Dendrogeomorphic Analysis Research Articles

Chronic rockfall in abandoned quarry: Chronology, modelling and implications

Abandoned quarries are a relatively common form of anthropogenic relief. The rocks disturbed by previous mining activities could foster conditions favourable for future dynamic geomorphic processes on quarry faces. Nevertheless, the activity of such processes, particularly that of rockfall, has not yet been studied in detail. This is of particular importance because human infrastructure is often still present in abandoned quarries, and rockfall can cause distinct natural hazards in such conditions. This study focuses on a multidisciplinary analysis of rockfall in selected abandoned quarry in the region of the Carpathians. A dendrogeomorphic analysis of 807 tree-ring series from 130 disturbed trees occupying the talus provided evidence of 264 tree injuries caused by falling rocks during the last 22 years. The reconstructed frequency of rockfall in quarries is several times higher than that of rockfall from natural localities within the studied region but also in Europe. Next, the seasonal dating of rockfall hits to trees displayed the prominence of such events during the tree dormancy period (ca. October to April), suggesting freeze–thaw cycles as the main trigger (the same as for natural localities in the region). Three species of broad-leaved trees of comparable age were used for the reconstruction, but no differences among the species were detected. The number of rockfall hits on tree trunks decreased significantly with increasing distance from the rockfall source zone, suggesting strong gradation of rockfall activity, compared to natural localities. Generally, the multidisciplinary results suggest very unstable conditions of rocks in abandoned quarries, predisposing them to frequent gravitational events. In contrast, ecological succession after mining termination gradually offers a protective function of trees.

Exploring mining-induced subsidence effects on Alnus glutinosa (L.) Gaertn. through dendrogeomorphic analysis and mining data

Mining operations can have a significant impact on the environment, which may result in surface lowering due to overburden disturbance, commonly referred to as mining subsidence. This issue has been extensively studied in many engineering studies. However, the dendrogeomorphic approach can provide additional insights into this issue. This study examined the tree-ring response of black alder (Alnus glutinosa (L.) Gaertn.) to subsidence. The analysis of subsidence activity was exclusively conducted through tree-ring eccentricity (EC). Using the Knothe method, the theoretical radius of main subsidence influence range was determined to differentiate between periods of mining influence and non-mining influence. Significantly higher event-response (It) index (p = 0.0001) was calculated during years with mining influence. Nevertheless, the use of the It index alone is insufficient for reconstructing the subsidence events as the process is continuous. When comparing the recorded reactions of the onset of EC across different subsidence rates from 2012 to 2015, no distinctive differences were found in tree growth response. The use of dendrogeomorphology and tree-ring eccentricity has proven valuable in studying the subsidence process. Nonetheless, new challenges persist, particularly regarding secondary processes like landslide activity or residual subsidence.

Anatomical growth response of Fagus sylvatica L. to landslide movements

Determining the age of landslide events is crucial for determining landslide risk, triggers, and also for predicting future landslide occurrence. Currently, the most accurate method for dating historical landslide events is dendrogeomorphic analysis. Unfortunately, the standard use of macroscopic growth responses of damaged trees for dating landslide activity suffers from many shortcomings. Thus, the aim of this study is to analyze in detail the growth response of trees to landslide movements at the anatomical level, a completely groundbreaking methodological approach. Ten specimens of European beech (Fagus sylvatica L.) were analyzed at two sampling heights, growing in two morphologically contrasting zones of the landslide area. Detailed anatomical analysis was focused on changes in morphometric parameters of the vessels and in the number of radial rays. The period (2008–2012) with the occurrence of the largest landslide movement (2010) recorded by long-term monitoring was analyzed. The results obtained revealed different anatomical responses in trees growing in different morphological zones of landslide. The tree responses on the ridge corresponded to the manifestations of tension wood formation, which corresponded to the stem tilting due to the landslide block movement. In the case of the trees in the trenches, root damage due to the subsidence of the landslide block blocked the flux of phytohormones, and their accumulation caused a significant reduction in the parameters of vessels and an increase in the number of rays. The study also includes recommendations in the future application of anatomical analyses in landslide research resulting from the obtained results. Thus, the obtained findings will improve the acquisition of chronological data for the purpose of landslide risk assessment.

The tree-ring anatomy of Robinia pseudoacacia L. As a data source for dendrogeomorphic reconstruction of past landslide movements in the landslide hot-spot of central Europe

The chronology of past landslide movements is a key source of information for effective landslide hazard determination. Dendrogeomorphological methods are currently an effective tool for obtaining chronological data but still encounter limitations in extracting landslide signals from yearly series. These limitations can be reduced by introducing anatomical analyses of the yearly series. Black locust (Robinia pseudoacacia L.), a widespread tree species occupying landslides on river banks, has been surprisingly rare in dendrogeomorphology. Thus, this study focused on the effectiveness of using both standard macroscopic signals (tree-ring eccentricity) and anatomical growth responses of this species in a dendrogeomorphic analysis of landslide movements at one of the landslide hot spots in Central Europe (the Váh River bank). Analysis of the eccentricity of tree rings from 67 individuals of R. pseudoacacia allowed us to reconstruct 13 landslide events over the last 49 years. From an anatomical point of view, a strong association was found between a significant reduction in vessel lumen area and the eccentricity of the tree rings and the tilting of the tree stems. A possible cause of the anatomical response is a combination of root damage from subsurface landslide movements and occasional tension wood formation following tree tilting. The constructed chronology confirmed the known years of landslide activity and generally complemented the known chronology. One of the results of the study is the implication for effective evaluation of the spatial extent of landslide activity, considering tree-ring-based magnitude-frequency relationships. The results of this study will thus allow further extension of the use of dendrogeomorphological methods both methodologically and biogeographically.

Snow avalanches and debris flows in the Mediterranean conditions of the southern coast of the Crimean Mountains: Dendrogeomorphic reconstruction

The southern coast of the Crimean Mountains is affected by both debris flows and snow avalanches. However, there is no systematic record of these dangerous natural hazards. Therefore, this study is aimed at compiling a chronology of snow avalanches and debris flows at one of the most exposed sites on the southern coast of the Crimean Mountains. Dendrogeomorphic analysis of 310 individuals of black pine (Pinus nigra ssp. Pallasiana) revealed 639 growth disturbances induced by 55 debris flow events (since 1705) and three snow avalanche events (since 1959). A multidisciplinary approach combining precise intra-seasonal tree-ring dating, event-response index thresholds, geomorphological mapping, and testing of spatial patterns of disturbed trees was used to distinguish the two processes. This method has confirmed more than 85% of the known events and has thus proven to be highly effective and can thus be used to compile a catalogue of natural hazards in the region of interest. A limitation for dating snow avalanches was the inertia of growth disturbances induced by debris flows preceding avalanche events. Based on the analysis of meteorological data, possible triggers and predisposing factors for the occurrence of the events of both studied processes are also discussed.

Quantitative anatomy or macroscopic parameters of compression wood of Picea abies (L.) H. Karst.? Defining the optimal parameters for dendrogeomorphic purposes

Summary The quantification of the intensities of tree growth responses to the impact of geomorphic processes is a modern research trend in dendrogeomorphology. It enables a more sensitive assessment of the activity of the studied geomorphic process compared to the traditional use of growth disturbances. The advanced definitions of individual intensity classes of growth disturbances are based exclusively on macroscopic observations. This study evaluates the possibility of anatomical quantification of compression wood (CW) intensity in the case of common spruce (Picea abies (L.) Karst.) and compares it with subjective macroscopic evidence of CW with respect to stem tilting intensity. In total, 25 disturbed (tilted) individuals of P. abies occupying a landslide active during July 1997 were sampled, and 21 of them were analysed. The intensity of external disturbance (stem tilting) was compared against the macroscopic (intensity and duration) and microscopic (quantitative change of the tracheid lumen area and the cell wall proportion) parameters of compression wood suitable for practical application in common dendrogeomorphic analysis. Generally, the macroscopic indices of CW were strongly correlated with stem tilting. The intensity of the anatomical growth response was stronger in the earlywood zone than in the latewood zone. Nevertheless, their dependence on stem tilting was not detected. Results suggest that CW classification based on the quantification of anatomical changes is not possible for dendrogeomorphic purposes. Nevertheless, based on the obtained results, the present study suggests preferring the most intensively tilted trees during future dendrogeomorphic research to obtain the most intensive macroscopic and subjective observable anatomical evidence for studying compression wood.

Active geomorphic hazards in the Sâmbăta Valley, Făgăraș Mountains (Romania): A tree-ring based approach

The present study addresses, for the first time, the problem of spatio-temporal reconstruction of geomorphic processes using tree-rings in the Sâmbăta Valley (Romanian Carpathians). The dendrogeomorphic analysis was conducted in two different sites, one affected by snow avalanches and the other by rockfall. A total number of 130 Picea Abies were sampled in the two sites. The results yield 13 major snow avalanches between 1950 and 2020 and a return period of 3.3 years. The winters with the highest activity index were 1988, 1997 and 2012. The rockfall reconstruction highlights several years of intense activity: 1952, 1955, 2003 and 2012. Thus, the results of the present study provide evidence of active geomorphic processes in the studied area, indicating that tourists are highly exposed to geomorphic hazards, as both sites interfere with popular hiking trails. (Because Sâmbăta Valley is one of the most intensely frequented by tourists in the Făgăraș Mountains, it is a need for warning signs to be installed on the exposed trails.

Debris Flow Peak Discharge Reconstruction Using Geological, Historical, and Dendrogeomorphic Analyses in the Cauca River Canyon, Northern Andes, Colombia

Debris Flow Peak Discharge Reconstruction Using Geological, Historical, and Dendrogeomorphic Analyses in the Cauca River Canyon, Northern Andes, Colombia

Improving regional flood risk assessment using flood frequency and dendrogeomorphic analyses in mountain catchments impacted by tropical cyclones

River floods frequently occur when tropical cyclones hit land. Nonetheless, systematic, long-term discharge data remain rather scarce in many tropical countries, which prevent proper analysis of peak discharges occurring during floods. The Térraba catchment is the biggest and most dynamic catchment in Costa Rica. In this study, we developed regional flood-frequency analyses combining tree-ring based estimation and measurement of peak discharge at monitoring stations during tropical cyclones to derive flood quartiles. Flood quartiles were combined with the Topographic Wetness Index (TWI) to determine regional flood hazards along floodplains. The flood risk assessment was based on a high-resolution mapping of infrastructure, population density (as a measure of exposure), and a social development index (to represent vulnerability). We show that peak discharge of cyclone-induced floods can be assessed accurately with flood-frequency analyses including dendrogeomorphic reconstructions and systematic discharge measurements. We also show that regional flood risk assessments can be performed in large-scale catchments if both coarse and detailed inputs are used. The results of this study will be useful for the development of flood risk schemes promoting resilience of local populations.

The dendrogeomorphic use of shrubs for the detection of landslide movements: A case study of Corylus avellana L.

Dendrogeomorphic-based landslide activity dating is a commonly employed scientific approach used to detect historical landslide events with subannual precision. Trees are dominantly used for this research, but in some localities, current dendrogeomorphic methods are not suitable due to their exclusive occupation by shrub species. Nevertheless, shrub species have recently been successfully used in dendroclimatic research. Thus, this study aims to evaluate the applicability of common shrub species in Europe (Corylus avellana L.) in dendrogeomorphic research on landslides. In total, 71 stem samples from 11 shrub groups were macroscopically and microscopically analysed. Nearly half of the samples provided evidence supporting the presence of tension wood as a response to stem tilting due to landslide movement. The intensity of tension wood expressed by changes in vessel lumen area corresponded to the intensity of stem tilting, but no relationship was found for ring eccentricity. Moreover, the eccentricity of rings was found to be inapplicable to landslide event detection due to the short length of the studied C. avellana ring series. Nevertheless, the analysis of tension wood provided evidence of two past landslide events (2010 and 2014) and the absence of chronological noise. Moreover, the onset of tension wood was very abrupt, enabling the dating of landslide movements with seasonal precision. This assumption was verified by an analysis of potential triggers: extreme precipitation and high river discharge. Moreover, the roots of C. avellana exposed during the triggering flood undercutting the landslide base expressed a similar anatomical response to exposure, as seen in the cases of broad-leaved tree species. The results of this study could distinctly extend the use of dendrogeomorphic analysis of landslides into regions with no tree vegetation.

Low-topography deep-seated gravitational slope deformation: Slope instability of flysch thrust fronts (Outer Western Carpathians)

Unlike high-relief mountain areas, low-relief hilly landscapes are usually rarely affected by deep-seated gravitational slope deformations (DSGSDs). However, low-topography flysch thrust fronts can create suitable structural conditions for DSGSDs. The study area of the Kavalčanky ridge represents a relatively low-lying (<120 m of local relief) DSGSD-affected ridge situated at the thrust front in the flysch Outer Western Carpathians (Czech Republic). With the aim of revealing the main controlling factors and temporal constraints of mass-movement activity, a multidisciplinary investigation of DSGSDs was performed. Typical DSGSD landforms were mapped using high-resolution LiDAR-based mapping. Structural analysis revealed the presence of specific flysch thrust structural conditions with competent sandstone units overthrusted on a weak tectonically disrupted claystone basement. Geophysical measurement with the use of electrical resistivity tomography (ERT) and ground penetrating radar (GPR) profiling confirmed the deep reach (>50 m) of the studied DSGSD. Together with slope stability finite element modelling, geophysics suggests that the shear zone of DSGSDs is represented by one of the thrust faults, whereas lateral limits are formed by a set of conjugate strike-slip faults. Radiocarbon dating of bogs within the DSGSD body showed two phases of mass-movement activity corresponding to the Late Glacial-Holocene transition and Middle Holocene. The recent activity was excluded by dendrogeomorphic analysis. We concluded that the structural conditions involving high lithological complexity, tectonic weakening and the presence of major tectonic contacts might create conditions prone to DSGSDs even in relatively low-topography settings with mass movement activity, especially during humid and warmer Late Quaternary periods.

The age-dependent sensitivity of Picea abies (L.) H. Karst. to landslide movements

Dendrogeomorphic dating of past landslide events is a valuable tool for the assessment of landslide activity, providing unique data for the analysis of triggers or the modelling of landslide behaviour in the future. Unfortunately, tree-ring-based methods as well as dating approaches suffer from some limitations. One of the less frequently addressed limits of dendrogeomorphic analysis concerns the changing capacity of trees to record landslide events in their tree-ring series with increasing age. This study uses, to date, the most extensive database of tree-ring series (1736) of 868 disturbed individuals of Picea abies (L.) H. Karst. subjected to 20 landslides in the Outer Wester Carpathians for the assessment of their age-dependent sensitivity. The distribution of the total number of 1485 growth disturbances (reaction wood - RW and abrupt growth suppression - GS) throughout all decades of tree life shows evidence of distinct changes in the capacity for trees to record landslide signals with increasing age. The occurrence of RW dominated in the juvenile decades of tree life and then increased again in the 9th decade. The frequency of GS gradually increased and culminated during the 7th and 8th decades. The two intensities of growth disturbance (strong and moderate) expressed temporally balanced ratios, suggesting an effect of disturbance intensity rather than changing tree age. The important factors controlling age-dependent tree sensitivity to landslide movements based on the results seem to be changing stem elasticity, decreasing annual increment rates, root system development and increasing tree body weight. Moreover, this study demonstrates that landslide type (e.g., rigid block vs. plastic flow-like landslides) and bedrock geology distinctly modify age-dependent sensitivity that should be taken into account during the sampling of trees for landslide dating.

Old but still active: &gt; 18 ka history of rock slope failures affecting a flysch anticline

The Czech part of the Outer Western Carpathians (COWC) formed by flysch rocks is among the most landslide-prone areas in Europe. Recent LiDAR-based inventory mapping reveals that the area of the COWC encompasses more than 13,500 landslides, with some geological domains exhibiting > 20% of the surface area covered by landslides. Although mass movements represent crucial geomorphic agents in the COWC, their timing and especially lifespan remain elusive. In this study, we focused on rockslides affecting the flanks of the flysch anticline developed within Paleogene sandstones. The area (“Hradisko” ridge) is well known as the most extensive rock city in the COWC. Structural-geological investigation, kinematic analysis and the application of near-surface geophysics (ERT, GPR and seismic refraction) suggest that rockslides accompanied by toppling and lateral spreads are predisposed by the architecture of bedding planes, joints and inherited faults cross-cutting the anticlinal ridge. Based on 10Be exposure dating of five scarps and rockslide boulders (a total of 25 cosmogenic-dated samples), we were able to reconstruct the long-term history of rock slope failures. Although the obtained ages of headscarp exposures reveal consistent patterns suggesting major mass movement phases at ~ 18 ka and ~ 12 ka, the interpretation of boulder exposure ages is ambiguous. The study area has not been affected by catastrophic slope failure in historic times, but dendrogeomorphic analysis reveals surprisingly strong tree ring signals of mass movements within the last 150 years. These findings suggest that progressive failure is developing within some parts of the rockslide and/or slope portions above the major scarp. We conclude the following: (1) some rockslide spots within the COWC might express a very long history, encompassing the full Late Glacial–Holocene Period; (2) major mass movement activity in the study site temporally coincided with the major climatic changes; (3) rockslides with a very long history still represent potential hazards, although evidence of their active movement is not detected by standard geomorphic mapping techniques and (4) dating of the scarps should be preferred as even a higher number of boulders might be unreliable.

Impacts of land-cover changes on snow avalanche activity in the French Alps

Abstract Dendrogeomorphic analyses provide long and continuous chronologies of mass movements that are useful for the detection of trends related to climate change. Socio-environmental changes can, however, induce non-stationarities. This study addresses the following questions: (1) How does the evolution of forest cover induce non-stationarities in tree-ring based reconstructions of snow avalanche activity? (2) How are trends inherent to tree-ring approaches distinguishable from real fluctuations in avalanche activity? Using dendrogeomorphic techniques, we reconstructed snow avalanches in six adjacent paths in the French Alps. Results show two distinct trends in process activity between 1750 and 2016. In the southern paths, the frequency of snow avalanches increased sharply in the 1970s. The distribution of tree ages, as well as old topographic maps, allow an attribution of this trend to the destruction of large parts of the forest stand by a large snow avalanche in the 1910−20 s. This extreme event induced a sudden change in the capability of newly colonizing trees to record subsequent snow avalanches. In the northern paths, by contrast, progressive afforestation starting in the mid-19th century, as well as colonization of the release areas after World War II, resulted in a strong reduction in snow avalanche activity since the 1930s. Even if global warming remains a possible additional driver of snow avalanche activity at the study sites, the rural exodus and the abatement of pastoral practices during the 19th and 20th centuries are the main explanations for the observed trends in process activity. Results also illustrate a need to clarify the complex interrelations among forest evolution, global warming, social practices, and the process activity itself when interpreting trends in mass movements.

The sensitivity of dendrogeomorphic approaches to assessing landslide movements

An understanding of past landslide behaviour is crucial for the estimation of future landslide activity. In this respect, methods of absolute dating are frequently used for landslide history assessment. Dendrogeomorphic (tree ring-based) approaches are standard methods for the spatio-temporal reconstruction of landslide activity with annual to even sub-annual resolution and timespans of several centuries (depending on the tree age). Nevertheless, the verification of the effectiveness, sensitivity and precision of these dendrogeomorphic approaches has not been addressed in more detail despite their wide spectrum of limitations. This study makes an assessment of tree ring-based chronologies of landslides based on a comparison with the independent data of past landslide activity from instrumental monitoring. The chronology of past landslide events was created from 528 tree ring series from 132 individuals of Norway spruce (Picea abies (L.) Karst.) growing on a monitored landslide in the Outer Western Carpathians (the Ondřejník Mt.) and was expressed as the yearly variation in the standard event-response index. The landslide was monitored at two sites via technical levelling (since 2007) and an automated wire extensometer (since 2014). The obtained results show a high sensitivity of reaction wood formation to movements on the order of several mm (e.g., the abrupt acceleration of movements in February 2017, as recorded by the extensometer). Moreover, a one-year delay in the growth response to movement occurring in the middle part of the growing season of the previous year was detected. This fact fully supports the introduction of the dendrological year to dendrogeomorphic research. The partial lack of correspondence between tree ring records and data from more distant monitoring sites suggests the need for separate dendrogeomorphic analyses of the individual homogenous parts of landslide areas. Finally, the use of four instead of two increment cores per tree did not provide any extra event years but increased the values of the event-response index of the dated events.

Tree-ring reconstruction of snow avalanche activity: Does avalanche path selection matter?

In the current context of anthropogenic global warming, one of the purposes of dendrogeomorphic analyses is to provide long and continuous chronologies of mass movements, so as to detect potential trends or shift related to increasing temperatures. However, on documented slopes, the comparison between historical archives and tree-ring records suggests that dendrogeomorphic reconstructions systematically underestimate the natural activity of the process under investigation. In the specific case of snow avalanches, underestimation generally amounts to 50% and the main causes generally given for this difference are related to the magnitude of past events. In this study, we hypothesize that the morphometric characteristics of avalanche paths and their forest cover could have significant impacts on the length and reliability of tree-ring reconstructions. In order to test this hypothesis, we selected four adjacent, albeit differently structured, avalanche paths from the Queyras massif (French Alps), with the aim to compare their potential for a continuous reconstruction of past avalanche activity. On the most active avalanche paths characterized by high-altitude release areas (covered only by shrubby vegetation), tree-ring reconstructions do not exceed one century in length, with recurrence intervals of high magnitude events >25 years. By contrast, on forested couloirs where lower slopes and forest coverage up to the release areas limits the intensity of events, the frequency of reconstructed snow avalanches is 2.5 times higher, the reconstructions span longer periods and the convergence rate with historical archives attest to the reliability of the dendrogeomorphic approach. These results suggest that a careful selection of couloirs is essential and that priority should be given to forested sites as (i) they allow for exhaustive and (ii) reliable reconstructions over (iii) long periods of time.

Tree-ring eccentricity in the dendrogeomorphic analysis of landslides – A comparative study

The chronological aspects of landslide research are of great interest regarding the hazard assessment of landslide triggers. Dendrogeomorphic methods provide a good ratio of chronology length and dating resolution. An alternative to the standard dendrogeomorphic approaches used in landslide research (based on reaction wood analysis – a reaction to stem inclination) is the analysis of tree-ring eccentricity. This study tests three frequently used dendrogeomorphic approaches based on tree-ring eccentricity and verifies the results by comparing them with the results obtained by the standard dendrogeomorphic expert approach. In total, 125 individual Fagus sylvatica L. trees growing on the block-type landslide of Lipová stráň in the foothills of the Orlické hory Mts. (central Europe) were sampled and used for the analysis. The first tested approach identifies landslide signals based on the analysis of mean eccentricity values from two moving windows. The second tested approach determines landslide signals from a series of eccentricity values based on the use of an eccentricity threshold from reference trees. The last tested approach organizes the eccentricity values into categories and identifies landslide signals based on the defined temporal patterns of the individual categories. The limitations of each approach are revealed, and recommendations for increasing the robustness of each approach are presented. Moreover, the sensitivity of F. sylvatica trees to landslide signals was tested using each approach. The obtained results suggest the need for a specific sampling design (regarding tree age) for each tree-ring eccentricity-based approach.

Quantification of cliff retreat in coastal Quaternary sediments using anatomical changes in exposed tree roots

AbstractSea cliffs represent 80% of the world's coasts and can be found virtually in all types of morphogenetic environments. Most studies on rocky environments focused on the impacts of modern sea level rise on cliff stability derived from sequential surveys, direct measurements or erosional features in anthropogenic structures. In this study, we explore the potential of dendrogeomorphic techniques to quantify multidecadal changes in coastal environments on Porquerolles Island (France). We sampled a total of 56 cross‐sections from 16 Pinus halepensis Mill. roots growing on former alluvial deposits and on sandy‐gravelly cliffs to quantify mean annual cliff retreat rates as well as changes in cliff geometry. Anatomical changes in roots have been used successfully in the past to quantify continuous denudation rates on slopes, channel incision and gullying processes but the approach has not been used so far in a coastal cliff context. At Porquerolles Island, reconstructed rates of cliff retreat cover 30–40 years and show average erosion rates between 0.6 and 3.9 cm yr−1 (average: 2.1 cm yr−1). Highest rates are observed at Pointe de la Tufière (2.6–3.9 cm yr−1), a small rock promontory that is more exposed to wave and storm surges than the remainder of the study area. By contrast, lower erosion rates are recorded at cliffs protected by the La Courtade pocket beach (0.6–1.9 cm yr−1). This contribution demonstrates that dendrogeomorphic analyses of roots clearly have a significant potential and that they are a powerful tool for the quantification of multidecadal rates of cliff retreat in areas where measurements of past erosion are lacking. More specifically, the approach also has clear advantages over the shorter time series obtained with repeat monitoring (e.g. terrestrial laser scanning, sensors, erosion pins) or over longer, but more coarsely resolved records obtained from aerial photographs or radio‐nuclides. © 2018 John Wiley &amp; Sons, Ltd.

Tree-ring reconstruction of reactivation phases of the Schimbrig landslide (Swiss Alps)

One of the most important issues of current landslide research is related to the dating of their reactivations, both in spatial and in temporal terms. Landslide chronologies thus play a key role because they provide very essential information on past activity and thereby contribute substantially to hazard assessment, in particular in areas with intense anthropogenic uses. In most cases, however, knowledge of past landslide activity remains incomplete and archive records of past activity typically over represent the largest and miss the smaller events. In this paper, a chronology of past reactivations of the Schimbrig landslide (Swiss Alps) has been derived from the dendrogeomorphic analysis of 184 disturbed P. abies trees growing on the landslide body. In total, 318 growth disturbances identified and dated in the tree-ring series enabled reconstruction of 26 reactivation phases of the landslide body between 1859 and 2010 (mean return period: 0.17 event yr‑¹). Given the spatio-temporal completeness of the reconstruction, probabilities of landslide reactivation were computed and mapped using a Poisson distribution model for an event to occur within 5, 20, 50, and 100 years. Probabilities of landslide reactivation increase from 0.33 for a 5-year period to 0.99 for a 100-year period. High-resolution maps also indicate a strong spatial variability with lower values computed on the margins and in the central portion of the landslide body whereas less frequent reactivations are predicted in its lower part. The field-based reconstruction proposed in this paper provides quantitative probability maps of reactivation derived directly from the frequency of past events that appear complementary to conventional susceptibility maps, used for landslide zoning that provides an estimate of where landslides are expected to occur. This approach is considered a valuable tool for land managers in charge of protecting and forecasting people and their assets from the negative effects of landslides as well as for those responsible for land use planning and management. It demonstrates the reliability of dendrogeomorphic mapping that should be used systematically in forested shallow landslides.

Disentangling the impacts of exogenous disturbances on forest stands to assess multi-centennial tree-ring reconstructions of avalanche activity in the upper Goms Valley (Canton of Valais, Switzerland)

Abstract The purpose of dendrogeomorphic analyses is to amplify the signal related to the geomorphic process under investigation, and to minimize the noise induced by other signals in the tree-ring series. Yet, to date, no study accounts specifically for interferences induced by climate conditions or exogenous disturbances and which can, potentially, affect the quality of tree-ring based process reconstructions. In this paper, we develop a specific procedure allowing evaluation of the quality of reconstructions in five avalanche paths at Oberwald (Swiss Alps). The study is based on possible interferences between snow avalanches, climatic conditions and ecological signals in the tree-ring series. Analysis of past events was based on tree-ring series from 564 heavily affected, multi-centennial European larch trees ( Larix decidua Mill.) growing near or next to the avalanche paths. A total of 2389 growth disturbances, such as scars, tangential rows of traumatic resin ducts, compression wood as well as abrupt growth suppressions or releases, were identified in the samples, indicating 43 destructive snow avalanches since AD 1780. At the same time, 31 potential events, which were detected with the conventional Shroder index value, were rejected from the final reconstruction due to potentially strong interferences between the different signals. This high rejection rate underlines the necessity to systematically–and carefully–discriminate ecological and climatic noise from avalanche-related disturbances. This discrimination is even more so crucial as a significant proportion of dendrogeomorphic studies in the Alps are based on L. decidua trees which are cyclically affected by larch budmoth outbreaks.