldron
(1977) model, the extent of root reinforcement de-
pends on tensile strength, density and depth of
roots, which vary significantly depending on spe-
cies, local environmental characteristics and spa-
tial variability of vegetation properties (density,
age, fire events, erosion, trees health, etc.). Root
density, in particular, shows an extremely large
spatial variability, both in the vertical and in the
horizontal planes. Despite the large amount of
studies investigating such an issue (Bohm, 1979;
Danjon et al., 1999; Glinski and Lipiec, 1990;
Kramer and Boyer, 1995; Keyes and Grier, 1981;
Jackson et al., 1996; Libundgut, 1981; McMinn,
1963; Paar, 1994; Sainju and Good, 1993; Watson
and O Loughlin, 1990), most of them focus on
eco-physiologic behaviour of vegetation and do
not provide data useful for root reinforcement
estimation. Such studies, in fact, deal with nutri-
ent and organic matter input to the soil, soil fer-
tility maintenance and with carbon sequestration
so, as a consequence, they only consider small
size roots (<1–2 mm) in the upper soil layers.
Because of a renewed interest in understanding the role of vegetation on slope stability and shal- low landsliding, the number of studies on such an issue is increasing (Abernethy and Ruthefurd,
2001; Bischetti et al., 2002; Roering et al., 2003;
Schmidt et al., 2001; Zhou et al., 1998). Neverthe-
less, due to the complexity of reinforcement mech-
anisms, the variety of species and environments
and the spatial variability of characteristics driv-
ing the processes, such work can be considered
eminently site-specific and more experimental data
are still needed for a whole comprehension and
generalisation of the phenomenon.
The present study, focusing on some typical
Alpine and Prealpine tree species in Northern Italy,
aims to expand the knowledge on root strength
and root density values contributing in gathering
enough data to achieve such a generalisation.
Materials and methods
Study sites
Since 2000, the Authors have been focusing their attention on the effect of roots on slope stability starting from a series of data and observations collected in the Central Italian Alps and Prealps within the Lombardy region territory. Most of the work was done in the areas of Valdorena, Morterone, Alpe Gigiai and Valcuvia (Figure 1);
additional samples for beech were also collected in the areas of Valsassina and Val Intelvi.
Valdorena is a right-hand flank tributary of the Camonica Valley (Oglio River). Most of the slopes are dominated by superficial overconsoli- dated periglacial materials and till deposits, with a thickness of more than 20 m, intensively ero- ded by the fluvial incision. Such deposits mainly consist of Late Pleistocene and Holocene till deposits, rock glaciers (Holocene), talus, land- slides and alluvial deposits, whereas gneisses are the dominant lithotype outcropping. At sampling points located between 1525 and 1575 m a.s.l., soils consist of a mixture of sand and gravel in a silty matrix. The mean annual precipitation mea- sured at the nearest rain gauge of Edolo (690 m a.s.l.) in more than 40 years of observa- tions, is equal to about 1000 mm. Precipitation mostly occurs as snowfall from November to March and as rainfall in spring and summer, with a maximum between May and September. At this site we focused on root systems of Green alder (Alnus viridis (Chaix) D.C.) and Willow (Salix spp).
Morterone area is located in the Taleggio val- ley, a right hand flank tributary of Val Brembana (Brembo river). Slopes are formed by glacial and fluvio-glacial deposits with variable thickness overlying sedimentary rocks (mainly formed of fine and coarse-grained limestone with interbeds of marl and nodular chert and, occasionally, inter- calation of calcareous breccias). At sample points located at 1100 m a.s.l., the soil consists of very
Figure 1. Location map of the study areas and species. Beech: Morterone, Valsassina, Alpe Gigiai, Val Intelvi and Valcuvia; Red willow, goat willow and green alder: Valdorena; European ash and hazel: Valcuvia; Norway spruce: Valcuvia and Alpe Gigiai; European larch: Alpe Gigiai.
poorly graded material, generally defined as a silt with clayey sand and fine gravel. Annual precipita- tion measured at the nearest raingauge of Vedese- ta is equal to 1828 mm and it mostly falls as rainfall in autumn and spring, but during summer many heavy storms may occur. Mean annual air temperature is 6.1 C with an average summer temperature of 14.3 C and an average winter temperature of –1 C. At this site we focused on root strength and RAR distribution of European beech (Fagus sylvatica L.), which is the dominant species sometimes with intrusion of other tree spe- cies such as European white birch (Betula pendula Roth.), European ash (Fraxinus excelsior L.) and Sycamore maple (Acer pseudoplatanus L.).
Alpe Gigiai is located on the North-Western side of Como Lake, in an area called Alto Lario. The area is characterised by a steep and dissected topography, with slopes commonly ranging from
20 to 30 . Glacial and fluvio-glacial deposits with variable thickness overlie rocks in the lower part of the area, whereas the higher portion is covered by colluvial deposits. Rocks outcropping in the study area are represented by gneiss belonging to Falda Adula. At sample points soils consist of a gravel-sand mixture with silty ma- trix. The mean annual precipitation of the area is equal to 1604 mm. Mean annual air temperature is estimated in 9.0 C with an average summer temperature of 18.5 C and an average winter temperature of )0.1 C. At this site we focused on root strength and RAR distribution of beech, Norway spruce and European larch.
Valcuvia is a left-hand flank tributary of the Maggiore Lake (Varese, Northern Italy). The study sites are located in the St. Giulio creek catchment (about 5 km2) which is characterised by a steep and highly dissected topography, a typical V-shaped valley and slopes commonly ranging from 25
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