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The role of border areas for dendrochronological investigations on catastrophic snow avalanches: A case study from the Italian Alps
Catena 87 (2011) 209–215
Contents lists available at ScienceDirect
Catena j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t e n a
The role of border areas for dendrochronological investigations on catastrophic snow avalanches: A case study from the Italian Alps Valentina Garavaglia ⁎, Manuela Pelfini Università degli Studi di Milano, Dipartimento di Scienze della Terra “Ardito Desio”, via Mangiagalli 34, 20133 Milano, Italy
a r t i c l e
i n f o
Article history: Received 22 February 2010 Received in revised form 1 June 2011 Accepted 3 June 2011 Keywords: Snow avalanches Tree rings Dendrogeomorphology Italian Alps
a b s t r a c t Snow avalanches are common in mountain environments, sometimes affecting inhabited areas and infrastructures (e.g. roads, bridges, and ski slopes). Their study is widespread and involves the use of a variety of different techniques, including dendrochronological methods. The aim of such investigations is principally to date past events, but also to detect variation in avalanche frequency as well as their spatial distribution. Trees affected by snow avalanches generally have scars, tilted trunks and broken branches, which allow past events to be dated to within a year. In this work tree rings were used to investigate a disruptive snow avalanche which occurred in 2001 in Val Mala in the Italian Alps. The event almost completely removed forest along the flow path, while the snow powder component of the avalanche also impacted on the adjacent forest. Comparison of tree reaction in surviving plants along the flow path and vegetation on the border showed i) the production of reaction wood even in apparently undisturbed trees and ii) the usefulness of border plants for dating past events. Different dendroecological indicators were investigated (i.e. reaction wood, scars, traumatic resin ducts, and variations in stem eccentricity) and reaction wood is evidently concentrated from 2001 not only in damaged trees but also in adjacent plants. The potential to investigate past snow avalanches by collecting samples from trees growing in border areas is presented: in the specific case of Val Mala, vegetation bordering the flow paths reacted to produce reaction wood in 2001, in a similar manner to plants along the flow track, demonstrating their usefulness in such investigations. Their response to the 2001 event confirms the possibility of applying dendrochronological techniques to trees adjacent to the flow path, even if they appear morphologically undisturbed, and proposes to use only border areas in cases of an absence of sufficient trees along the avalanche track. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Snow avalanches are widespread processes in Alpine environments (Strunk, 1991), commonly affecting valley slopes. Often they occur in areas frequented by tourists or habited regions, damaging constructions such as roads and ski slopes and sometimes claiming lives (Bründl et al., 2004; Casteller et al., 2007). Reconstructing their frequency and spatial distribution represents an important phase of risk mitigation and useful for landscape planning. Moreover, in the last decade, snow avalanches have been investigated more intensively because of their relationship with climate change: they are triggered by snow cover instability that is determined by weather conditions and, like other physical systems such as, for example, permafrost, are sensitive to climatic variation (Germain et al., 2009). Tree rings represent a good proxy data for reconstructing past avalanche frequency, as well as for estimating their frequency variation related to climate change. Not only do they provide information ⁎ Corresponding author at: Dipartimento di Scienze della Terra “A. Desio”, via Mangiagalli 34, Milano 20133, Italy. Tel.: + 39 0250315514; fax: + 39 02 50315494. E-mail address: [email protected] (V. Garavaglia). 0341-8162/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2011.06.006
at an annual resolution, but also the possibility to study both the temporal distribution of these slope processes and their spatial occurrence (Reardon et al., 2008), thus obviating the problem of fragmentary historical archives. Avalanche paths are easily identifiable using aerial photographs since they generally form stripes parallel to the slope, characterized by lower and younger tree vegetation with respect to the adjacent forest (Germain et al., 2009; Pelfini and Orombelli, 1997). Nevertheless, in the case of catastrophic events the removal of the entire tree layer tends to impede the collection of dendrochronological data, even if the presence of seedlings may enable the study of slope recolonization (Garbarino et al., 2010; McCarthy and Luckman, 1993; Pelfini and Orombelli, 1997). When applied to snow avalanches, dendrochronological dating is based generally on the presence, along avalanche paths, of tilted trees, damaged branches and wounded stems (Bezzi et al., 2004; Carrara, 1979; Pelfini et al., 2001; Strunk, 1993), i.e. morphologies associated with anatomical features that can be detected and accurately dated in tree rings. For example, reaction wood (compression wood in conifers and tension wood in broadleaves) is a growth anomaly typically used in the dating of snow avalanches (Butler and Sawyer, 2008).
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In this paper the impact of a catastrophic avalanche which occurred in the Central Italian Alps is investigated, on the basis of i) samples collected from surviving trees on the running area and ii) apparently undisturbed trees in the border areas, where the snow powder component of the avalanche impacted on vegetation but left no characteristic morphologies. Growth anomalies in undisturbed trees in border areas were studied in more detail, including: traumatic resin ducts, reaction wood, stem eccentricity, callus tissue and wounds. The aim of this study is to explore the possibility of studying avalanches using only trees in forests adjacent to the avalanche path. 2. Study area Located in Stelvio National Park, Upper Valtellina (Central Italian Alps) (Fig. 1), Val Mala rises from 1050 m a.s.l. at the village of Morignone (destroyed by the 1987 Val Pola rock avalanche (Crosta et al., 2004)) to Mount Mala, at an altitude of 2943 m a.s.l. Val Mala was shaped by glacial and gravitational processes, although at present the Mala torrent and cryoclastic processes are the two principal factors acting on the valley (Pozzi et al., 1990). On the valley bottom a large fan formed by debris flow and avalanche deposits is present. A tributary of the Adda River, the Mala torrent originates from Mount Mala flowing west-northwest, but at 1700 m a.s.l. it turns to west-
southwest. The entire slope is covered by a well developed forest dominated by Picea abies L. Karst., with a reduced component of Larix decidua Mill and Pinus silvestris L. The catastrophic 1987 Val Pola rock avalanche (Crosta et al., 2004; 2006) also affected the Val Mala slope, with the debris mass falling from the Val Pola slope running up on the opposite valley flank, damaging the tree layer. 2.1. The 2001 Val Mala avalanche On 9th January 2001, a portion of snowpack with a volume of 1,045,519–1,151,843 m 3 (Ciucci, 2005) (Fig. 2) detached at an altitude of between 2900 m and 2450 m a.s.l. With the snow avalanche triggered by exceptional snowfalls which characterized the winter of 2000–2001, the detached snow slab fragmented in several parts due to the steep slope (~35°). The snow avalanche thus evolved into a mixed type with both hydrodynamic and aerodynamic characteristics (Ciucci, 2005) (Fig. 2). The two components separated at 1700 m a.s.l., where the Mala torrent changes its flow direction from W-NW to W-SW. The snow powder avalanche damaged the forest on the left bank, whereas the slab avalanche ran along the stream path, affecting the tree layer on the right bank. Norway spruces and European larches were swept away by the extraordinary avalanche with, according to Ciucci (2005), thousands of trees felled. Seedlings have now begun to colonize the avalanche flow
Fig. 1. Panoramic view of the study area, taken on the 17th January 2001 after the snow avalanche events. The valley bottom appears bare and without vegetation as a result of the 1987 Val Pola rock avalanche (photograph kindly provided by L. Bonetti). The locations of Val Pola and Val Mala are shown in the square at upper left.
V. Garavaglia, M. Pelfini / Catena 87 (2011) 209–215
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Fig. 2. Detachment area, snow-powder and snow-flow avalanche tracks. The photograph at bottom right represents a detail of the area in the rectangle, after tree layer removal. The black circle corresponds to the location of the village of S. Antonio Morignone before the 1987 rock avalanche. The table contains quantitative information regarding the avalanche's spatial extent.
path, with the few Norway spruces that survived the event widespread on the slope (Ciucci, 2005). 3. Material and methods After the 2001 snow avalanche, a total of 17 cross-sections were collected from fallen Norway spruces distributed on the slope. These samples represent the last remaining evidence of the forest destroyed by the avalanche, since all available fallen trunks were removed in the years following the event. During the summers of 2008 and 2009, 71 P. abies L. Karst were sampled along the 2001 snow avalanche track. Using an increment borer, at least two cores (5 mm in diameter) were extracted from each tree. Seventeen of the sampled Norway spruces had no evident damage, in part belonging to the right border area which was not directly affected by the slab avalanche (but which was impacted by the snow powder avalanche). All increment cores and cross-sections were prepared using standard methods (Schweingruber, 1988; 1996; Stokes and Smiley, 1968).
Growth curves were constructed using the LINTAB–TSAP system and Windendro software (Régent Instruments Inc., 2001; Rinn, 1996). Five sampled trees were excluded from the analysis because of sample fragmentation or bad wood conservation. At least four growth curves were drawn from each cross-section, selecting perpendicular radii. The trees felled during the 2001 snow avalanche were subsequently found dispersed on the slope, so the original valley side home of each specimen was identified on the basis of ring width, which is generally larger downslope if the average slope inclination is high (~35° in this study area). Cross-dating of the dendrochronological series was performed using COFECHA (statistical analysis) (Holmes et al., 1986) and TSAP software (visual analysis) (Rinn, 1996). A reference chronology built from P. abies L. Karst from Upper Valtellina (unpublished data) was used for cross-dating. In the absence of the pith, the lacking rings were estimated based on the method suggested by Villalba and Veblen (1997). Detailed analysis was carried out regarding the presence of i) scars exposed or hidden by bark (Johnson, 1968), ii) reaction wood (Carrara, 1979), iii) traumatic resin canals (Cherubini et al., 1997) and
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iv) variation in stem eccentricity (Casteller et al., 2007). Variation in stem eccentricity occurring after the 2001 snow avalanche was evaluated using the eccentricity index (EI) (Casteller et al., 2007): ∑1983–1986 ¼ ∑ trw years 1997–2000 dsr=∑ trw years 1997–2000 usr ð1Þ ∑1987–1990 ¼ ∑ trw years 2001–2004 dsr=∑ trw years 2001–2004 usr ð2Þ ΔEI% ¼ 100⁎ðEI2001–2004 −EI1997–2000 Þ=EI1997–2000
ð3Þ
where trw is tree ring width, drs is downslope radius and usr upslope radius. The time intervals 1997–2000 and 2001–2004 represent the
time frames before and after the considered landslide. With the aim of identifying different degrees of eccentricity variation, three thresholds for classifying weak, medium and strong EI variations (40%, 55% and 70%) (Rolland et al., 2001; Schweingruber et al., 1991) were applied. 4. Results 4.1. Tree ages Sampled trees cover the time interval between 1867 and 2007 and the cross-sections between 1911 and 2001. Cross sections are generally older (81–90 years old) than sampled trees, which majority is 41–50 years old and 51–60 years old (Fig. 3). Nevertheless, the age
Fig. 3. Spatial distribution of sampled trees based on age class. The allotment of trees in terms of age class is also represented in the graph.
V. Garavaglia, M. Pelfini / Catena 87 (2011) 209–215 Table 1 Distribution of dated anomalies in increment cores and cross sections. CW is the abbreviation of compression wood and TRD is traumatic resin duct.
CW TRD Scars Tot
Sampled trees
Cross sections
Tot
637 (84%) 86 7 730
124 (16%) – – 124
761 86 7 854
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4.2. Scars A total of seven scars were detected, dating to 1999, 2001, 2002, 2003 and 2007. These scars occur in the increment cores belonging to trees located in the accumulation area. However since in the years 1999, 2002, 2003 and 2007 at least two trees exhibited wounds, it is therefore not possible to associate them with snow avalanche events. Moreover, no scars are associated with the run up of the 1987 Val Pola rock avalanche (Crosta et al., 2004; 2006). 4.3. Compression wood and traumatic resin ducts
Fig. 4. Temporal variation in the percentage of trees exhibiting compression wood (CW) in sampled Norway Spruces and cross-sections. The black bars represent the number of sampled Norway Spruces with traumatic resin ducts.
of the trees is not statistically different in the two groups (t-test, p N 0.05), suggesting that in the past (since ~ 160, considering the age of the oldest studied tree) similarly disruptive avalanches, able to remove the entire forest layer, did not occur in Val Mala. Tree-age spatial distribution does not suggest any particular tree clustering (Fig. 3), although a major concentration of 17–37 year-old trees does appear in the inner part of the avalanche path, near the route of the Mala torrent. This therefore seems to be a recurrent avalanche path along which vegetation has not been removed by avalanches at an altitude of 1300 m a.s.l. for 17–37 years.
A total of 854 growth anomalies were identified and dated (Table 1). Their presence in the sampled trees is largely restricted to the more extended time interval covered by the increment cores rather than the cross-sections. In fact, cross-section size impeded accurate polishing, causing problems with the identification of traumatic resin canals (TRD). The concentration of compression wood (CW) evidently increases from 2001, occurring in at least 40% of trees in this year. This percentage is maintained over the following years, with a slight decrease starting from 2005 (Fig. 4). The effect of the 2001 snow avalanche is clearly detectable by the presence of compression wood, with no more than 7 sampled trees showing TRDs in other years. The spatial distribution of reaction wood presented in Fig. 5 shows an increase of this growth anomaly in the right border area in 2001, indicating that the adjacent forest was impacted by the avalanche. From these results, compression wood can be said to be the growth anomaly that better expresses the reaction of the forest layer to the snow avalanche and therefore its distribution more useful in delineating the event's extent. 4.4. Variation in stem eccentricity Changes in stem eccentricity due to avalanche impact are expected along its track, but their presence may potentially also extend to border areas. Indeed, the presence of reaction wood suggests that snow-flow effects did impact vegetation adjacent to the avalanche track.
Fig. 5. Spatial distribution of trees containing compression wood in 2000 and 2001 (black circles). The increase in the number of trees with reaction wood is evident.
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either to the 2001 snow avalanche or to trunk size. As suggested by Casteller et al. (2007), after avalanche impact, older trees with larger stem circumferences show less eccentricity variations than younger, smaller trees. In this study, no more than 5 trees with a circumference of 25–69 cm present changes in stem eccentricity, while trees belonging to the other selected circumference classes do not present eccentricity variations in 2001 (Table 2). This suggests that variation in stem eccentricity is not associated with tree circumference. With regards to the cross-section data, there are no particular concentrations of trees exhibiting variations in stem eccentricity during the covered period: no evidence is present of events or processes affecting stability preceding the 2001 avalanche (Fig. 6). Taking stem eccentricity to be an indicator of the 2001 snow avalanche, no evident tree reaction is detectable.
5. Discussion
Fig. 6. Percentage of trees and cross-sections exhibiting variation in stem eccentricity. EI represents the Eccentricity Index, the dashed bar the year 2001.
Table 2 Variations in stem eccentricity in trees with different circumferences. Few trees in each class present high values of EI: it seems that there is not a relation between eccentricity variation and trunk size. cfr (cm)
No. of trees
No. of trees with variations in stem eccentricity in 2001
25–69 69.1–105 105.1–161 N161 Tot
19 15 16 16 66
5 (26%) 3 (20%) 3 (19%) 6 (18%) 17
Almost 40% of Norway Spruces present stem modifications, starting in 1999 but also in 2000 (Fig. 6), with another major concentration in 2003. These variations in stem eccentricity do not seem to be related
The tree-ring analysis carried out in this work enabled the identification of a reduction in tree age near the Mala torrent, evidencing a probable ancient avalanche path aligned with the route of torrent flow. Moreover, the age of sampled trees and cross-sections shows that processes similar to the 2001 event have not occurred during the last ~160 years. The presence of reaction wood is a growth anomaly typically used in dating snow avalanches (Butler and Sawyer, 2008), but is also produced by a variety of processes including soil creep, slumps, wind and snow creep (Carrara, 1979). In this study, the strong increase in compression wood, starting in 2001, is likely related to the avalanche of that year, confirming the supposed impact of the generated air displacement. Moreover, the simultaneous presence of CW in trees in border areas shows the impact of snow avalanches even on apparently undisturbed trees. Traumatic resin ducts are generally considered useful indicators of past avalanches (Cherubini et al., 1997) but in this study a reduced number of TRDs was dated in the increment cores, excluding them from acting as good indicators of the 2001 avalanche. In contrast, variations in stem eccentricity are not related to tree circumference: in theory younger trees should react to tilting and trunk eccentricity more easily with respect to older specimens. Nevertheless,
Fig. 7. Spatial distribution of trees with compression wood and variation in stem eccentricity in 2001 and 2002. Few trees present both anomalies (black circles), suggesting that the detected stem variations are not directly related to avalanche occurrence.
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in this study no appreciable dominance of young trees exhibiting stem eccentricity with respect to older plants was observed. Variation in stem eccentricity may also result from irregular distribution of nutrients and/ or from uneven development of crown and roots, but in these cases, they are generally not accompanied by reaction wood formation (Braam et al., 1987). Moreover, the absence of correlation between the occurrence of compression wood and changes in stem eccentricity in both 2001 and 2002 (Fig. 7) confirms that, in this case, stem eccentricity is unlikely to be a strong indicator of avalanche activity. Historical archives record two avalanches taking place in 1917 and 1951, but no evidence of these are found in the sampled trees. It is possible that the two events affected reduced areas at higher altitudes, or that their magnitude was insufficient to involve the sampled areas. Such a non-destructive character of past avalanches could also explain the reduced number of traumatic resin canals observed. 6. Conclusions When applied to snow avalanches, tree rings are a useful instrument, with analysis of damaged trees (see for example Corona et al., 2010; Germain et al., 2009) potentially revealing the year of event occurrence. Applying such a dendrochronological approach to the 2001 Val Mala snow avalanche enabled information to be collected where data were absent due to the removal of the majority of trees. The use of crosssections provided data regarding forest conditions before the 2001 avalanche took place, while growth anomalies from surviving Norway Spruces highlighted the reaction to this event. Positive results from apparently undamaged trees adjacent to the avalanche path highlights the opportunity for exploring new sampling strategies that involve only border areas, solving the problem of the absence of damaged trees on the flow path or accumulation area. Analysis of compression wood in ‘undisturbed’ trees confirmed the usefulness of this indicator, although it appears that variation in stem eccentricity must be combined with other growth anomalies if it is to be employed in future investigations. Despite being a restricted study area and the 2001 event certainly a local case, Val Mala represents a good research site and a solid example for the selection of sampling areas in snow avalanche investigation; i.e. an approach permitting the inclusion of apparently undamaged border areas in dendrogeomorphological analysis. Trees in marginal areas thus become a precious source of information with which to analyze catastrophic events that destroyed forests and erased other information associated with previous avalanches. The impact of the 2001 avalanche on border trees presented in this study demonstrates the possibility of applying dendrochronology to recurring avalanche paths where the high frequency of such events impedes colonization by tree vegetation. Considering the increasing amount of human activity taking place in mountain environments, it makes sense for snow avalanche processes to be monitored and investigated more thoroughly. The usefulness of tree rings in dating past events using damaged trees has been demonstrated in several case studies (e.g. Casteller et al., 2007; Corona et al., 2010). However, this study has shown the potential of new sampling areas, opening up the opportunity of also investigating adjacent forests. This method may permit the detailed mapping of avalanche paths, allowing the collection of information even if damaged woody vegetation is not available. As suggested by Corona et al. (2010: 117), “trying to identify anatomical differences related to dense and powder avalanches” may be the next step in improving these results and providing a more precise snow avalanche reconstruction. Acknowledgments The Authors thank the Stelvio National Park for permitting the samples collection, L. Bonetti for the assistance during the preliminary phases of the fieldwork and S. Lucarelli for taking part in this investigation.
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This research was funded by the PRIN 2008 project ‘Climate change effects on glaciers, permafrost and derived water resource. Quantification of the ongoing variations in the Italian Alps, analysis of their impacts and modelling future projections’, national coordinator Prof. C. Smiraglia. References Bezzi, M., Ciolli, M., Comunello, G., Cherubini, P., Cantiani, M.G., 2004. Integration of dendrochronology and GIS techniques to study avalanche phenomena. Geomatics Workbooks 3. Available at: http://geomatica.como.polimi.it/workbooks/n3/articoli/ mcmbmgcpcgc.pdf. Braam, R.R., Weiss, E.E.J., Burrough, P.A., 1987. Spatial and temporal analysis of mass movement using dendrochronology. Catena 14, 573–584. Bründl, M., Etter, H.J., Steiniger, M., Klinger, C., Rhyner, J., Ammann, W.J., 2004. IFKIS—a basis for managing avalanche risk in settlements and on roads in Switzerland. Nat. Hazard Earth Sys. 4, 257–262. Butler, D.R., Sawyer, C.F., 2008. Dendrogeomorphology and high-magnitude snow avalanches: a review and case study. Nat. Hazard Earth Sys. 8, 303–309. Carrara, P.E., 1979. The determination of snow avalanche frequency through tree-ring analysis and historical records at Ophir, Colorado. Geological Society of America Bulletin 90, 773–780. Casteller, A., Stöckli, V., Villalba, R., Mayer, A.C., 2007. An evaluation of dendroecological indicators of snow avalanches in the Swiss Alps. Arctic, Antarctic, and Alpine Research 39 (2), 218–228. Cherubini, P., Schweingruber, F.H., Forster, T., 1997. Morphology and ecological significance of intra-annual radial cracks in living conifers. Trees Structure and Function 11, 216–222. Ciucci, E., 2005. Dinamica ed effetti della valanga 2001 in Val Mala (Valtellina, So), analisi delle condizioni climatiche e nivologiche. Master Thesis., University of Milan, Milan. Corona, C., Rovera, G., Lopez Saez, J., Stoffel, M., Perfettini, P., 2010. 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Contents lists available at ScienceDirect
Catena j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c a t e n a
The role of border areas for dendrochronological investigations on catastrophic snow avalanches: A case study from the Italian Alps Valentina Garavaglia ⁎, Manuela Pelfini Università degli Studi di Milano, Dipartimento di Scienze della Terra “Ardito Desio”, via Mangiagalli 34, 20133 Milano, Italy
a r t i c l e
i n f o
Article history: Received 22 February 2010 Received in revised form 1 June 2011 Accepted 3 June 2011 Keywords: Snow avalanches Tree rings Dendrogeomorphology Italian Alps
a b s t r a c t Snow avalanches are common in mountain environments, sometimes affecting inhabited areas and infrastructures (e.g. roads, bridges, and ski slopes). Their study is widespread and involves the use of a variety of different techniques, including dendrochronological methods. The aim of such investigations is principally to date past events, but also to detect variation in avalanche frequency as well as their spatial distribution. Trees affected by snow avalanches generally have scars, tilted trunks and broken branches, which allow past events to be dated to within a year. In this work tree rings were used to investigate a disruptive snow avalanche which occurred in 2001 in Val Mala in the Italian Alps. The event almost completely removed forest along the flow path, while the snow powder component of the avalanche also impacted on the adjacent forest. Comparison of tree reaction in surviving plants along the flow path and vegetation on the border showed i) the production of reaction wood even in apparently undisturbed trees and ii) the usefulness of border plants for dating past events. Different dendroecological indicators were investigated (i.e. reaction wood, scars, traumatic resin ducts, and variations in stem eccentricity) and reaction wood is evidently concentrated from 2001 not only in damaged trees but also in adjacent plants. The potential to investigate past snow avalanches by collecting samples from trees growing in border areas is presented: in the specific case of Val Mala, vegetation bordering the flow paths reacted to produce reaction wood in 2001, in a similar manner to plants along the flow track, demonstrating their usefulness in such investigations. Their response to the 2001 event confirms the possibility of applying dendrochronological techniques to trees adjacent to the flow path, even if they appear morphologically undisturbed, and proposes to use only border areas in cases of an absence of sufficient trees along the avalanche track. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Snow avalanches are widespread processes in Alpine environments (Strunk, 1991), commonly affecting valley slopes. Often they occur in areas frequented by tourists or habited regions, damaging constructions such as roads and ski slopes and sometimes claiming lives (Bründl et al., 2004; Casteller et al., 2007). Reconstructing their frequency and spatial distribution represents an important phase of risk mitigation and useful for landscape planning. Moreover, in the last decade, snow avalanches have been investigated more intensively because of their relationship with climate change: they are triggered by snow cover instability that is determined by weather conditions and, like other physical systems such as, for example, permafrost, are sensitive to climatic variation (Germain et al., 2009). Tree rings represent a good proxy data for reconstructing past avalanche frequency, as well as for estimating their frequency variation related to climate change. Not only do they provide information ⁎ Corresponding author at: Dipartimento di Scienze della Terra “A. Desio”, via Mangiagalli 34, Milano 20133, Italy. Tel.: + 39 0250315514; fax: + 39 02 50315494. E-mail address: [email protected] (V. Garavaglia). 0341-8162/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2011.06.006
at an annual resolution, but also the possibility to study both the temporal distribution of these slope processes and their spatial occurrence (Reardon et al., 2008), thus obviating the problem of fragmentary historical archives. Avalanche paths are easily identifiable using aerial photographs since they generally form stripes parallel to the slope, characterized by lower and younger tree vegetation with respect to the adjacent forest (Germain et al., 2009; Pelfini and Orombelli, 1997). Nevertheless, in the case of catastrophic events the removal of the entire tree layer tends to impede the collection of dendrochronological data, even if the presence of seedlings may enable the study of slope recolonization (Garbarino et al., 2010; McCarthy and Luckman, 1993; Pelfini and Orombelli, 1997). When applied to snow avalanches, dendrochronological dating is based generally on the presence, along avalanche paths, of tilted trees, damaged branches and wounded stems (Bezzi et al., 2004; Carrara, 1979; Pelfini et al., 2001; Strunk, 1993), i.e. morphologies associated with anatomical features that can be detected and accurately dated in tree rings. For example, reaction wood (compression wood in conifers and tension wood in broadleaves) is a growth anomaly typically used in the dating of snow avalanches (Butler and Sawyer, 2008).
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In this paper the impact of a catastrophic avalanche which occurred in the Central Italian Alps is investigated, on the basis of i) samples collected from surviving trees on the running area and ii) apparently undisturbed trees in the border areas, where the snow powder component of the avalanche impacted on vegetation but left no characteristic morphologies. Growth anomalies in undisturbed trees in border areas were studied in more detail, including: traumatic resin ducts, reaction wood, stem eccentricity, callus tissue and wounds. The aim of this study is to explore the possibility of studying avalanches using only trees in forests adjacent to the avalanche path. 2. Study area Located in Stelvio National Park, Upper Valtellina (Central Italian Alps) (Fig. 1), Val Mala rises from 1050 m a.s.l. at the village of Morignone (destroyed by the 1987 Val Pola rock avalanche (Crosta et al., 2004)) to Mount Mala, at an altitude of 2943 m a.s.l. Val Mala was shaped by glacial and gravitational processes, although at present the Mala torrent and cryoclastic processes are the two principal factors acting on the valley (Pozzi et al., 1990). On the valley bottom a large fan formed by debris flow and avalanche deposits is present. A tributary of the Adda River, the Mala torrent originates from Mount Mala flowing west-northwest, but at 1700 m a.s.l. it turns to west-
southwest. The entire slope is covered by a well developed forest dominated by Picea abies L. Karst., with a reduced component of Larix decidua Mill and Pinus silvestris L. The catastrophic 1987 Val Pola rock avalanche (Crosta et al., 2004; 2006) also affected the Val Mala slope, with the debris mass falling from the Val Pola slope running up on the opposite valley flank, damaging the tree layer. 2.1. The 2001 Val Mala avalanche On 9th January 2001, a portion of snowpack with a volume of 1,045,519–1,151,843 m 3 (Ciucci, 2005) (Fig. 2) detached at an altitude of between 2900 m and 2450 m a.s.l. With the snow avalanche triggered by exceptional snowfalls which characterized the winter of 2000–2001, the detached snow slab fragmented in several parts due to the steep slope (~35°). The snow avalanche thus evolved into a mixed type with both hydrodynamic and aerodynamic characteristics (Ciucci, 2005) (Fig. 2). The two components separated at 1700 m a.s.l., where the Mala torrent changes its flow direction from W-NW to W-SW. The snow powder avalanche damaged the forest on the left bank, whereas the slab avalanche ran along the stream path, affecting the tree layer on the right bank. Norway spruces and European larches were swept away by the extraordinary avalanche with, according to Ciucci (2005), thousands of trees felled. Seedlings have now begun to colonize the avalanche flow
Fig. 1. Panoramic view of the study area, taken on the 17th January 2001 after the snow avalanche events. The valley bottom appears bare and without vegetation as a result of the 1987 Val Pola rock avalanche (photograph kindly provided by L. Bonetti). The locations of Val Pola and Val Mala are shown in the square at upper left.
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Fig. 2. Detachment area, snow-powder and snow-flow avalanche tracks. The photograph at bottom right represents a detail of the area in the rectangle, after tree layer removal. The black circle corresponds to the location of the village of S. Antonio Morignone before the 1987 rock avalanche. The table contains quantitative information regarding the avalanche's spatial extent.
path, with the few Norway spruces that survived the event widespread on the slope (Ciucci, 2005). 3. Material and methods After the 2001 snow avalanche, a total of 17 cross-sections were collected from fallen Norway spruces distributed on the slope. These samples represent the last remaining evidence of the forest destroyed by the avalanche, since all available fallen trunks were removed in the years following the event. During the summers of 2008 and 2009, 71 P. abies L. Karst were sampled along the 2001 snow avalanche track. Using an increment borer, at least two cores (5 mm in diameter) were extracted from each tree. Seventeen of the sampled Norway spruces had no evident damage, in part belonging to the right border area which was not directly affected by the slab avalanche (but which was impacted by the snow powder avalanche). All increment cores and cross-sections were prepared using standard methods (Schweingruber, 1988; 1996; Stokes and Smiley, 1968).
Growth curves were constructed using the LINTAB–TSAP system and Windendro software (Régent Instruments Inc., 2001; Rinn, 1996). Five sampled trees were excluded from the analysis because of sample fragmentation or bad wood conservation. At least four growth curves were drawn from each cross-section, selecting perpendicular radii. The trees felled during the 2001 snow avalanche were subsequently found dispersed on the slope, so the original valley side home of each specimen was identified on the basis of ring width, which is generally larger downslope if the average slope inclination is high (~35° in this study area). Cross-dating of the dendrochronological series was performed using COFECHA (statistical analysis) (Holmes et al., 1986) and TSAP software (visual analysis) (Rinn, 1996). A reference chronology built from P. abies L. Karst from Upper Valtellina (unpublished data) was used for cross-dating. In the absence of the pith, the lacking rings were estimated based on the method suggested by Villalba and Veblen (1997). Detailed analysis was carried out regarding the presence of i) scars exposed or hidden by bark (Johnson, 1968), ii) reaction wood (Carrara, 1979), iii) traumatic resin canals (Cherubini et al., 1997) and
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iv) variation in stem eccentricity (Casteller et al., 2007). Variation in stem eccentricity occurring after the 2001 snow avalanche was evaluated using the eccentricity index (EI) (Casteller et al., 2007): ∑1983–1986 ¼ ∑ trw years 1997–2000 dsr=∑ trw years 1997–2000 usr ð1Þ ∑1987–1990 ¼ ∑ trw years 2001–2004 dsr=∑ trw years 2001–2004 usr ð2Þ ΔEI% ¼ 100⁎ðEI2001–2004 −EI1997–2000 Þ=EI1997–2000
ð3Þ
where trw is tree ring width, drs is downslope radius and usr upslope radius. The time intervals 1997–2000 and 2001–2004 represent the
time frames before and after the considered landslide. With the aim of identifying different degrees of eccentricity variation, three thresholds for classifying weak, medium and strong EI variations (40%, 55% and 70%) (Rolland et al., 2001; Schweingruber et al., 1991) were applied. 4. Results 4.1. Tree ages Sampled trees cover the time interval between 1867 and 2007 and the cross-sections between 1911 and 2001. Cross sections are generally older (81–90 years old) than sampled trees, which majority is 41–50 years old and 51–60 years old (Fig. 3). Nevertheless, the age
Fig. 3. Spatial distribution of sampled trees based on age class. The allotment of trees in terms of age class is also represented in the graph.
V. Garavaglia, M. Pelfini / Catena 87 (2011) 209–215 Table 1 Distribution of dated anomalies in increment cores and cross sections. CW is the abbreviation of compression wood and TRD is traumatic resin duct.
CW TRD Scars Tot
Sampled trees
Cross sections
Tot
637 (84%) 86 7 730
124 (16%) – – 124
761 86 7 854
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4.2. Scars A total of seven scars were detected, dating to 1999, 2001, 2002, 2003 and 2007. These scars occur in the increment cores belonging to trees located in the accumulation area. However since in the years 1999, 2002, 2003 and 2007 at least two trees exhibited wounds, it is therefore not possible to associate them with snow avalanche events. Moreover, no scars are associated with the run up of the 1987 Val Pola rock avalanche (Crosta et al., 2004; 2006). 4.3. Compression wood and traumatic resin ducts
Fig. 4. Temporal variation in the percentage of trees exhibiting compression wood (CW) in sampled Norway Spruces and cross-sections. The black bars represent the number of sampled Norway Spruces with traumatic resin ducts.
of the trees is not statistically different in the two groups (t-test, p N 0.05), suggesting that in the past (since ~ 160, considering the age of the oldest studied tree) similarly disruptive avalanches, able to remove the entire forest layer, did not occur in Val Mala. Tree-age spatial distribution does not suggest any particular tree clustering (Fig. 3), although a major concentration of 17–37 year-old trees does appear in the inner part of the avalanche path, near the route of the Mala torrent. This therefore seems to be a recurrent avalanche path along which vegetation has not been removed by avalanches at an altitude of 1300 m a.s.l. for 17–37 years.
A total of 854 growth anomalies were identified and dated (Table 1). Their presence in the sampled trees is largely restricted to the more extended time interval covered by the increment cores rather than the cross-sections. In fact, cross-section size impeded accurate polishing, causing problems with the identification of traumatic resin canals (TRD). The concentration of compression wood (CW) evidently increases from 2001, occurring in at least 40% of trees in this year. This percentage is maintained over the following years, with a slight decrease starting from 2005 (Fig. 4). The effect of the 2001 snow avalanche is clearly detectable by the presence of compression wood, with no more than 7 sampled trees showing TRDs in other years. The spatial distribution of reaction wood presented in Fig. 5 shows an increase of this growth anomaly in the right border area in 2001, indicating that the adjacent forest was impacted by the avalanche. From these results, compression wood can be said to be the growth anomaly that better expresses the reaction of the forest layer to the snow avalanche and therefore its distribution more useful in delineating the event's extent. 4.4. Variation in stem eccentricity Changes in stem eccentricity due to avalanche impact are expected along its track, but their presence may potentially also extend to border areas. Indeed, the presence of reaction wood suggests that snow-flow effects did impact vegetation adjacent to the avalanche track.
Fig. 5. Spatial distribution of trees containing compression wood in 2000 and 2001 (black circles). The increase in the number of trees with reaction wood is evident.
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either to the 2001 snow avalanche or to trunk size. As suggested by Casteller et al. (2007), after avalanche impact, older trees with larger stem circumferences show less eccentricity variations than younger, smaller trees. In this study, no more than 5 trees with a circumference of 25–69 cm present changes in stem eccentricity, while trees belonging to the other selected circumference classes do not present eccentricity variations in 2001 (Table 2). This suggests that variation in stem eccentricity is not associated with tree circumference. With regards to the cross-section data, there are no particular concentrations of trees exhibiting variations in stem eccentricity during the covered period: no evidence is present of events or processes affecting stability preceding the 2001 avalanche (Fig. 6). Taking stem eccentricity to be an indicator of the 2001 snow avalanche, no evident tree reaction is detectable.
5. Discussion
Fig. 6. Percentage of trees and cross-sections exhibiting variation in stem eccentricity. EI represents the Eccentricity Index, the dashed bar the year 2001.
Table 2 Variations in stem eccentricity in trees with different circumferences. Few trees in each class present high values of EI: it seems that there is not a relation between eccentricity variation and trunk size. cfr (cm)
No. of trees
No. of trees with variations in stem eccentricity in 2001
25–69 69.1–105 105.1–161 N161 Tot
19 15 16 16 66
5 (26%) 3 (20%) 3 (19%) 6 (18%) 17
Almost 40% of Norway Spruces present stem modifications, starting in 1999 but also in 2000 (Fig. 6), with another major concentration in 2003. These variations in stem eccentricity do not seem to be related
The tree-ring analysis carried out in this work enabled the identification of a reduction in tree age near the Mala torrent, evidencing a probable ancient avalanche path aligned with the route of torrent flow. Moreover, the age of sampled trees and cross-sections shows that processes similar to the 2001 event have not occurred during the last ~160 years. The presence of reaction wood is a growth anomaly typically used in dating snow avalanches (Butler and Sawyer, 2008), but is also produced by a variety of processes including soil creep, slumps, wind and snow creep (Carrara, 1979). In this study, the strong increase in compression wood, starting in 2001, is likely related to the avalanche of that year, confirming the supposed impact of the generated air displacement. Moreover, the simultaneous presence of CW in trees in border areas shows the impact of snow avalanches even on apparently undisturbed trees. Traumatic resin ducts are generally considered useful indicators of past avalanches (Cherubini et al., 1997) but in this study a reduced number of TRDs was dated in the increment cores, excluding them from acting as good indicators of the 2001 avalanche. In contrast, variations in stem eccentricity are not related to tree circumference: in theory younger trees should react to tilting and trunk eccentricity more easily with respect to older specimens. Nevertheless,
Fig. 7. Spatial distribution of trees with compression wood and variation in stem eccentricity in 2001 and 2002. Few trees present both anomalies (black circles), suggesting that the detected stem variations are not directly related to avalanche occurrence.
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in this study no appreciable dominance of young trees exhibiting stem eccentricity with respect to older plants was observed. Variation in stem eccentricity may also result from irregular distribution of nutrients and/ or from uneven development of crown and roots, but in these cases, they are generally not accompanied by reaction wood formation (Braam et al., 1987). Moreover, the absence of correlation between the occurrence of compression wood and changes in stem eccentricity in both 2001 and 2002 (Fig. 7) confirms that, in this case, stem eccentricity is unlikely to be a strong indicator of avalanche activity. Historical archives record two avalanches taking place in 1917 and 1951, but no evidence of these are found in the sampled trees. It is possible that the two events affected reduced areas at higher altitudes, or that their magnitude was insufficient to involve the sampled areas. Such a non-destructive character of past avalanches could also explain the reduced number of traumatic resin canals observed. 6. Conclusions When applied to snow avalanches, tree rings are a useful instrument, with analysis of damaged trees (see for example Corona et al., 2010; Germain et al., 2009) potentially revealing the year of event occurrence. Applying such a dendrochronological approach to the 2001 Val Mala snow avalanche enabled information to be collected where data were absent due to the removal of the majority of trees. The use of crosssections provided data regarding forest conditions before the 2001 avalanche took place, while growth anomalies from surviving Norway Spruces highlighted the reaction to this event. Positive results from apparently undamaged trees adjacent to the avalanche path highlights the opportunity for exploring new sampling strategies that involve only border areas, solving the problem of the absence of damaged trees on the flow path or accumulation area. Analysis of compression wood in ‘undisturbed’ trees confirmed the usefulness of this indicator, although it appears that variation in stem eccentricity must be combined with other growth anomalies if it is to be employed in future investigations. Despite being a restricted study area and the 2001 event certainly a local case, Val Mala represents a good research site and a solid example for the selection of sampling areas in snow avalanche investigation; i.e. an approach permitting the inclusion of apparently undamaged border areas in dendrogeomorphological analysis. Trees in marginal areas thus become a precious source of information with which to analyze catastrophic events that destroyed forests and erased other information associated with previous avalanches. The impact of the 2001 avalanche on border trees presented in this study demonstrates the possibility of applying dendrochronology to recurring avalanche paths where the high frequency of such events impedes colonization by tree vegetation. Considering the increasing amount of human activity taking place in mountain environments, it makes sense for snow avalanche processes to be monitored and investigated more thoroughly. The usefulness of tree rings in dating past events using damaged trees has been demonstrated in several case studies (e.g. Casteller et al., 2007; Corona et al., 2010). However, this study has shown the potential of new sampling areas, opening up the opportunity of also investigating adjacent forests. This method may permit the detailed mapping of avalanche paths, allowing the collection of information even if damaged woody vegetation is not available. As suggested by Corona et al. (2010: 117), “trying to identify anatomical differences related to dense and powder avalanches” may be the next step in improving these results and providing a more precise snow avalanche reconstruction. Acknowledgments The Authors thank the Stelvio National Park for permitting the samples collection, L. Bonetti for the assistance during the preliminary phases of the fieldwork and S. Lucarelli for taking part in this investigation.
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