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Ecological Engineering 61P (2013) 633 645

Contents lists available at ScienceDirect

Ecological Engineering

journa l h om epage: www.elsev ier .com/ locate /eco leng

orest protection and protection forest: Tree root degradation over hydrologicalhallow landslides triggering

ederico Preti

ipartimento di Economia, Ingegneria, Scienze e Tecnologie Agrarie e Forestali, Sezione di Ingegneria dei Biosistemi Agrari e Forestali,niversit di Firenze, via S. Bonaventura 13, 50145 Firenze, Italy

r t i c l e i n f o

rticle history:eceived 21 July 2012eceived in revised form 19 October 2012ccepted 12 November 2012vailable online 20 December 2012

eywords:rotection forestoot degradationhallow landslides triggering

a b s t r a c t

The potential use of protection forests to combat shallow slope instabilities is becoming increasinglyimportant and considerable, especially in the light of the recent landslides and debris/mud flows in regionstriggered by rainfalls with increased intensity. Tree vegetation has been constantly subjected to silvicul-tural activity both in exclusively productive forest areas and in more conservative ones meant to contrasthydrogeological risk. It is important to quantify the root system dynamics in order to correctly evaluatethe impact of wood felling or plants death on slope stability. Based on field investigation (on experimentalplots and 29 occurred landslides) and numerical modelling (on slope stability and root distribution), theaim of this work is to determine the effects of the evolution of the mechanical characteristics of rootsystems (and consequently on landslide probability). The paper investigates variations over time in thehazard of rainfall-triggered landslides as a result of root degradation after forest cutting (or death). Thecase under study is related to experimental investigations aimed at determining the tensile strength andelasticity of root samples of trees dead within a decade, which correspond to decreasing values of soilcohesion (root reinforcement). Two kinds of samples were taken into account: living beech roots fromprotected wood areas to determine the current characteristics and roots from dead beeches (felled inprevious years and at present in degradation) to analyse the evolution of root mechanical characteristics.To analyse the stability of representative slopes, we calculated the return time associated with a rainfallevent, which in saturated conditions would lead to the attainment of the limit value of the safety factor

and the associated hazard for different rainfall durations during a fixed period of time. Information aboutthe increasing risk of collapse with the degradation of root system was obtained and compared withlandslides occurrence in forested slopes of the study area. The results of the present paper show thatsuch slopes may remain stable if they are covered with intact protective vegetation, but they will becomeunstable if the conditions of the forest deteriorate or after a wooded area dies off: within a decade of tree

rotec

ssfrfaeaT

death the root system of p

. Introduction

Forest cover reduces shallow landslide hazard by influencingoth hydrological and geo-mechanical factors which contributeo slope stability. From a hydrological perspective, forest coverffects the soil moisture regime as it increases both transpirationates during interstorm periods and the evaporation rates byanopy interception, as well as it enhances the formation ofell drained soil surface horizons (OLoughlin, 1974; Waldron

nd Dakessian, 1981; Ziemer, 1981; Watson and OLoughlin,985; Alila et al., 2009; Preti et al., 2011). As a consequence, therequency of occurrence of high soil water potentials in shallow

Tel.: +39 3209223758.E-mail address: federico.preti@unifi.it

stspl22

925-8574/$ see front matter 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.ecoleng.2012.11.009tion forests loses most of its soil-stabilising function. 2012 Elsevier B.V. All rights reserved.

oils is significantly reduced, with a favourable effect to the slopetability during rainstorms. From a geo-mechanical perspective,orest cover also reinforces the soils explored and bounded by itsoot system, which improves the slope stability independentlyrom the actual soil water content (e.g. Selby, 1993; Nilaweerand Nutalaya, 1999; Abernethy and Rutherfurd, 2001; Schmidtt al., 2001; Simon and Collison, 2002; Frei et al., 2003; Graynd Barker, 2004; Fournier et al., 2006; Reubens et al., 2007).his geo-mechanical effect is generally parameterised in slopetability models by an additional apparent cohesion provided byhe root system to the soil. However, the geometry of the rootystem is the results of eco-hydrological processes, i.e. of the soil

lant atmosphere interactions, which in turn are influenced byocal climatic regimes and soil hydraulic properties (Laio et al.,006; De Beats et al., 2008; Preti et al., 2010; Giadrossich et al.,012).

dx.doi.org/10.1016/j.ecoleng.2012.11.009http://www.sciencedirect.com/science/journal/09258574http://www.elsevier.com/locate/ecolengmailto:federico.preti@unifi.itdx.doi.org/10.1016/j.ecoleng.2012.11.009

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34 F. Preti / Ecological Engi

It is important to consider the entire living cycle of the forestystem when one has to analyse the role of the forest cover onlope stability. Particularly from a forest management perspective,t is important to assess the effect of the death of the forest cover onhe slope stability, as it can occur after a sudden perturbation, suchs tree cut, wildfire and plant disease attack. After tree death, theeduction of overload due to biomass reduction generally is notignificant for the slope stability (e.g. Preti, 2006; Schwarz et al.,010a,b). The favourable effects on the soil water potential regimere lost immediately (e.g. Selby, 1993). The roots is subjected to arogressive decomposition, eventually causing gaps in the inter-ocking root system of neighbouring individual trees (Burroughsnd Thomas, 1977), which inevitably implies a reduction of theoot tensile strength and of the apparent cohesion provided by theoot system to the soil, with a progressive increase of the landslideazard over time (Ziemer, 1981; Sidle et al., 2005; Sidle, 1992; Sidlend Ochiai, 2006; Ammann et al., 2009). This scenario is particu-arly critical in places where the timing of the natural renovationf the forest, with a fully developed root system, starting from newpruces and pioneer flora, encompass several hydrological years.

Detailed studies of rotting processes in tree roots and theirffects on the strength of the roots have so far been carried outainly in North America, New Zealand and Asia (OLoughlin andatson, 1979; Ziemer, 1981; Ekanayake et al., 1997; Watson

t al., 1997, 1999). All these studies have been motivated by thebservation of increased occurrence of shallow landslides aftereforestation. In Alaska, roots of Tsuga heterophylla [Raf.] Sarg. eicea sitchensis [Bong.] Carr showed a reduction in root strengthy a 2025% only three years after the trees death (Ziemer, 1981).oreover, the frequency of landslides increased by 3.8 times after

large-scale decline of Chamaecyparis nootkatensis [D.Don] SpachJohnson and Wilcock, 2002). Little information is available on theonthly degradation rate of root tensile strength of a decaying dead

oot (Schmidt et al., 2001). Ammann et al. (2009) examined theecrease in root tensile strength of decomposing stumps, after theying-off or cutting-down of red firs (Picea abies Karst.) in Swissountain forests. Tensile strength and elasticity was measured on

oot segments sampled 8 to 12 years after tree death and com-ared with living ones, although these were taken from living treesocated at altitudes different from the dead ones (Ammann et al.,009). Genet et al. (2006, 2008) have shown the effect of altituden the characteristics and resistance of the root system.The reinforcement provided by the roots to the soil can be

ssessed in various ways (Sidle and Ochiai, 2006): (1) laboratoryeasurements of tensile or cutting strength on single roots withifferent diameters; (2) laboratory direct shear tests on soil sam-les with roots; (3) in situ measurements using cut boxes appliedo the soil horizon explored by roots; (4) laboratory measurementsf cutting strength opposed by a roots column; (5) uprooting evi-ences of stumps or plants; (6) back analysis on collapsed slopesfter storm events.The last approach implies the application of a hydrological

odel for predicting the spatial distribution of the soil moisturefter the storm event (e.g. Borga et al., 1998; Chirico et al., 2003a,b;ndriola et al., 2009).In this study we examine the strength reduction of roots and

ts influence on the slope stability of a beech forest in Northernuscany (Italy). The study is structured as follows. We first presenthe experimental plots in the study area and the recently occurredandslides. Then we illustrate the model employed for predictinghe root system geometry from local climatic and pedological data.

ollowing we illustrate the root sample design and laboratory mea-urements of tensile strength on sample roots. Finally a model forstimating the strength decrease of a cut or senescent root overime and its effect on slope stability when combined with rainfall

fmrp

g 61P (2013) 633 645

vents of different scale is presented and discussed, by examiningandslide events occurred in the study area.

. Study sites

The study area is the Northern Toscana (Fig. 1a). The experi-ental plots are located within the Serchio River watershed, in theortion of the Apennine range separating Emilia Romagna fromoscana in Central Italy (Casone di Profecchia Fig. 1b). The geologi-al setting consists of sandstones layers averagely inward dipping,ocally covered by debris subjected to runoff erosion. The bedrocks sub-outcropping. The top soil consists in a transient high forest ofeech, showing standards of relevant size in its upper portion. Theeech basal area per hectare G is equal to 33 m2/ha in the inves-igated area. The data examined in this study have been collectedainly in experimental plots located in the Province of Lucca, in

he Castiglione Garfagnana district, at an altitude between 1290nd 1533 m a.s.l., with an average slope of 1525% and peak slopealues up to 4550% (Fig. 1c).In December 2009, one month after our field investigation, an

nusual hydrological event occurred in northern Tuscany. A wide-pread and heavy snowfall occurred on December 18th and 19th,ith air temperature significantly under the average value of theeriod. Following, around Christmas time a heavy rainfall occurred,oined with a sudden temperature increase. The rainfall height wasf 10-years return time, but the total available water at the sur-ace was much higher, as the sudden temperature increase meltedhe snow accumulated in the previous week. The combination ofhese two events is infrequent in central Italy and caused relevantoods, especially along the Serchio river, and over 800 slope insta-ilities, such as shallow land-slides, debris-flows and localized treealling in the northern Apennines of Tuscany. Fig. 1d illustrateshe locations of 29 investigated landslides (red triangles). Particu-arly, Crespole (near Pescia in Fig. 1d) landslide has very similar soilnd slope characteristics of the representative one for the presentaper, at an average altitude of 690 m a.s.l., along a north-west fac-ng slope, with an average slope angle of s = 36.80. The slope wasriginally covered by trees which have been clear cut 9 years priorhe landslide occurred and during field surveys we could verify thathe coppices were highly degraded.

. Materials and methods

.1. Data collection

Beech (Fagus sylvatica L.) sample roots have been collected fromoth living and dead beech plants, felled in 2008, 2006 and 2004.ther sample roots dead in 2002 have been collected in nearbyreas at same elevations, assuming that they are representativef roots with similar characteristics, following Genet et al. (2006,008). Other samples have been taken in the upper, middle and theower part of the natural beech range of the Serchio Valley. Livingeech roots have been taken from areas at elevations of 800, 1450nd 1600 m a.s.l., corresponding to the minimum, intermediate andaximum limits of the elevation distribution of beechwoods in the

tudy region.The beech roots have been carefully dug out to a depth of

.0 m by hand and they were identified morphologically follow-ng Kutschera and Lichtenegger (2002). Provided that beech rootsan expand over large extents, even larger that the coppice spacing,ne can be easily get confused in the identification of the coppice

rom which the sampled root is originated. In order to avoid anyistake, we traced each collected sample back to one of the main

oots which can be clearly attributed to the corresponding cop-ice. The root samples have been collected with a minimum length

F. Preti / Ecological Engineering 61P (2013) 633 645 635

Fig. 1. Study area localization (a and b) and example of beech vegetation on slope (c); 29 other landslide sites (red triangles (d)). (For interpretation of the references to colori

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n this figure caption, the reader is referred to the web version of the article.)

f 4 cm and a diameter ranging from 1.9 to 9.5 mm, excluding theark. The roots were kept in closed bags before the tests to maintainheir initial moisture content. A total of 276 samples was collectednd tested. Root water content has been measured with the ovenrying method and has been expressed as the ratio of the mass ofater to the mass of the solid material.For tensile strength measurements, we used the the Amsler uni-

ersal testing machine, with maximum load set at 40 kN, equippedith a pressure transducer for loads recording and a potentiometricisplacement transducer for elongation measurements, connectedo a computer for data acquisition (Preti and Giadrossich, 2009).eformation measurements were performed by fixing the trans-ucer directly on the roots with two light clamps adapted to griphe root. The results of the root tensile tests were used to calcu-ate the tensile strength at the moment of maximal tensile forcepplied. Tensile strength Tr (N/mm2) is calculated as the ratio ofhe maximum tension force F (N) to the initial root cross-sectional

rea A0 (mm2) (DIN 52188, 1979 in Preti and Giadrossich, 2009).

r = FA0

(1)

r

fm

We calculated the Modulus of Elasticity (MoE or Youngs mod-lus) expressed as follows:

oE =

= FD0A0x

(2)

here and are, respectively, the stress and strain of the root, Fs the pullout force difference in the near-linear elastic range (N),0 is the initial distance between the two clamps (mm), x is thelongation in the near-linear elastic range (mm). The experimentaloE data values can be expressed by a non-linear function of theiameter d as follows (Operstein and Frydman, 2000; Fan and Su,008; Schwarz et al., 2010a,b):

oE = cd1 (3)here c is a parameter depending on species and the degree ofegradation. Root tortuosity may affect the apparent MoE because

tortuous root may stretch with reduced stress transmission to the

oot tissue (Commandeur and Pyles, 1991; Schwarz et al., 2010a,b).

The following geotechnical properties have been measuredrom soil sample measurements at Casone di Profecchia experi-ental plots: angle of internal friction = 30; saturated specific

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36 F. Preti / Ecological Engi

eight sat = 2.2 kN/m3; submerged specific weight = 1.2 kN/m3;oil porosity = 0.35, water content at field capacity = 0.28.

We characterized the 29 landslide slope mechanical parame-ers too, according to the field surveys, as follows: soil cohesions = 0 kPa (coefficient of variation C.V. = 0), friction angle = 29.96

C.V. = 0.192), specific weight of saturated soil sat = 2.2 kN/m3, soilorosity = 0.35, slope angle 36.80 (C.V. = 0.218), root cohesionv at 1 mt of depth = 3.52 kPa estimated as in Eq. (8) (C.V. = 1.159),verage rooting depth b = 0.83 m (C.V. = 0.958) estimated as in Eq.10), unit tensile strenght of roots Tr(1) = Tr1 = 41 MPa (according toq. (11)).The measured tree basal area at Casone di Profecchia site is

= 33 m2/ha, while the average value at the abandoned landslideites is G = 17 m2/ha and at Crespole site (near the landslide scarp)

= 10 m2/ha.

.2. Root reinforcement

For the slope stability analysis of vegetated slopes, we have touantify root reinforcement, as increase of shear strength or addi-ional soil cohesion Cv. Additional soil cohesion due to root tensiletrength can be calculated using different models on different rootoadings and breakage assumptions (Ji et al., 2012). Particularlyiber or Root Bundle Models could be implemented if all necessaryarameters are available (Schwarz et al., 2010a, 2010b). The modelhoice was dictated by the need to keep the numerical model asimple as possible, with limited number of parameters. A modifiedersion of the well-known Wu (1976) and Waldron (1977) theoret-cal model (Preti, 2006; Schwarz et al., 2010a,b; Ji et al., 2012) haseen here used considering the MohrCoulomb failure criterion as:

sr = cv + cs + tan (4)here cv is the additional cohesion due to the presence of roots, cs

he bare soil cohesion, the normal stress on the shear plane, and the bare soil friction angle.When soil mass movement occurs, such as translational shallow

andslides, shear forces are developed and roots crossing the slipurface are mobilized in tension.

Resulting additional cohesion cv is thus expressed as:

v = kktR (5)here tR is the mobilized root tensile strength per unit area ofoil and k is a correction factor used to account for the roots thatre not oriented perpendicular to the slipping surface, usually con-idered equal to 1.2 (ranging from 1.0 to 1.3, Waldron, 1977; Wut al., 1979). The parameter k = 0.4 is an empirical correction factorntroduced by Preti (2006) in order to correct the bias due to theverestimation of the cohesion values with WM. Ji et al., 2012 foundhat this simply model revision provided the most conservative cvstimates as compared with other six models.The mobilized root tensile strength

R = Tr RAR (6)s defined as the product of the mean tensile strength of roots (Tr)ultiplied by the root area ratio (RAR), i.e. the relative surface ratiof the soil profile occupied by roots. In our case, for each sampleite, RAR is calculated using the following equation:

AR =nArj (7)j=1A

here n is the number of root sections observed in a vertical cross-ectional area A, Arj is the cross-sectional area of j-th root.

t

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g 61P (2013) 633 645

In order to account for the variability of the root strength asunction of the root size, the last equation is rewritten as follows:

v = kkn

j=1Trj

ArjA

(8a)

here N is the number of diameter classes, j the current diameterlass, Trj and Arj the mean root tensile strength and the mean rootross-sectional area respectively in the class j.

For technical purposes, root cohesion at z soil depth can be cal-ulated as follows:

v(z) = kkTrRAR(z) (8b)

The root area ratio can be also computed as function of the soilepth, RAR(z), by computing the ratio of the sum of root cross sec-ions at given depth z (Ar(z)) and the total vertical extent of soilxplored by the root system A = Ars.The variation of the root density Ar(z) over depth can be mod-

lized by a negative exponential function with two parametersor the applications of technical interest: Ar0 is the area of rootsxtrapolated at the initial depth and b is the average rooting depthepending on hydrological and pedological characteristics (Pretit al., 2010). The scaling factor Ar0 is specie-specific and plant-age-ependent; in fact, the basal stump area and the above-groundiomass are different from specie to specie, increasing over timeo a mature state. As a consequence, RAR(z) can be assessed by theollowing formula:

AR(z) = Ar0Ars

e(1/b)z = RAR0e(1/b)z (9)

RAR0 is the root area ratio at the surface and can be estimatedy the tree basal area (RAR0 = G/10.000, Preti et al., 2010).The parameter b represents the average rooting depth and it is

erived from the long-period soil water balance in the followingorm (in water controlled eco-systems and during the vegetativerowing season; Preti and Giadrossich, 2009; Preti et al., 2010):

= (fc w)(1 0/Tp)

(10)

here 0 is the mean frequency of rain events (n. events/day)n growing vegetative phase; is the mean intensity of rainfallvents (mm/event) on growing vegetative phase; Tp is the potentialvapotranspiration ratio (mm/day) on growing vegetative phase;

is the effective soil porosity; fc and w is the moisture contentst field capacity and the wilting point, respectively.The calculated b values are in good agreement with all the

bserved root profiles.The expected value of the mean root tensile strength for a given

oot diameter can be expressed as follows

r = Tr1d (11)

here Tr1 is the tensile strength for diameter of unit length and ds the root diameter. The parameters Tr1 and can be obtained by

linear regression model after logarithmic transformation.In 3 plants sampled at 800, 1450 and 1645 m a.s.l., Tr1 is esti-

ated equal to 42.6 N/m2 while is estimated equal to 0.51. Thesealues are comparable with other authors values for the samepecies (e.g. Tr = 41, 57d0,98 at 1100 m a.s.l. in Bischetti et al., 2005,ven if in a different diameter range of thinner roots 1.78 1.19 mm

han the ones tested in the present study).

Based on the field surveys, in the study area the following dataave been found and they have been used for the model simula-ions:

F. Preti / Ecological Engineerin

0

0,5

1

1,5

2

2,5

0 1 3 5 7

degradation years

Ten

sile s

tren

gth

rati

o T

r/T

r2009

Fig. 2. Box-plot of tensile strength ratio [Y-axis, Tr values divided by Tr(2009)]ofq

-

-

4

4

b

T

wT

dttTtTd

F3

ver degradation time [X-axis: 0 = 2009, 1 = 2008, 3 = 2006, 5 = 2004 and 7 = 2002]or all samples. The plotted values are: minimum and maximum, first quartile, thirduartile, median. Regression line Tr(DY)/Tr(2009) = 0.1168 DY + 1.0322 R2 = 0.9958.

root cohesion at 0 depth cv(0) = 178.8 kPa as maximum value cor-responding to the tree basal area of the slope under examinationG = 33 m2/ha as in the experimental site, neglecting the reduc-tion coefficient k in Eqs. (5) and (8) and considering Tr = 45 kPa

(according to literature data).

root cohesion at 0 depth cv(0) = 28.1 kPa, corresponding to youngforest case with tree basal area G = 5.2 m2/ha, considering onlystandard plants and/or newly-sprouting spruces and pioneer

ave

(a)

(b)

ig. 3. (a) Tr (d) regression power lines for different degradations years (Eq. (11)). (b) Unit = 2006, 5 = 2004 and 7 = 2002]: decay rate = 9.1%; annulment (intercept of X-axis) after 1g 61P (2013) 633 645 637

flora and the same hypotesis as above for k and Tr or, alter-natively, to a tree basal area G = 17.5 m2/ha (representative ofthe investigated landslide areas) with conservative k = 0.4 andTr = 33.4 kPa (average value of measured data).

. Results and discussion

.1. Root reinforcement decay after tree death

The decay of root tensile strength over time can be representedy the following linear function:

rDY = Tr0[1 (DY DY0)] (12)here Tr0 is the root tensile strength at the reference year DY0 whilerDY = Tr(DY) is the tensile strenght at year DY, after the root death.In Fig. 2 the Box-plot of the relative tensile strength by

ividing for the 2009 value over time Tr(DY)/Tr(2009) (degrada-ion years DY) is presented, showing the expected reduction inensile strenght with respect to the reference year DY0 = 2009.he plotted values are: minimum and maximum, first quartile,hird quartile, median. The median values have regression liner(DY)/Tr(2009) = 0.1168 DY + 1.0322 with R2 = 0.9958. The yearlyecreasing rate is 11% and the total decay time is ca. 9 years.

The regression power curves Tr(d) (Eq. (11)) for several years

ppear to have similar trends: by analysing their log-transformedalues (Fig. 3a), we found that the angular coefficients are almostqual, resulting in the parallelism of the curves, and that the Y-axis

tensile strength (d = 1 mm) [Mpa] over degradation time. [X-axis: 0= 2009, 1 = 2008,1 years R2 = 0.9957.

638 F. Preti / Ecological Engineering 61P (2013) 633 645

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 2 4 6 8 10Degradation years

Te

ns

ile

str

en

gth

ra

tio

diameter 4 mm

New Zeland Beech

(Sidle & Ochiai, 2006)

y=-0.0962x+1

R2=0.9435

y=-0.1214x+1

R2=0.9713

y=-0.0969x+1

R2=0.9055

F 9) oni r studo

itd9(a

d(

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is attributed to variations in tissue density and cellulose contentand are affected by branching order (Genet et al., 2005; Wang andYen, 1974; Schwarz et al., 2010a,b). In our results, both Tr (Fig. 2)

0

0,5

1

1,5

2

2,5

3

0 1 3 5 7Degradation time [year]

Ela

sti

cit

y r

ati

o (

Mo

E/M

oE

2009) ig. 4. Tensile strength ratio [Y-axis, Tr values divided by the maximum or the Tr(200n samples grouped by a diameter threshold (< and >4 mm) and comparison with othef X-axis) after 10.4 and 8.3 years, respectively.

ntercepts Tr1 (root resistance for d = 1 mm as the interpolation inhe power regression curves Tr(d) for each degradation year) areecreasing over time (Fig. 3b): an annual linear decrease of about.1% can be observed with a complete annulment after ca. 11 yearsregression equation Tr(d) = 4.4528 DY + 49.066 characterized by

very high R2 = 0.9957).Eq. (11) and Eq. (12) imply that roots of different diameters

egrade in a similar way, consistently with findings by Ziemer1981) and Ammann et al. (2009).

Results in Fig. 4 show that the dimensionless tensile strengthecreases after cutting also for diameters ranging between 1.9 and

mm (almost exactly as in Sidle and Ochiai, 2006 for the samepecies) and for those between 4 and 9.5 mm (being 4 mm theedian diameter), reaching an almost null value after more or less

decade with a decreasing rate of ca. 10% and significant R2 val-es (0.900.97). These results are in agreement with other authorsnes too as e.g. Ziemer (1981) and to Ammann et al. (2009), even ifomparisons have to be made paying attention due to the fact thatensile strength is affected by differences in the selection of rootamples (altitudes, diameter and curvature), root moisture contentnd testing methods.Thus, if we assume that RAR is constant over time, the apparent

oil cohesion will also decrease linearly over time, according to Eqs.8) and (11). The hypothesis that RAR is constant over time implieshat the hillslope should be affected by external events, such asandslides, which may affect the root mass distribution as naturalonsequence of forest adaptation to disturbing forces (Johnson andilcock, 2002; Saklas and Sidle, 2004; Preti et al., 2010)A similar analysis can be done also for MoE:

oEDY = MoE0[1 (DY DY0)] (13)

Knowledge of the elasticity is important for quantifying pro-esses inducing mechanical activation of the rootsoil interfacend its shear strength (Mickovski et al., 2007, 2008; Schwarz et al.,010a,b), but there is a lack of information concerning MoE valuesr the slope of stressstrain relationship during root tensile testsCommandeur and Pyles, 1991; Operstein and Frydman, 2000; Tosi,

007; Fan and Su, 2008; Ammann et al., 2009). In Fig. 5 all elasticityeasurements have been analysed over the diameter using Eq. (2b)

the coefficient c assuming value equal to 893 in our study case foriving beech, while e.g. is equal to 696 for different plant species as

FMatD

e] over degradation time [X-axis: 0 = 2009, 1 = 2008, 3 = 2006, 5 = 2004 and 7 = 2002]ies (Sidle and Ochiai, 2006): degradation rate = ca. 10 and 12%; annulment (intercept

esbania cannabina, Medicago sativa, Rosmarinus officinalis, Pistaciaentiscus and Cistus in Schwarz et al., 2010a,b). In Fig. 5 the Box-plotf MoE measurements is presented: a roughly 4% linear decreasef the median values can be observed, with extrapolated null valuefter ca. 25 years (R2 = 0.7). The unit MoE (interpolation for d = 1 mmy the hyperbolic regression curves MoE(d) in Eq. (3) for each degra-ation year) over degradation time is shown in Fig. 6 with an annualinear decrease of about 4.8% with annulment after 20.9 years and2 = 0.81. (Apparent) MoE could be lower if we also consider rootlongation of tortuous roots (Schwarz et al., 2010a,b): in our testse avoided this problem measuring directly on stretched-out roots,fter by excluding very tortuous roots. For each year of degradation,n Fig. 7 the decreasing of average MoE values is analysed for twoata-sets, grouped by a 4 mm threshold (median diameter value):

lower starting value for larger root diametres (64%) and a smallerecreasing rate (ca. 3% and 6%, with intercept of X-axis equal to ca.3 and 18 years, respectively), contrarily to the Tr case (Fig. 4).The dependency of stressstrain relationship on root diameterig. 5. Box-plot of Module of Elasticity ratio [Y-axis, MoE values divided byoE(2009)] over degradation time [X-axis: 0 = 2009, 1 = 2008, 3 = 2006, 5 = 2004nd 7 = 2002]. The values are minimum and maximum, first quartile, third quar-ile, median. Regression line MoE(DY)/MoE(2009) = 0.0304 DY + 0.769 R2 = 0.7075.egradation rate = 4%; annulment (intercept of X-axis) after 25 years.

F. Preti / Ecological Engineering 61P (2013) 633 645 639

y = -46,649x + 976,05

R2 = 0,8105

0

200

400

600

800

1000

1200

0 1 2 3 4 5 6 7 8

Degradation time [year]

Ela

sti

cit

y [

Mp

a]

Fig. 6. Unit Module of Elasticity (d = 1 mm) over degradation time [X-axis: 0 = 2009,1c

aasadpjtcttataobi

mtwpscw8tis

F0ta

0

20

40

60

80

100

120

140

160

0 1 5 7degradation years

mo

istu

re c

on

ten

t [%

]

F1r

4

taP

F

wdcs

(t

Gesi

z

ww

= 2008, 3 = 2006, 5 = 2004 and 7 = 2002]. Degradation rate = 4.8%; annulment (inter-ept of X-axis) after 20.9 years R2 = 0.8105.

nd MoE (Fig. 5) decrease with increasing root degradation time,nd consequently the examined samples of old plants have lowertrength and stiffness properties than of younger plants. A drynd decayed root should be very fragile and so undergo scarceeformation until it reaches a quick break, with extremely lowlastic field. In our case, as the roots were kept in closed bagsust after sampling and before testing, they should have conservedheir original moisture content. It has to be noticed that MoE isonsidered not dependent on the root state of decay accordingo other studies, but they were carried out after short degrada-ion times and for other species (OLoughlin and Watson, 1979) ort different altitudes and at very low, not constant and far fromhe actual soil moisture conditions (Ammann et al., 2009). Actu-lly there might be a combination effect of the moisture contentf the root itself: degradation time being equal, the MoE shoulde inversely proportional to moisture, at least within determinedntervals.

A root from a dead plant for a few years has a slightly loweroisture content (Fig. 8) and so there would be various fac-

ors at work: root degradation lowers the MoE (stiffness), butater loss could increase it. If there was a combination of com-ensating factors (moisture + degradation) it might be possible toee with a multiple regression the weight of each componentompared to the MoE, but the moisture content of our samplesas almost always higher than 50% (average values higher than0%): due to these high values, an influence of the moisture on

he Tr and MoE was not expected. Possibly effects could ver-fy only at lower values (under 3040%), if achievable into theoil.

y = -5,7428 x + 189 ,54R = 0,251 5

y = -16,125x + 288,21

R = 0,8029

0

50

100

150

200

250

300

350

0 1 2 3 4 5 6 7 8Degradation time [year]

Ela

sti

cit

y [

Mp

a]

diameter 4 mm

ig. 7. Average Module of Elasticity [MPa] in Y-axis over degradation time [X-axis: = 2009, 1 = 2008, 3 = 2006, 5 = 2004 and 7 = 2002] in samples grouped by a diameterhreshold (< and >4 mm): degradation rate = 6 and 3%; annulment (intercept of X-xis) after 18 and 33 years, respectively.

btenfdc

h

wt

2gT

weoH

ig. 8. Box-plot of moisture content (Y-axis) over degradation time [X-axis: 0 = 2009, = 2008, 5 = 2004 and 7 = 2002] for all samples (2006 dry weight not available);egression line for average values gives y = 2.6872x + 105.71 R2 = 0.8868.

.2. Slope stability analysis

The failure condition is examined using the MohrCoulomb cri-erion within an infinite slope model accounting for the apparentdditional cohesion provided by the root system (e.g. Sidle, 1992;reti and Giadrossich, 2009):

s = cv + z cos2 S tan

satz sin S cos S(14)

here S is the slope angle; z is the breaking or failure surfaceepth (m); is the soil friction angle; is the submerged spe-ific weight (kN/m3) = sat w (specific water weight); sat is thepecific saturated weight (kN/m3).

Eq. (14) is written in the hypothesis that the soil cohesion is nullas in the study case) and the vegetation weight is not relevant forhe slope stability (e.g. Preti, 2006).

In the hypothesis of vertical infiltration according to areenAmpt approximation with a piston like front (which is thexact solution for soil with diffusivity equal to a Diract function ataturation) and no infiltration excess, the depth of saturation frontncreases according to the relation

(t) = h(t)

S i

(15)

here i and S are the initial and saturated soil water content,hile h(t) is the rainfall height after time t.Other solutions have been proposed for assessing the slope sta-

ility factor during a storm event and particularly interesting arehose solutions based on a linear approximation of the Richardsquation (Iverson, 2000; DOdorico et al., 2005), even if consideredot necessary for the purposes of the present paper. Rainfall heightor a given duration t can be usefully expressed by rectangularesign hyetograph according to local intensity-duration frequencyurve:

(t) = a(rT )tn (16a)

here n is a constant parameter and a(rT) is a scaling factor, func-ion of the return period rT.

In the present study we considered the equation h =1.985r0.167T t

0.537 corresponding to rainfall data recorded at theauge closest to the experimental plots (Preti et al., 1996; Regioneoscana, 2007).To analyse the effect on the stability of a representative slope,e calculated the return time rT associated with a single rainfallvent, which in saturated conditions would lead to the attainmentf the limit value of the safety factor Fs and the associated hazardna for different rainfall durations during a fixed period of time.

640 F. Preti / Ecological Engineering 61P (2013) 633 645

0,00

0,20

0,40

0,60

0,80

1,00

1,20

1,40

1,60

1,80

0 1 2 3 4 5 6 7 8 9 10

Degradation years

Lan

dslid

e d

ep

th [

m]

Cv(0) = 28,08 kPa

Cv(0) = 0 kPa

Cv(0) = 178,8 kPa

Cv(0) = 54 kPa G = 10 mq/ha

December 2010 landslide measured scarp depth (rainfall 316 mm)

Fig. 9. Landslide failure depth associated to root decay over root degradation time in agreement with the values registered in Preti et al. (2001), Casagli et al. (2006), Schwarzet al. (2010a,b) and recently measured in the landslides occurred in the area under examination. Slope characteristics: soil cohesion c = 0 kPa, soil angle of internal friction = 30 , saturated specific weight = 2. 2 kN/m3, submerged specific weight = 1.2 kN/m3 (porosity = 0.35), slope angle = 32; root cohesion Cv up to 0 m of depth,r ges or d to c

c

H

Hl

d

ccrCfti3mFddi

csvFod

cF((

(dooCa(

F(

sat

espectively = 0 kPa; 28.08 kPa and 178.8 kPa. In the case of felling, the effect of chaned point represents the measured landslide scarp depth at Crespole slope subjecte

The corresponding hazard in a fixed period (range time) wasalculated as:

na = 1 (1 1

rT

)na(16b)

na is the probability of occurrence of an event with a return periodarger than or equal to rT in a period of na years.

Information about the increasing risk of collapse with the degra-ation of root system was obtained.Figs. 9 and 10 show the value of the critical z depth and

orresponding rainfall for several years of root degradationonsidering three representative conditions of root reinforcement,espectively: Cv(0) = 0 kPa that means complete absence of roots;v(0) = 28.08 kPa that would give at 1 m depth Cv(1) = 1 kPa (felledorest and/or newly sprouting spruces and pioneer flora case withree basal area G = 5.2 m2/ha) and Cv(0) = 178.8 kPa correspond-ng to the tree basal area of the slope under examination G of3 m2/ha and to a value of Cv(1) = 6.38 kPa (in agreement with oureasurements and the back analysis in Schwarz et al., 2010a,b).

igs. 11 and 12 show the behaviour of the return time over rootegradation time and the behaviour of the hazard over root degra-ation time for different durations (Eqs. (14)(16)). For examplen Fig. 11a the value of the return time of a 48-hours rainfall is

r2

s

0

100

200

300

400

500

600

0 1 2 3 4 Degradat

Rain

fall lan

dlisd

e t

rig

geri

ng

[m

m]

Cv(0) = 28 ,08 kPaCv(0) = 0 kPa

Cv(0) = 178 ,8 kPa

ig. 10. Critical rainfall triggering landslides over root degradation time in agreement w2010a,b) and in recent field surveys.s

f above-ground biomass overload was conservatively considered not relevant. Thelear cut just 9 years before.

ritical for slope instability (triggering threshold for shallow land-lides) and decreases in value from 100 years (in the case of healthyegetation) to 10 years in about a decade of degradation, while inig. 11b a 48-h rainfall has a return time 100 years (in the casef healthy vegetation) and

F. Preti / Ecological Engineering 61P (2013) 633 645 641

11

00

10

00

01

00

00

00 0 1 2 3 4 5 6 7 8 9 10

Degradation years

Re

turn

tim

e

1 hour 3 hours 6 hours12 hours 24 hours 2 days

11

00

10

00

01

00

00

00 0 1 2 3 4 5 6 7 8 9 10

Degradation years

Degradation years

Re

turn

tim

e

1 hour 3 hours 6 hours12 hours 24 hours 2 days

1

10

100

1000

10000

0 1 2 3 4 5 6 7 8

retu

rn t

ime

Qua rtil e I

Qua rtil e II

Qua rtil e II I

Qua rtil e I

Qua rtil e II

Qua rtil e II I

(a)

(b)

(c)

Fig. 11. (a) Return time (years) of landslide triggering rainfall threshold associated to root decay over root degradation time. Slope characteristics in Fig. 9 caption with rootcohesion Cv(0) = 28.08 kPa and rainfall-duration equation h = 21.985rT0.167t0.537 with h = rainfall amount (mm), rT = return time (years), t = rainfall duration (h). (b) Return time(years) of landslide triggering rainfall threshold associated to root decay over root degradation time. Slope characteristics in Fig. 9 caption with root cohesion Cv(0) = 178.8 kPaand rainfall-duration equation h = 21.985r0.167

Tt0.537 with h = rainfall amount (mm), rT = return time (years), t = rainfall duration (h). (c) Return time (years) associated to FS = 1,

concerning the slope characteristics in Fig. 9 caption for the values of the median root cohesion and of the first and third quartiles of the experimental T data (tensile strenghtl es) rait

aiJtlRt

daf

ike in Fig. 2) per each degradation year with 24 h (blue curves) and 48 h (red curvhe reader is referred to the web version of the article.)

severe number of landslides per unit area were triggered offn abandoned forest areas (Johnson and Wilcock, 2002). On 19thune 1996 the Versilia-Garfagnana area of the Apuanian Alps (in

he same Province of Lucca, Fig. 1) has been affected after out-ier rainstorms (a 24-h total of 400 mm of rain was observed atetignano gauge). 15 people died and great damages were regis-ered. The majority were soil slips, but a fraction developed into

tem2

rr

nfall-duration. (For interpretation of the references to color in this figure caption,

ebris flows. The area is characterized by shallow soils (0.351.1 ms in Fig. 9) that develop on acid bedrock. The texture rangesrom sandyclayloam to loamy-sand. The slopes were originally

erraced for chestnut orchards that have been abandoned at thend of the 19th century and that have subsequently turned intoixed broad leaves woods with little undergrowth (Preti et al.,001). Another severe rainstorm of high intensity occurred on

642 F. Preti / Ecological Engineering 61P (2013) 633 645

0,00

0,01

0,10

1,00

0 1 2 3 4 5 6 7 8 9 10Degradation years

Ha

za

rd

1 hour 3 hours 6 hours12 hou rs 24 hou rs 2 da ys

0,00

0,01

0,10

1,00

0 1 2 3 4 5 6 7 8 9 10Degradation years

Ha

za

rd

1 hour 3 hours 6 hours12 hou rs 24 hou rs 2 da ys

(a)

(b)

Fig. 12. (a) Hazard of landslide associated to root decay over time for different rainfall durations in a time range na = 100 years. Slope characteristics in Fig. 11 captionw 0.167t0.537

H s in a tC ll am

1assdSe2foe1wf1Ioofwooa

eotTSa

sosstriasa

ith root cohesion Cv(0) = 28.08 kPa and rainfall-duration equation h = 21.985rT

azard of landslide associated to root decay over time for different rainfall durationv(0) = 178.8 kPa and rainfall-duration equation h = 21.985rT0.167t0.537 with h = rainfa

9th21st November 2000, in the same Province of Lucca (Fig. 1)nd in the close-by Province of Pistoia which triggered tens of land-lides. These landslides can be broadly defined as complex earthlidesearth flows, originating as rotational slides that developownslope into a flow (Casagli et al., 2006; Schwarz et al., 2010a,b).torms are fairly frequent in this area, but cyclonic storms ofxtreme intensity such as the one in November 20th and 21st,000, are rare. This storm precipitated around 200220 mm of rain-all within a period of 3840 h. Rainfall intensity hit a maximumf 17 mm/h, near Montecatini Terme and Pescia, and it has beenstimated that the event has a return time period of more than00 years. Intensive antecedent rainfall was also recorded over 3eeks prior to the event giving a total of 545 mm of precipitation

or November 2000, which is the highest quantity recorded since970, exceeding the monthly average by 328% (Casagli et al., 2006)n a little catchment near the village of Vinchiana in the Provincef Lucca (Fig. 1), a number of shallow landslides have occurred, onef them resulting in 2 human casualties during a moderate rain-all event that followed on three week period of prolonged rainfall,

ith a cumulative rainfall of 360 mm having a return time periodf more than 100 years. This landslide affected a small portionf the slope (area 1000 m2) between 175 and 260 m a.s.l. withn angle of 38 and a soil thickness of less than 1 m (Schwarz

ca

c

with h = rainfall amount (mm), rT = return time (years), t = rainfall duration (h). (b)ime range na = 100 years. Slope characteristics in Fig. 11 caption with root cohesionount (mm), rT = return time (years), t = rainfall duration (h).

t al., 2010a), as in Fig. 9. The vegetation cover consisted mainlyf chestnut trees (Castanea sativa Mill.), with presence of locustrees (Robinia pseudoacacia L.) and cluster-pines (Pinus Pinaster A.).he mobilized sediments reached the main stream as a debris flow.imilar scenarios were observed for other shallow landslides in thisrea.Important factors for evaluating the risk of landslides or ero-

ion after tree death are of course not only the time spanf root decomposition, but also the slope steepness and theoil material susceptibility to landslides. Slopes with a gradientteeper than the angle of internal friction of the soil in ques-ion are more susceptible to landslides as a consequence of theeduced protective function of the vegetation. Actually we foundn the case under examination the slope angle s = 32 > the soilngle of internal friction = 30 and in Schwarz et al. (2010a)s = 3538 > = 33.4 (in Casagli et al., 2006, = 29.935), in oururveys in Versilia (Preti et al., 2001) and in the recently affectedreas s = 3136 > = 24.530).The proposed methodology here presented has been tested byalculating the landslide depth for all the 29 investigated slopesnd obtaining the results shown in Fig. 13 (Montgomery, 1994).Considering Crespole landslide, we could assess the total root

ohesion equal to 3.52 kPa (C.V. = 1.159) up to 1 m of depth. At the

F. Preti / Ecological Engineerin

y = 0,96x

R2 = 0,63

0

0,5

1

1,5

2

2,5

3

3,5

4

0 1 2 3 4measured landslide scarp depth [m]

sim

ula

ted

la

nd

sli

de

de

pth

[m

]

Fig. 13. Comparison between the measured and calculated scarp depth obtainedwith the proposed method and considering the effect of vegetation. The stabilityanalyses of these slopes have been carried out with the following soil mechan-ical parameters observed in the field: soil cohesion c = 0 kPa (C.V. = 0), frictionangle = 29.96 (C.V. = 0.192), specific weight of saturated soil sat = 2.2 kN/m3,sdr

i1soocetistb

i

Ftstcsidru

mtii

ic(ssttisate

rmclnteilRep

gaistlis

oil porosity = 0.35, slope angle 36.80 (C.V. = 0.218), root cohesion Cv at 1 mepth = 3.52 kPa (C.V. = 1.159), average rooting depth b = 0.83 m (C.V. = 0.958), unitoot tensile strength Tr(1) = 41 MPa).

nitial condition (live plants), we would obtain by the model ca..6 m failure depth, while the observed failure depth of the Cre-pole landslide scarp is 0.9 m (Fig. 9). This depth can be predictednly if we assume that root strength is reduced precisely to 10%f the original value, due to 9 years of root degradation after treeut. We find similar results even if we account for the root lat-ral cohesion (Schwarz et al., 2010a,b). It is interesting to observehat the root lateral cohesion is not significant for the slope stabil-ty in this case as far as the root strength is above 90% of the roottrength when plants were alive. Significant differences between

he FS values computed with and without the lateral cohesion cane observed only for a root strength reduction factor of 10%.Finally Fig. 14 shows the effect of all experimental data variabil-

ty on the return time estimation: in the Box-plot the minimum and

0,00 1

0,01

0,1

1

10

100

1000

10000

0 1 2 3 4 5 6 7 8 9

Degradation years

Re

turn

tim

e

Serie1

Serie2

Serie3

Serie4

Serie5

'

ig. 14. Box-plot (plotted values are: minimum and maximum, first quartile,hird quartile, median) concerning the 29 investigated slopes with the cohe-ion degradation as in Fig. 3 (the degradation years here are 10, as we usedhe experimental degradation llnear raw applied to a decade), soil cohesion = 0 kPa (C.V. = 0), angle of internal friction = 29.96 (C.V. = 0.192), saturatedpecific weight sat = 2.2 kN/m3, submerged specific weight = 1.2 kN/m3 (poros-ty = 0.35), slope angle s = 36.80 (C.V. = 0.218), root cohesion Cv up to 1 mepth = 3.52 kPa (C.V. = 1.159), average rooting depth b = 0.83 m (C.V. = 0.958), unitoot tensile strenght Tr1 = Tr(1) = 41 MPa); for all the slopes we considered the sat-ration and FS = 1 conditions. Logarithmic scale gives distortion in box widths.

wontqestbta(

5

gp

iami

i

g 61P (2013) 633 645 643

aximum, first quartile, third quartile, median values of the returnime are represented concerning the 29 investigated slopes by vary-ng all the slope parameters and with the cohesion degradation asn Fig. 3.

That is, such slopes may remain stable if they are covered withntact protective vegetation, but they will become unstable if theonditions of the forest deteriorate or after a wooded area dies offSelby, 1993; Ammann et al., 2009): in our case from root cohe-ion Cv(1) = 6.38 kPa to soil cohesion null. The results of this studyhow that within a decade of tree death the root system of protec-ion forests loses most of its soil-stabilising function. The length ofime span depends on the decomposition rate of the roots, whichn turn is a function of site parameters such as altitude (climate) oroil water regime and it can be assumed that, particularly at highltitudes, this period of time is not long enough for new genera-ions of trees to have grown enough to have the same stabilisingffect on the soil (Ammann et al., 2009).The model here used considers, at present, the hydrological

esponse of the soil which is not dependent on the topological andorphological connections (the slope being the only morphologi-al parameter employed). Since this problem represents a crucialimit to the procedure applied, a control is required in order to allowot using a distributed hydrological approach and consequentlyo overcome this shortcoming (e.g. Borga et al., 2004; Erminit al., 2005). The upslope contributing area represents obviously anmportant hydrological factor selected for this analysis. In particu-ar, as widely recognized (e.g. Kirkby, 1971; Tarboton et al., 1992;inaldo et al., 1995), it controls the water flow at a point, influ-ncing how soil saturates, with particular reference to landslidingrocesses.Moreover, slope stability analysis in areas dominated by active

eomorphologic processes (such as soil erosion and landsliding)nd covered by vegetation, is often impeded by the lack of reliablendirect methods for the spatial estimation of soil depth. Actuallyoil thickness (or the potential failure depth) z can vary as a func-ion of many different and interplaying factors, such as underlyingithology, climate, gradient, hillslope curvature, upslope contribut-ng area, and vegetation cover, making the distributed estimation ofoil depth challenging and often unreliable. While the relationshipith gradient and curvature should reflect the kinematic stabilityf the regolith cover, allowing greater soil thicknesses over pla-ar and concave areas, the distance from the hill crest (or fromhe valley bottom) accounts for the position within the soil topose-uence. This last parameter is fundamental: according to Catanit al. (2010) points having equal gradient and curvature can haveignificantly different soil thickness due to their dissimilar posi-ion along the hillslope profile. Finally, slope stability appears toe especially important particularly where installations and infras-ructures (such as road networks, railway lines, electrical lines, etc.)re present and it has to be carefully considered regarding the latterBorga et al., 2004).

. Conclusions

Steep slopes are often covered by tree and shrub vegetation thatives an increased stability, technically and legaly recognized asrotection forests.The potential use of protection forests to combat shallow slope

nstabilities here investigated is becoming increasingly importantnd relevant, especially in the light of the recent landslides and

ud flows in various regions triggered by rainfalls with increased

ntensity.Thus in the analysis of slope stability it is necessary to take

nto account that the characteristics of vegetation (above and

6 neerin

boTwpsdirtmJ2fmo

faottftotbaffaasyi

utleCeuPdt

A

AdTNp

R

A

A

A

A

B

B

B

B

C

C

C

C

C

D

D

E

E

F

F

F

F

F

F

F

G

G

G

G

G

I

J

44 F. Preti / Ecological Engi

elow-ground biomass, root density, etc.) do not remain constantver time, but evolve along with their effects on slope stability.ree vegetation is also subject to dying (beetle outbreak, storms,ild-fires, etc.) and cutting operations (forestry production,lant felling, fire control strips, replacement of species, etc.). Theoil root reinforcement (soil-bolstering function) of dead treeseclines as their strength and stiffness decreases. If the vegetations not replaced soon enough, erosion processes may be aggravated,esulting in increased weathering and more water penetrating intohe soil through spaces created by the decayed roots: this, in turn,ay trigger off shallow landslides (OLoughlin, 1974; Sidle, 1992;

ohnson and Wilcock, 2002; Reubens et al., 2007; Ammann et al.,009). Consequently, after a large-scale tree die-off in mountainorests, a triggering event such as heavy rain or massive snowelting can be a serious threat to slope stability as in Tuscanyccurred in the last decades.Based on field investigation and numerical modelling, a model

or estimating the decrease in cut/senescent root strength over timend its effect on slope stability when combined with rainfall eventsf different scale is presented. The goal was the evaluation of varia-ions over time in the shallow landslide hazard associated with theriggering rainfall threshold as a result of root degradation afterorest cutting (or death). The study case is related to experimen-al investigations to determine the tensile strength and elasticityf root samples of plants dead within a decade, which correspondo decreasing values of root cohesion. Tr and MoE were tested onoth living beech roots and degraded ones and they decreasedlmost linearly over the number of years after death. The rain-all return time corresponding in saturated conditions to safetyactor FS = 1 and the associated hazard for different durations in

fixed period of time have been calculated, obtaining results in good agreement with recent inventories at regional scale: soillope stabilization by plants would enter a critical phase someears after a plant die-off, caused either naturally or by humanntervention.

Further study will be performed to assess the role of parameterncertainty in the overall model prediction as well as to explorehe possibility of distibuted mapping the relative hazard of shal-ow landslide over large spatial scales (Wu and Sidle, 1995; Roeringt al., 2003; Pollen and Simon, 2005; Schwarz et al., 2010a, 2010b;atani et al., 2010), also by using remote sensing data (Forzierit al., 2009, 2011a,b, 2012), to design soil-bioengineering meas-res (Petrone and Preti, 2008, 2009; Stokes et al., 2009; Preti andetrone, 2012), and to investigate the damages to forest cover itselfue to slope erosion and instability or other related processess ashe transport ones (e.g. Rossi Pisa et al., 1999; Ziemer et al., 1991).

cknowledgments

Special thanks to Ergys Alliu (DEISTAF Thesis student),ndrea Dani (DEISTAF Fellowship funded by Fondazione Cassai Risparmio di Trento e Rovereto), Marco Togni (DEISTAF Woodechnnology Lab), Giovan Battista Chirico (DIAAT, Universit diapoli Federico II), and to DREAM Italia (funding the researchroject Dissesto idro-geologico e vegetazione).

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Forest protection and protection forest: Tree root degradation over hydrological shallow landslides triggering1 Introduction2 Study sites3 Materials and methods3.1 Data collection3.2 Root reinforcement

4 Results and discussion4.1 Root reinforcement decay after tree death4.2 Slope stability analysis

5 ConclusionsAcknowledgmentsReferences