Tree-ring based assessment of rockfall frequency on talus slopes at Solà d'Andorra, Eastern...

Geomorphology 118 (2010) 393–408

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Geomorphology

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Tree-ring based assessment of rockfall frequency on talus slopes at Solà d'Andorra,Eastern Pyrenees

J. Moya a,⁎, J. Corominas a, J. Pérez Arcas b, C. Baeza a

a Department of Geotechnical Engineering and Geosciences, Technical University of Catalonia (UPC), 08034 Barcelona, Spainb Mekano4, 08190 Sant Cugat del Vallés, Spain

⁎ Corresponding author. Tel.: +34 93 401 07 36; fax:E-mail address: [email protected] (J. Moya).

0169-555X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.geomorph.2010.02.007

a b s t r a c t
a r t i c l e i n f o
Article history:Received 27 April 2009Received in revised form 29 January 2010Accepted 9 February 2010Available online 13 February 2010

Keywords:RockfallFrequencyDendrogeomorphologyPyreneesAndorra

The determination of the frequency of rockfall events continues to challenge quantitative hazardassessments in most mountain areas. Dendrogeomorphological analysis was used to assess rockfallfrequency on talus slopes at Solà d'Andorra (Eastern Pyrenees, Andorra). Rockfall events were dated at twosites: one at the outlet of a chute, and the other below a rock wall. The impact wounds visible on the treesurface were analyzed with a seasonal temporal resolution. At each site, trees were sampled in three foreststrips 15 to 30 m wide located at different heights on the talus. All the trees with visible injuries in the stripswere sampled.Rockfall frequency cannot be assessed by a simple analysis of time series of tree damage. A satisfactoryassessment requires a prior interpretation of the location of the damaged trees in relation to a number ofrockfall events. At Solà d'Andorra, the rockfall chronology was reconstructed for the last 25 years by datingvisible wounds. For older periods, the tree wound record of rockfalls is incomplete owing to progressiveclosure of wounds. Wounds exceeding 40 years in age had already disappeared from the tree surface in thestudy area. Our results show a clear reduction of the rockfall frequency down the talus, a noticeable lateralchange in frequency, and an influence of source morphology (chute or wall) on rockfall activity on the talus.

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1. Introduction

The magnitude–frequency relationships for landslide events areindispensable input data for a quantitative hazard assessment.Although rockfall is a common landslide mechanism in mountainousareas, determining its frequency is still a challenge.

The future occurrence of rockfalls may be determined either byanalyzing the potential of slope failure at a cliff or by the statisticaltreatment of observed events (Corominas andMoya, 2008). A numberof researchers have performed a probabilistic assessment of thepotential for slope failure by considering the spatial variability of jointsets in rock masses, the uncertainty in their geomechanical propertiesand the occurrence probabilities of triggers such as earthquakes orrainfall (e.g., Gokceoglu et al., 2000; Genevois and Romeo, 2003). Theanalysis of the stability of cliffs in mountainous areas with thisapproach is not easy owing to difficult access to rockfall source areas.In order to obtain realistic results, these approaches often requirecalibration using inventories of rockfall events (e.g. Hantz et al., 2003).

Alternatively, rockfall frequency may be estimated from pastevents (e.g. Bunce et al., 1997; Hungr et al., 1999; Guzzetti et al., 2003;Guzzetti and Reichenbach, 2004). In this case, the key issue is the

construction of a complete time series of events. Some rockfallinventories have been used to estimate event frequency or sedimen-tation rate; they are based on a direct observation over a period oftime or on the number of landed clasts in marked plots (e.g. Rapp,1960; Luckman, 1976; Gardner, 1980, 1982, 1983; Matsuoka andSakai, 1999; Matsuoka, 2008). With this approach it is not easy toextend the observation period for more than a few years. This is animportant drawback because a short series of rockfalls cannot showthe temporal variability of the process (Fig. 1).

The rockfall inventories recorded by administrative units such asroad-maintenance services or forest agencies are the ideal source datafor frequency assessment. The available historical records of rockfallscorrespond to events affecting railways and highways (e.g. Bunceet al., 1997; Hungr et al., 1999; Dussauge-Peisser et al., 2002; Guzzettiet al., 2003). As regards natural slopes, many rockfalls are notrecordedwith the result that frequency is underestimated (Wieczorekand Jäger, 1996; Bell and Glade, 2004).

Dendrogeomorphology has been successfully used to date differ-ent types of rapid mass movements such as debris flows or snowavalanches (Potter, 1969; Burrows and Burrows, 1976; Hupp et al.,1987; Van Asch and Van Steijn, 1991; Strunk, 1997; Blijenberg, 1998;Stoffel et al., 2005a, 2006; Bollschweiler et al., 2007; Butler andSawyer, 2008). It has also been used to identify landslides showingmultiple reactivations (Shroder, 1978; Braam et al., 1987; Corominasand Moya, 1999; Fantucci and Sorriso-Valvo, 1999; Stefanini, 2004).

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Fig. 1. Event frequency obtained using different time intervals with durations of 5 and20 years. The time series was generated by simulating a Poisson process with a mean of0.5 events per year. The frequencies calculated with shorter intervals are less accurateand show a large variance.

394 J. Moya et al. / Geomorphology 118 (2010) 393–408

More recently, dendrochronology has been applied to the study ofrockfall activity (Schweingruber, 1996; Moya and Corominas, 2004;Stoffel et al., 2005b; Perret et al., 2006; Stoffel, 2006; Schneuwly andStoffel, 2008).

The aims of this paper are: (a) to analyze rockfall activity at Solàd'Andorra (Eastern Pyrenees, Andorra) using dendrogeomorphologi-cal techniques; (b) to propose a sampling strategy of trees to preparethe possible most complete series of rockfalls based on injuries visibleon tree surface; (c) to calculate the change of rockfall frequency downthe talus; (d) to compare the frequency of rockfalls at two sites thatcorrespond to a chute and a wall, and (e) to discuss the potentialities

Fig. 2. Some types of damage caused by rockfalls on trees. (a) Rockfall that uprooted a treeblocks and the high density of the forest; the tree diameter is about 10 cm on average. The wthe wedge sample located in (a); the tree-rings that formed after the wound cover the old

and shortcomings of dendrochronological dating of rockfalls usingbroad-leaved trees.

2. Particularities of tree-ring dating of rockfalls

2.1. Rockfalls and tree damage

The impact of falling blocks injures trees in a variety of ways suchas wounding, branch breaking, beheading and tilting (Fig. 2). Thebreakage of the main branch or of the upper part of the trunk inhibitstree growth, resulting in an abrupt reduction of ring width (Potter,1969; Burrows and Burrows, 1976). Tilting of the tree leads to theformation of reaction wood (Alestalo, 1971; Shroder, 1978; Timell,1986; Hupp et al., 1987). Wounding causes local destruction ofvascular cambium (the tissue from which wood and bark aregenerated), leading to the disruption of the concentric ring growthpattern (Alestalo, 1971; Shroder, 1978; Osterkamp and Hupp, 1987;Shigo, 1989). Moreover, strong impacts on the trunk cause woundingwith erosion of wood. Procedures for dating such tree damage aredescribed in the literature (Alestalo, 1971; Shroder, 1978; Braam et al.,1987; Hupp et al., 1987; Lang et al., 1999; Stoffel et al., 2005b).Wounds are by far the most common damage inflicted by rockfalls ontrees, and can be dated with an accuracy of one dendrological season(Fig. 3). A dendrological season lasts for approximately six monthsand it may correspond to: a) the growing season (i.e. the period oftree-ring formation that takes place from spring to the end of the

and wounded others (Alzina talus, Solà d'Andorra, Eastern Pyrenees). Note the fallenedge sample of (b) was taken from a side of a wounded tree (white arrow). (b) View ofer rings forming an angle. The wound was inflicted 4 years before sampling.

Fig. 3. Dating of tree wounds with a seasonal resolution. (a) Wound whose edge is located between two tree-rings, indicating occurrence of wound during the dormant season. (b)Wound whose edge is located within a tree-ring, indicating occurrence of wound during the growing season.

395J. Moya et al. / Geomorphology 118 (2010) 393–408

summer in temperate climates) or b) the dormant season (a periodwithout tree-ring growth that in temperate climates usually extendsfrom autumn to winter). For dating, it is useful to express years indendrochronological terms. A dendrological year includes both adormant season and the subsequent growing season (Moya andCorominas, 1996; Corominas and Moya, 1999). For example, a woundinflicted in December of 1982 can be dated to the dormant season ofthe 1982–83 dendrological year.

2.2. Mode of rockfall movement and evidence of events

For some types of mass movement, the extent of the area affectedmay be easily identified by geomorphological reconnaissance. Forinstance, a debris flow is usually characterized by a distinct sourcearea, a main track and a depositional toe. Affected trees are found onthe flanks of the main track and in the depositional area (Fig. 4a).Earthflows and rotational slides result in considerable disruption ofthe original topographic surface. The head of a slide undergoesrotation and depletion of the ground surface whereas the foot mayshow bulging and cracking. As a result, trees located on a landslide

Fig. 4. Location of trees injured by different types of mass movements. (a) Single-event debritilted over the slide mass. Tilted or wounded trees can also be found along the edge of the traby recent events; in other areas trees grow on older debris-flow deposits and are undisturbundisturbed trees.

surface may display massive tilting, which is easily recognizable. Alarge number of trees in the area affected by a landslide showsynchronous responses. Large rockslides or rock avalanches can alsobe dated without difficulty provided that living trees are close to thepath or are partially buried by debris (i.e. Moore and Mathews, 1978;Butler et al., 1986).

By contrast, rockfalls are generally small-size slope movements.They usually mobilize a few cubic meters of material and involvesingle or a few blocks. A rockfall event does not always leave acontinuous recognizable track or a continuous deposit. Blocks left byconsecutive events may not be differentiated if the events occurwithin a short time span (a few months). Consequently, the numberand magnitude of rockfalls cannot usually be determined from theobservation of deposits on the ground. The fragmental character ofrockfall could also result in only a small number of trees being affected(Fig. 4c). Thus, tree injuries are seldom associated with a given blockor deposit unless the rockfall occurred very recently, retaining freshfeatures or blocks in contact with injured trees. Therefore, dating ofrockfalls from deposits is not easy and other approaches should beadopted.

s flow or mudslide. Disturbed trees are either partially buried by debris-flow deposits orck. (b) Fan formed by repeated debris flows. Disturbed trees are located in areas affecteded. (c) Talus slope formed by repeated rockfalls. Disturbed trees are scattered between

Fig. 5.Morphological stages of closure of wounds. (a) Very recent (b1 year) wound with a fresh appearance showing scratches in the bark and wood. (b) A very young wound withcallous growing over the exposed wood. The wood is discoloured, and the callous is identifiable by its younger bark. (c) Closed wounds are still identifiable by their younger bark. (d)Old wound is still identifiable from a longitudinal suture in the bark.


2.3. Loss of rockfall evidence due to wound closure

Trees close their wounds by means of an overgrowth of wood(Shigo, 1989). Rings appearing after the injury overlap the wound andform an angle with the older rings (Fig. 2). Wounds close progres-sively, and old wounds are more difficult to recognize (Fig. 5). In thispaper, we use the term “visible wound” (or “visible scar”) for a woundthat can be identified on the tree surface regardless of its status such asopen (with visible wood; Fig. 5a,b) or closed but still recognizable byits younger bark (Fig. 5c) or by a longitudinal suture (Fig. 5d).

Old closed wounds cannot be recognized from the tree surfacealthough they remain within the tree trunk as internal (hidden) scars.Potter (1969), Hupp et al. (1987) and Hétu (1990) found that woundscaused by rapid mass movements 45 to 55 years ago are still visible.Wounds caused by fire or flood may remain open for a longer timebecause these processes injure the base of the trunk in a recurrent

Fig. 6. Study area. (a) Map of Solà d'Andorra with the location of rockfalls witnessed by the iabove the built-up area of Santa Coloma in the Forat Negre basin.

way, preventing the pre-existing wounds from closing (Sigaffoos,1964; Dieterich and Swetnam, 1984). Wounds that disappear fromthe tree surface blur the evidence of past rockfalls. Loss of traces oftree disturbance with time is a well known limitation of dendrogeo-morphology (i.e. Shroder, 1978; Osterkamp and Hupp, 1987). This isespecially relevant to dating rockfalls because wounds are thepredominant tree injury.

Small wounds can close in a few years, which suggests that rockfallfrequency cannot be determined with only visible wounds (Schwein-gruber, 1996). Detection of a hidden scar in a tree requires the taking ofmultiple sets of cores at different heights of the trunk or felling the treeto obtainmultiple cross-sections (Stoffel et al., 2005b; Stoffel and Perret,2006). In some coniferous trees such as Pinus sp., Larix sp. or Picea sp.hidden scars can be detected indirectly using traumatic resin ducts(Wimmer et al., 1999). Traumatic resin ducts, which also form inresponse to wounding, may be present in a tree-ring several meters

nhabitants of Santa Coloma in the last 50 years. (b) Two lines of rockfall fences installed


from the wound (Stoffel and Perret, 2006; Bollschweiler et al., 2008;Stoffel, 2008). Rockfall chronologies spanning 400 years have beenreconstructed taking cross-sections or sets of cores in resin duct-bearingspecies (Stoffel et al., 2005b; Perret et al., 2006).However, resin ducts donot form in most tree species including broad-leaved ones.

3. Study area

Solà d'Andorra is a long steep slope in the Gran Valira River Valley(Andorra, Eastern Pyrenees) (Fig. 6). Themaximumaltitude of the slopeis 2332 m a.s.l. (Carroi peak) while the valleyfloor is around 980 m a.s.l.The length of the slope face is more than 2 km. Steep chutes (couloirs)develop across the slope. The steepest parts of the slope, which exceed60° locally, are located close to the valley bottom where erosion byPleistocene glaciers generated rock walls 400 m in height (Copons,2004). The bedrock is a massive granodiorite with well-developed setsof joints (Copons, 2004; Copons et al., 2005). Abundant fresh scars andopen cracks on the slope provide evidence of intense rockfall activity(Fig. 7), which produced taluses at the foot of the rock walls and at the

Fig. 7. Location of the two study sites (Alzina and Basera Mateu) at Solà d'Andorra and geomomany planar and wedge failure scars (Forat Negre chute). (c) Open cracks providing eviden

outlets of the chutes. Long-term rockfall activity at Solà d'Andorra isevidenced by blocks on the talus slopes affected by different degrees ofweatheringand lichen cover. Old debris-flowdeposits are also locatedatthe bottom of the slope (Fig. 6a); they probably resulted from theerosion of glacial sediments during deglaciation. There are no recentdebris-flow deposits on the slope.

The towns of Santa Coloma and Andorra la Vella are located on thevalley floor at the foot of Solà d'Andorra. The built-up area extends tothe bottom of the talus, increasing the risk of rockfall. Local peoplehave witnessed more than 30 rockfalls in the last 50 years (Coponset al., 2005; Fig. 6). Most of these events reached the bottom of thetalus and, in some cases, caused damage to buildings. Recent rockfallevents involved rock masses smaller than 25 m3 and up to a few tensof blocks. The volume of the individual blocks usually ranges between0.5 and 25 m3. However, in February 2003, a mass of 300 m3 includedblocks up to 70 m3 (Copons et al., 2005). In April 2008, a mass of150 m3 was released with blocks up to 32 m3. The most damagingrockfall occurred on 21 January 1997, when a block of 25 m3 struck abuilding, and seriously injured one person. After this event, a number

rphic evidence of recent rockfalls. (a) General view of Solà d'Andorra. (b) Hillslope withce of slope instability.

Table 1Morphological characteristics of the two study sites, and number of historical rockfallevents witnessed in recent decades (Source of historical data: Copons et al., 2005).

Site Talus Source zone Witnessedrockfalls

Length(downslope)(m)

Width(m)

Type Maximumelevation(m a.s.l)

Alzina 220 40 to 80 Couloir 1360 01/21/199704/04/1999

Basera Mateu 150 420 Wall 1430 Early20th centuryEarly 1990s

Fig. 8. Flow chart showing the method for assessing rockfall frequency at So


of risk mitigation measures including the construction of protectiverockfall fences (Fig. 6b) and the establishment of a surveillance planwere implemented (Copons et al., 2005; Copons and Vilaplana, 2008).The first line of the rockfall fence was installed at the bottom of thetalus in 1999, located 10 to 20 m upslope from the built-up area.Another set of fences was installed in 2002 on the talus apex, at theoutlet of the most dangerous chutes (Forat Negre and Alzina, Fig. 6).The risk of rockfalls was quantitatively assessed after the constructionof the fences (Corominas et al., 2005).

Rockfall frequency analysis was performed at two test sites, on theAlzina and Basera Mateu taluses (Figs. 6 and 7). These sites wereselected because they have different geomorphic characteristics(Table 1). The Alzina talus is located at the outlet of a chute whereas

là d'Andorra. Some steps shown in the chart are presented in Section 4.


the Basera Mateu talus is situated directly below the cliff. Tworockfalls were observed on the Alzina talus in the last 40 years. One isthe aforementioned January 1997 event (Fig. 6). The other event,which occurred in April 1999, reached the bottom of the talus near thebuilt-up area. This event involved a rock mass of 5 m3 and a few tensof boulders. Historical data for Basera Mateu are imprecise butinhabitants in the valley witnessed one event in the early years of the20th century and another in the early 1990s. No information on themagnitude of the rockfall events for Basera Mateu is available.

The historical record of rockfalls for Solà d'Andorra is relativelyshort and includes mainly small-size events (b25 m3; and allb300 m3). However, old rockfall scars and old fallen blocks with aweathered surface and considerable lichen cover indicate a long-termgeomorphological record that may include large rockfalls. Neverthe-less, it is not easy to determine the magnitude of rockfalls fromgeomorphological and sedimentological observations.

In the forest stand of the study area, broad-leaved species, Englishoak (Quercus robur L.) and Holm oak (Quercus ilex L.), account for 99%of the forest on the taluses. Quercus robur is the most dominantspecies, forming 75% of the forest stand at Alzina, and the whole standat Basera Mateu. Forest density, measured on six plots of 100 or150 m2, ranges between 950 and 2700 trees ha−1. The trees areusually thin; the diameter at breast height (DBH) is 10 cm on average(Fig. 2).

4. Materials and methods

4.1. Preliminary forest reconnaissance: selection of datable features

A preliminary dendrogeomorphological reconnaissance of theforest was made to identify the type of tree injury and to interpretits relationship with rockfalls (Fig. 8). This reconnaissance isnecessary to rule out other processes causing damage to trees.

In the study area, wounding is the most dominant type of treedamage. Our field survey indicated that there were no active

Fig. 9. Sketch illustrating the strategy of tree sampling in horizontal strips located atdifferent heights of talus. A rockfall mass may split into different blocks with divergenttrajectories on the talus, as shown by the arrows. In order to intercept all the potentialtrajectories, the sampling strips must extend across the whole width of the talus.

geomorphic processes other than rockfalls causing damage to treeson the talus (e.g. snow avalanches and debris flows), and no apparentsigns of biological or anthropic processes injuring trees (e.g. browsingand felling). Other forest stands close to Solà d'Andorra showed nogeomorphic evidence of recent rockfalls. Young oaks exhibited asinuous trunk with a mild to moderate leaning, which could beattributed to phototropic processes or to genetic malformation(Timell, 1986). Given the ambiguous origin of tree tilting at Solàd'Andorra, the use of tilted trees was ruled out for dating rockfalls.

Traumatic resin ducts do not form on broad-leaved trees, whichare dominant in the study area. Dating of old and hidden wounds bytakingmultiple sets of cores at different heights of the trunk (Stoffel etal., 2005b; Perret et al., 2006) is time-consuming, and cutting trees isnot permitted in the study area. The chronology of rockfalls hadtherefore to be reconstructed using only visible wounds.

4.2. Strategy of tree selection for sampling

The number of trees damaged by a rockfall event is highly variableowing to the dispersion of rock trajectories, and the number of fallenblocks per event. When a few falling blocks progress through a forest,they usually impact on a small number of trees, as witnessed at Solàd'Andorra. If a forest is not sufficiently dense or a plot selected forsampling is too small, impacts may not be found. Although studies ona limited number of trees (Gsteiger, 1993; Schweingruber, 1996)provide evidence for past rockfall activity, they do not supply a goodrecord for a reliable estimate of rockfall frequency. Sampling all thetrees in a forest stand would compensate for this shortcoming.However, since this is very time-consuming, selective samplingshould be undertaken. Schneuwly and Stoffel (2008) performed aregular tree sampling along horizontal profiles (transects) to ensurean even distribution of sampled trees. These authors chose a profileinterval of 15 m in which a tree was selected every 10 m. Satisfactoryresults cannot be obtained in our study area using the approach ofthese authors given that a small number of trees per event aredamaged because rockfalls usually involve a few blocks.

In this study, an exhaustive sampling of all the visible tree injurieswas performed in broad horizontal forest strips on the talus. The stripswere arranged parallel to the contour lines (Fig. 9). Each stripincluded the whole section of the talus so that all the rockfalltrajectories could be detected. In the study area, the presence of forestgaps reduced the length of some strips in the northeastern part of theAlzina talus and in the southwestern part of the Basera Mateu talus(Fig. 10). The width of the strips was considerable, 15 to 30 mdepending on the forest density, in order to increase the chance ofdetecting rockfalls.

Three sampling strips were established at different heights at eachsite (Figs. 9 and 10). The highest strip is located on the talus apex.Sampling with multiple strips has two aims: to identify the highestnumber of rockfall events, and to observe rock-arrival frequencychanges downslope.

A total of 276 trees were sampled, 136 at Alzina (87 Quercus roburand 49 Quercus ilex) and 140 at Basera Mateu (all Quercus robur). If atree had several wounds, all of them were sampled. A total of 375samples were taken. The characteristics of the strips at the two talusesare shown in Table 2.

4.3. Extraction and analysis of samples

Almost all the tree injuries sampled were wound scars that werevisible on the trunk surface (N99% of the samples). All the visiblewounds exceeding 5 cm and located at a height below 2 m weresampled. Wounds smaller than 5 cm were not sampled because theycould have been caused by small rock fragments (stones).

Wedge-shaped samples were extracted with a small handsawalong the side of the scar (Fig. 2). Each sample contained both the

Fig. 10. Location of tree-sampling strips at Alzina (a) and Basera Mateu (b). Talus sectors at Basera Mateu are also shown.


wound surface and the overgrowing callous tissue. In order to date thedisturbance, the sample had to include the edge of the wound (Fig. 3).A few beheaded trees were also found. In this case, a cross-sectionaldisc sample was cut at the broken tip of the trunk. Our field work alsoincluded the mapping of all trees sampled (by compass and tapemeasurements), the description of tree morphology, and theannotation of the vertical position of the sampled injury.

Sampleswere dried at room temperature and polishedwith a bandsander until a flat and smooth surface was obtained. After this, thesamples were analyzed under a binocular microscope. The descriptionand synchronization of samples were carried out using skeleton plots(Stokes and Smiley, 1968; Schweingruber et al., 1990).

We found that Quercus ilex was unsuitable for dating purposes.Many false tree-rings were found in the samples because it is a broad-

leaved but evergreen species. A false ring is a change in cell structurewithin an annual growth layer which resembles the boundary of atrue annual ring. False rings can be detected during synchronization ifthey are occasional in the tree-ring series. However, when theyappear frequently, the series becomes undatable. This occurred in 27of the 49 samples of Quercus ilex.

4.4. Determination of the number of rockfall events

It is not unusual to find injuries affecting different trees in the samedendrological season. One question is whether these injuries wereproduced by several rockfalls or by a single rockfall composed ofdifferent blocks. In some situations (e.g., Perret et al., 2006), each rockimpact on a tree is believed to represent one rockfall, assuming that

Table 2Characteristics of strips for tree sampling.

Talus Strip Distance fromcliff face (m)

Strip size Forest density(trees ha−1)

Number ofinjured trees

Injured treesper ha

Length (m)a Width (m)b Area (ha)

Alzina Upper 0 42 30 0.131 970 32 244Mid 80 38 25 0.098 2100 66 673Lower 125 47 25 0.107 2690 38 355

Basera Mateu Upper 5–15 230 15 0.345 1200 to 1900 112 325Mid 40 157 15 0.235 28 119Lower 70 154 15 0.231 0 0

a Measured along the contour lines.b Measured downslope.


every rockfall consists of a single block. This assumption maximizesthe frequency of rockfalls and minimizes their size. In the study area,this cannot be assumed because most recorded events mobilizedseveral blocks (Copons, 2004; Copons et al., 2005; Corominas et al.,2005). The surveillance plan set up by the Andorran administrationhas provided valuable data on rockfalls such as the number of blocksinvolved in each event, locations of fallen blocks and trajectories.These data show that all the events were composed of several blockswith divergent trajectories (e.g., Fig. 11). Such rockfalls are very likelyto produce multiple impacts on trees, even within the same samplingstrip.

Consequently, the number of rockfall events cannot be obtaineddirectly from the number of injuries, and a more detailed analysis ofthe age and spatial distribution of the injuries is necessary. Twoapproaches to identifying rockfalls based on the characteristics of thesource area (chute or cliff) and the spatial distribution of tree injurieswere adopted. At Alzina, it was assumed that all the injuries producedin the same dendrological season were due to the same rockfall if thespatial distribution of the injuries could be explained by a single ormultiple divergent block trajectories. This implies that consecutiverockfalls that occurred in the same season and showed a similartrajectory cannot be differentiated.

In large source areas, like the Basera Mateu rock wall, which is230 m long, the occurrence of two or more rockfalls in the sameseason is frequent. At Basera Mateu, the same approach as the one atAlzina was adopted with the difference that the talus was previously

Fig. 11. Location of blocks (solid circles) fallen during two recent rockfalls. The event inthe Alzina chute occurred on 4 April 1999, and the one close to the Forat Negre chute on17August 2003. Both events were composed of several blocks and only large ones areshown in the figure. Small numbers indicate the volume of blocks in m3.

divided into eight sectors each of whichwas fed by awatershed on thewall (Fig. 10b). Consequently, synchronous tree damages occurring indifferent talus sectors are assumed to be caused by different rockfallevents.

4.5. Validating the strip width

Sampling in wide forest strips sought to maximize the chance ofdetecting rockfalls although some rockfalls might not have producedany impacts on trees. The question here is whether the tree-samplingstrips are wide enough for a complete record of rockfalls. To answerthis question, the relationship between the strip width and therockfall frequency obtained must be determined. This can be doneempirically by sampling all the trees with (visible) injuries in a verywide strip. From this original strip, progressively thinner strips aredefined and the trees enclosed in each strip are used for assessingrockfall frequency. Forest density was expected to influence the resultgiven that it determines the chance of impact on trees.

This method was applied at Alzina using three very wide (40 m)strips in which the forest density was approximately constant. Thisapproach involved sampling all (265) trees with injuries at Alzina.Forest density was determined for each strip in a square plot of about150 m2. The change in rockfall frequency from amaximum stripwidthof 40 m to a minimum of 2.5 m was analyzed using increments of2.5 m. The results obtained for Alzina was considered to berepresentative of Basera Mateu given the similarity of the forestdensity on the two taluses.

5. Results

5.1. Temporal distribution of tree injuries and rockfall events

5.1.1. General aspectsFigs. 12 and 13 show the temporal distribution of injured trees at

Alzina and Basera Mateu, respectively. The oldest visible wound wasproduced 40 years ago. The time range covered by the dendrochro-nological record resembles that of the historical rockfall record at Solàd'Andorra.

Between the dendrological years 1961–62 and 2001–02, 12rockfalls reached the Alzina talus, and 77 reached the Basera Mateutalus. The three events at the two sites witnessed by the inhabitants ofSanta Coloma during the last 50 years were also confirmed by tree-ring analysis. Figs. 12 and 13 indicate that rockfall occurrence ismarkedly seasonal; 90% of the events occurred during the dormantseason (October to March). There is almost no difference in theseasonality of rockfalls between the two taluses. The dormant seasonaccounted for 11 of 12 rockfalls (91.7%) at Alzina and 69 of 77 (89.6%)at Basera Mateu. The marked seasonality may be linked to weatheringprocesses such as freezing and thawing. Copons (2004) andCorominas et al. (2005) compared the historical series of rockfalls atSolà d'Andorra with the rainfall record. They concluded that rainfallfavoured rockfalls because most recent events occurred during orshortly after wet episodes. However, no rainfall threshold for rockfalls

Fig. 12. Chronology of tree injuries in strips at Alzina. The injuries occurred in the dormant season are shown above tick marks, and those in the growing season are shown betweenthe marks. A small arrow indicates a rockfall not detected in the strip but in another strip downslope. H: witnessed rockfall, st: number of trees sampled, re: number of rockfalls, dcf:distance from the bottom of the cliff.


was obtained, suggesting that rainfall may be necessary but notsufficient for triggering rockfalls in the study area.

Sampling in three strips on the talus enabled us to detect severalrockfalls that were not recorded on the talus apex. These events areindicated by arrows in Figs. 12 and 13. Events undetected in one stripbut detected in another were considered when assessing rockfallfrequency for the strip.

5.1.2. Determination of reference time intervalsFigs. 12 and 13 show that the recurrence of the rockfalls has not

been constant in the last 40 years; the frequency increased at the endof the 1970s. At Alzina, none of the 12 rockfalls identified on the apexoccurred before 1978–79 (Fig. 12). At Basera Mateu, only nine of the77 events recorded on the apexwere older than this year (Fig. 13). The

Fig. 13. Chronology of rockfalls at Basera Mateu. The rockfalls that occurred in the dormant sthemarks. Small arrows indicate rockfalls not detected in the strip but in another strip downsof the cliff, H?: historic rockfall of imprecise age.

change in rockfall frequency around the year 1978–79 can be clearlyvisualized by plotting the cumulative number of rockfalls against time(Fig. 14). The slope of the two lines in the figure corresponds to themean frequency of rockfalls.

The change in the rockfall frequency obtainedmay reflect a changein the environmental conditions or the disappearance of wounds withtime. The masking of wounds owing to tree growth affected therecord given that almost all samples analyzed (99%) had visiblewounds. Some wounds produced before 1978–79 were recognizableonly as a longitudinal suture in the trunk (Fig. 5d). However, a moreprogressive loss of rockfall evidence is expected before 1978–79 ifdisappearance of wounds is the only factor to be considered.

Rapid tree colonization during the 1970s could also account for theincrease in damaged trees per year. The colonization and resulting

eason are shown above tick marks, and those in the growing season are shown betweenlope. st: number of trees sampled, re: number of rockfalls, dcf: distance from the bottom

Fig. 14. Cumulative number of rockfalls detected on the apex of Basera Mateu talus. Thefrequency of rockfalls is shown by the slope of the curve. It changedmarkedly in 1978–79.

Fig. 16. Selection of the time interval for frequency calculation. The time interval usedfor the calculation must begin one year after the oldest event (λb); otherwise thefrequency will be overestimated (λa).


increase in tree number may have led to more injuries recorded insubsequent years. The aerial photographs taken in 1948, 1972, 1985and 2008 show that the distribution of forest gaps did not change inthe last 60 years (Fig. 15). Rather, they suggest an increase in forestdensity between 1948 and 1972. Tree colonization did not thereforeaccount for the change in the record of damaged trees in the late1970s. Anyway, the record prior to 1978–79 is probably veryincomplete. Consequently, only the period after 1979 was consideredto calculate rockfall frequency.

Fig. 15. Series of aerial photographs of Solà d'Andorra taken betw

In 1978–79, rockfalls occurred at the two sites investigated. Inorder to obtain a more accurate value of rockfall frequency, data wereanalyzed only after these rockfall events (see explanation in Fig. 16).Although the time span considered (23 years) is relatively short, itresembles that of Hungr et al. (1999), who used a 22-year record ofrockfalls and rockslides.

5.2. Spatial distribution of rockfall frequency

5.2.1. Rockfall frequency on the talus apexOn the talus apex, rockfall frequency is 2.69 events per year at

Basera Mateu, which corresponds to a return period of 0.4 years,whereas the frequency is 0.48 events per year at Alzina (a returnperiod of 2.1 years) (Table 3). These frequencies reflect an influence ofthe source size since Basera Mateu is much bigger than Alzina. Tocompensate for this influence, the frequency was standardized usingthe surface area of the source zone, i.e. the total area of the rock face atBasera Mateu and the area of rock walls along the chute at Alzina. Thestandardized frequency (λs) at Basera Mateu is about three times as

een 1948 and 2008. Arrows A and B mark the same points.

Table 3Rockfall frequencies at the talus sites investigated. The strips located on the talus apex arehighlighted.

1) Rockfalls occurred from the year 1979–80 till the year 2001–02 (23 years).2) Measured as the area of exposures along the chute walls.3) Length of the strips is indicated in Table 2.4) No events are recorded.

Table 4Minimum width of strips required to avoid missing rockfalls. The main characteristicsof the forest in each strip were measured in three plots at Alzina. Forest density isshown separately for Quercus robur, the tree species used for dating.

Plot Tree number Mean treediameter(cm)

Plotarea(m2)

Forest density(trees ha−1)

Minimumstripwidth(m)

Allspecies

Quercusrobur

Allspeciesa

Quercusrobur

Upper 12 10 9.7 124 970 800 20Middle 33 22 10.0 152 2100 1450 17.5Lower 39 22 9.0 145 2690 1500 15

a Quercus ilex is present on the talus but was not useful for dating as explained in thetext.


high as that at Alzina (0.413 and 0.148 events year−1 ha−1; Table 3).Blocks detached from the wall fall directly on the talus at BaseraMateu, whereas some fallen rocks may be retained at the bottom ofthe chute at Alzina. However, absence of large blocks in the screedeposits of the chute suggests that only small-size rockfalls areretained.

For hazard zoning, it is also necessary to evaluate rockfallfrequency at different locations in the depositional zone (CorominasandMoya, 2008). To do this, it is necessary to normalize the calculatedfrequency by the length of the strip. The strip on the apex is 230 mlong at Basera Mateu, whereas it is 42 m at Alzina. The rockfallfrequency per unit length of the strip (λl) at Alzina resembles that atBasera Mateu (1.14×10−2 and 1.17×10−2 events year−1 m−1,respectively; Table 3).

5.2.2. Change in rockfall frequency along the talusGeomorphological evidence and the simulation of rockfall trajec-

tories suggest a reduction of frequency down the talus slope (e.g.Selby, 1993). Nevertheless, the reduction of frequency has beenpoorly documented.

Rockfall frequency at the two sites investigated is actually higheron the talus apex. Rockfalls reached the apex of the Alzina talus every2.5 years on average (Table 3), but only four of the 12 rockfallsrecorded on the apex reached the bottom of the talus (Fig. 12). Two ofthese events were observed by the inhabitants in January 1997 andApril 1999 (“H” in Fig. 12). The January 1997 event injured a largenumber of trees on the talus (43 of them were enclosed in the threestrips sampled, and 14 in the lower strip). Our data show that blocksfrom the event in 1978–79 also reached the lower strip and damaged

Fig. 17. Frequency of rockfalls at the bottom of Alzina talus for different strip widths. Noincrease in frequency is observed after the arrow. Forest density forQuercus robur is 500trees ha−1.

several trees (11 in this strip, and a total of 16 in the three strips). Thereturn period of such rockfalls with a long run-out is estimated to bebetween 18 and 24 years.

Rockfall frequency also decreased downslope at Basera Mateu,even more markedly than at Alzina. On the apex of the Basera Mateutalus, 77 rockfalls have been recorded in the last 40 years, but only 17of them reached the middle strip and none the lower strip (Fig. 13).This indicates that most blocks were retained at the top of the talus.The historical record for this talus is consistent with this inference. It isreasonable to assume that the size of fallen blocks is greater at Alzinathan at Basera Mateu given that the run-out distance is influenced byblock size (Whaley, 1984; Corominas, 1996; Erismann and Abele,2001).

5.3. Validating the strip width

The analysis of the three strips at Alzina showed that the rockfallfrequency obtained increases with the strip width until a constantvalue is attained (Fig. 17). The point at which the frequency reaches aconstant value indicates the minimum strip width to be sampled.Table 4 shows the minimum strip width at Alzina for different valuesof forest density. For a forest with an average tree diameter (DBH) ofabout 10 cm and a density of 1500 trees ha−1, the width of the stripshould not be less than 15 m, or less than 20 m if the density does notexceed 800 trees ha−1. These values of forest density correspond toQuercus robur, which were the only trees showing datable tree-rings.The strips sampled before the validation at Alzina and Basera Mateuhave widths from 15 to 30 m and forest densities from 800 to 1900trees ha−1 (Tables 2 and 4). Thus, these results suggest that the stripsare sufficiently wide to obtain representative frequencies of rockfallactivity.

6. Discussion

6.1. Spatial distribution of rockfalls

The historical record of rockfalls at Solà d'Andorra spans the last50 years and includes the rockfalls that had a large run-out distanceand reached the talus bottom. Although this record is of considerablevalue to risk assessment (Corominas et al., 2005), it only yields aminimum frequency of rockfalls in the source zone.

At Alzina, rockfall deposits are also found as thin and narrowveneers at the bottom of the chute. These deposits correspond to thetail of rockfalls that travelled beyond the chute outlet, and to small-size rockfalls that came to rest in the chute. The deposition in thechute screens the smallest rockfalls. The Alzina talus is mostly fed byrockfalls made up of several large blocks. The Basera Mateu talus, incontrast, is located directly below the cliff face and receives blocks ofall sizes. Accordingly, the number of fallen blocks reaching the BaseraMateu talus is expected to be higher than that reaching the Alzinatalus. Indeed, the Alzina chute generates fewer rockfalls than BaseraMateu in the same time interval. The fact that there is no significant


difference in rockfall frequency per unit length of the talus at the twosites is due to the funnelling of fallen blocks in the Alzina chute. Thisindicates that rockfall frequency at the talus is influenced by themorphology of the source zone.

6.2. Completeness of the rockfall record derived from visible wounds

The completeness of a rockfall series that is reconstructed usingdendrogeomorphology depends on the amount of evidence of treedamage and on our ability to extract information about rockfalls fromtree-rings. In the study area, wounds were datedwith a high temporalresolution (a dendrological season). However, this resolution is notenough to differentiate between rockfalls in one season. In our study,this uncertainty is partially reduced by adding geomorphic observa-tions of recent inventoried rockfalls.

The sampling of all the visible wounds at Alzina allowed thedetailed analysis of the spatial distribution of tree injuries (Fig. 18).The locations of some injured trees define a linear path. It is highlyunlikely that two (or more) rockfalls in one season followed the samepath. As stated in Section 4.4, all the injuries in one dendrologicalseason were ascribed to one event if the spatial distribution of theimpacts could be explained by a single or several divergent blocktrajectories. This assumption can be tested by calculating theprobability of spatial coincidence of two rockfalls of similarmagnitudeon the talus for a given cross-section (ps), using data of the April 1999event. This probability was calculated on the assumption that blocksdescended the maximum slope. Therefore, ps can be expressed by l1l2/L2, where L is the width of the talus, and l1 and l2 are the widths of the1999 event and a similar event that might had happened in the sameseason, respectively. For the apex of the Alzina talus L=42 m, l1 and l2are 15 m, yielding a value of 0.127 for ps. Thus, the probability ofcoincidence is low but not negligible at Alzina.

Several rockfalls may occur in one season in the case of large walls,where different catchments may be identified. The strip at the top ofthe Basera Mateu talus is 230 m long, and the rockfall frequency is 2.7events per year. The probability of two or more events in the sameseason is 0.75 assuming that occurrence of rockfall events follow aPoisson distribution. This suggests that in the case that tree injuries ofthe talus are analyzed as a whole, many rockfall events will remainundetected. To overcome this restriction, as mentioned before, it is

Fig. 18. Trees injured by falling blocks in three dormant seasons at Alzina (1986–87, 1998–9path.

necessary to divide the talus according to the number of catchments.At Basera Mateu, a local analysis of the sectors differentiated on thetalus yielded 77 rockfalls whereas only 22 events could be detectedusing a global analysis of the whole talus (Fig. 19).

As stated above, the completeness of rockfall records derived fromvisible wounds is dependent on the disappearance of wounds fromthe tree surface. The duration of this process depends on many factorssuch as wound size and thickness of the tree-rings that overlap thewound. Stoffel and Perret (2006) highlighted on that point theimportance of bark thickness and the rate of bark renewal, whichdepends on the tree species. Infection of wounds may affect theclosure time (Shigo, 1989). At Solà d'Andorra, the oldest visiblewound had an age of 40 years; large wounds that had remained openfor 24 years were also found, but small ones closed within only 1 year.Some other studies of mass movements (Potter, 1969; Hupp et al.,1987; Hétu, 1990) have also reported the age of the oldest visiblewounds to be between 45 and 55 years. In the Swiss Alps, Stoffel andPerret (2006) found that 16–90% of the wounds in a tree were hiddenscars and that the percentage of hidden wounds increased with treeage. Thus, the absence of rockfalls before 1961–62 in our recordprobably resulted fromwound disappearance. The existence of a poorrockfall record between 1961–62 and 1978–79 at Solà d'Andorra mayalso reflect a change in rockfall frequency. Some of the small woundsmay have disappeared since 1978–79. Nevertheless, the record issufficiently complete to yield a reliable frequency of rockfalls largeenough to become a risk for the population.

Although the forest density is high at Solà d'Andorra, some fallenrocks travelled through a strip without impacting any trees. At thetwo study sites, some rockfalls were detected in the middle strip butnot in the upper one. Some other rockfalls may have gone unnoticedin the upper strip and could have come to rest without reaching themiddle strip. In order to estimate rockfall frequency more accurately,it is necessary to calculate the probability of impact of falling blocks ontrees. To this end, Stoffel et al. (2005b) proposed the use of rockfallfrequency divided by the total diameter of trees exposed to fallingblocks. However, the effectiveness of this method remains to beverified.

In summary, given the absence of impact on some trees and giventhe occurrence of more than one event in one season, it is possible thatour rockfall record obtained by dendrogeomorphology for Solà

9 and 2001–02). In all cases the trajectories of the impacts define a more or less linear

Fig. 19. Location of injured trees at Basera Mateu illustrating the usefulness of a local analysis by talus sectors with respect to a global analysis of the whole talus. (a) Location of 11injured trees (solid circles) at Basera Mateu in the dormant season of 1983–84. Trees were injured in five sectors; therefore five rockfalls were calculated for this season. Small opencircles show uninjured trees in the season. (b) Chronology of injured trees on the apex of Basera Mateu talus obtained by a global analysis. Only 22 rockfalls can be identified (oneevent per season showing injured trees).


d'Andorra does not include all events in the last 23 years. The rockfallfrequency calculated in this work should therefore be regarded as aminimum value.

7. Conclusions

On forested slopes, dendrogeomorphological analysis of a talusenables us to reconstruct the chronology of rockfalls by dating visibletree injuries. The type of injuries available for dating rockfalls dependson the tree species. Quercus robur and Quercus ilex are the dominantspecies at the two study sites. Quercus ilex proved to be unsuitable fordendrochronological analysis. Although tilted trees have been used inthe literature for dating other types of mass movements, they cannotbe applied to rockfall dating in the study area because the origin oftrunk leaning of Quercus robur is unclear. Wounds can be used fordating rockfalls with a seasonal temporal resolution. Quercus roburdoes not show wounding-induced responses in the trunk such as thetraumatic resin ducts of some conifer species. Therefore, datingrockfalls should be undertaken using visible wounds on the treesurface. This method has been effective in the last 40 years given thatolder wounds become invisible due to closure. Sampling strips atdifferent altitudes of the talus enabled us to increase the likelihood ofdetecting rockfalls.

The frequency of rockfalls cannot be derived directly from thenumber of tree injuries. The number of rockfalls must be determinedby interpreting the temporal and spatial distribution of tree injuries asa number of rockfalls. The analysis of recent rockfalls indicates thatmost injuries formed in one season and in one catchment areacorrespond to one event.

From the foregoing account it follows that a strip width of 20 m islarge enough to record all the rockfalls that impacted on trees for aforest density exceeding 1000 trees ha−1. However, given the

possibility that some rockfalls are not included in the record, thefrequencies assessed in this paper must be considered as minimumvalues. The comparison of the rockfall frequency per unit length at thetwo study sites shows that the frequency at the talus is affected by themorphology of the source zone (chute or wall). Sampling of multiplestrips showed a marked reduction of rockfall frequency down thetaluses. The reduction is higher at Basera Mateu even though theforest has a lesser density (lower tree-impact probability) than atAlzina. This fact suggests that the size of fallen blocks is greater atAlzina given that the run-out distance of fragmental rockfalls isinfluenced by block size. However, this point has to be confirmedwithfurther studies.

Acknowledgments

This research has been partly supported by the Spanish ScientificCouncil (CICYT; project BIGRISK, no. BIA2008-06614), and by theSafeland project funded by the Commission of the EuropeanCommunities. We express our gratitude to Prof. Takashi Oguchi andDr. Mauro Soldati and to an anonymous reviewer for their helpfulcomments and suggestions.

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