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Abstract Investigations of mass movement hazards need to be supported by accurate data regarding past frequencies of landslide reactivations. Dating landslide events using tree rings is a reliable approach for vegetated slopes in temperate climates. Despite the enhanced use of dendrogeomorphic research during recent decades, little is known about the specific responses of Larix spp. to landslide movements. We investigated the active section (~0.6 ha) of a complex landslide in the Vsetínské vrchy Mts. (Outer Western Carpathians) that is overgrown by a forest stand composed of European larch (Larix decidua Mill.) and Norway spruce (Picea abies (L.) Karst.). Using the dendrometric and dendrogeomorphic data identified from 74 trees (33 L. decidua and 41 P. abies), including the number and types of growth disturbances (GDs) and compiled event chronologies, we compared the responses of both tree species to the same landslide reactivations. We observed the same mean tree stem inclination for both tree species, whereas the inclination azimuths differed. L. decidua generally showed a higher number of stem bends compared to P. abies. Similar proportions of subjectively identified GDs (compression wood, CW, and abrupt growth suppression, GS) and mathematically identified GDs (tree-ring eccentricities) were observed in both tree species. However, the total number of GD.tree−1 was slightly higher in L. decidua (4.1) than in P. abies (3.2). A higher abundance of GS was identified in L. decidua at the expense of CW compared to P. abies. The event chronology based on the mathematical approach revealed slightly more landslide signals than for the subjective approach, mainly in L. decidua. However, the largest known landslide reactivation (in 1997) was recorded in all cases in both tree species. Based on the CW analysis, it was shown that larger stem inclinations are associated with longer persistence of CW in P. abies (rather than L. decidua), resulting in the possible masking of younger landslide events. By contrast, P. abies compensates for that limitation by having a generally shorter mean duration of CW than L. decidua. Overall, it seems that the complexity of landslide movements and root system architecture influenced the number and character of GDs in both tree species. In addition, we did not find any relationships between the measured dendrometric parameters and number of GDs to better assess the most suitable trees for dendrogeomorphic dating prior to sampling. Therefore, we recommend sampling both tree species when they overgrow the same landslide body so that the final event chronology will be more robust and possible limitations of one tree species can be eliminated by the benefits of the second. Highlights • Growth responses of L. decidua and P. abies to landslide movements were compared. • L. decidua showed higher number of growth disturbances per tree than P. abies. • Growth suppression occurred more frequently in L. decidua than in P. abies. • Larger stem inclinations are associated with longer persistence of compression wood. • Both species are recommended for sampling when overgrowing the same landslide body. [ABSTRACT FROM AUTHOR]

Dating of geomorphic events using dendrogeomorphic methods has increased rapidly during the last years to provide a systematic overview on event frequencies, spatial reach and magnitudes. Despite the recent advances in methodology, multiple questions remain when it comes to the age-dependent sensitivity of trees which may further influence the quality of dating. Yet, the impacts of ageing effects on dendrogeomorphic reconstructions have not been assessed in detail so far. This study clearly shows the evolution of numbers, types, and intensities of growth disturbances (GDs) across the lifespan of three different tree species ( Larix decidua Mill., Picea abies (L.) Karst., and Pinus sylvestris L.) affected by debris flows in the European Alps. Based on the re-analyses of datasets from past dendrogeomorphic reconstructions and calculation of statistical tests we investigated decreased ability of all studied tree species to record the debris-flow impact with increasing age. The great abundance of GDs occurred between 31 and 60 year of a tree life. By contrast, below-average number of GDs started around the 10th decade. L. decidua and P. abies show almost twofold number of GD per tree and decade compared to P. sylvestris . Injuries and compression wood are the most frequent GDs at rather young ages of L. decidua and P. abies specimens while tangential rows of traumatic resin ducts and abrupt growth increases are much more scattered. The most balanced distribution of GD is obvious across the lifespan of P.sylvestris . Unequivocal signals (GD of intensity reactions: 5, 4, and 3) of all tree species occur at significantly younger ages than uncertain signals of debris flow impacts (GD of intensity reactions: 2, 1, and 0). Based on the findings of this study and with the aim of improving detection of debris flows we proposed options of sampling strategy and laboratory steps (namely identification of an event) for forest stands with different age structure. Whereas the study sites across the world may not provide the “ideal” age composition of forests due to, for instance, forest management interventions or presence of high-magnitude disasters, the list of recommendations provided here seems to be a good alternative when the conditions are somewhat limited. [ABSTRACT FROM AUTHOR]

Dendrogeomorphic reconstructions of debris flows and related hydro-geomorphic processes (i.e. debris floods and flash floods) are widely used in territories with incomplete chronologies and potential future hazards. In recent years, the tree-ring analysis of disturbed trees affected by dangerous geomorphic processes has developed into a discipline with sophisticated methodical approaches and with annual up to monthly accuracy of dating. Field sampling strategies are based on various ranges of sample depth (i.e., different amounts of sampled trees of different age structures). The age and diameter of trees influence the ability of trees to respond to geomorphic processes; therefore, we investigate age-dependent and diameter-dependent tree sensitivity. Our study, for the first time, addresses the sensitivity to hydro-geomorphic processes in detail. We tested 462 individuals of Norway spruce ( Picea abies (L.) Karst.) – a very common species used in the dendrogeomorphic reconstructions – in the apical part of the medium-high-mountains where there is widespread occurrence of debris flows and debris floods. The tree sensitivity analysis was performed based on the position of growth disturbances within the increment cores of trees (original age and original diameter of the tree before the tree was affected). P. abies shows very high tree sensitivity in the first two decades of the tree lifespan. With increasing age and diameter, trees are less sensitive recorders of debris flows/floods. Moreover, the intensity of growth disturbances significantly decreases with tree age. Mature trees (51–120 years) record the geomorphic events mainly through abrupt growth changes (suppression and release) and tangential rows of traumatic resin ducts. In contrast, scars, the onset of reaction wood and traumatic resin ducts are dominant growth disturbances in young, thin trees (11–30 years; diameter 0–15 cm). Different occurrences of particular growth disturbances as well as the different age- and diameter-dependent sensitivities emphasize the need for the inclusion of new variable that will take into account the amount of sensitive trees for each year. After that, adequate mixture of sampled tree (i.e. mixture of age classes), which is necessary for the completeness of debris flow/flood chronologies, could be balanced regarding their sensitivity. [ABSTRACT FROM AUTHOR]

Rockslides in the Variscan orogenic belt of the Central Europe are a rare and poorly studied phenomenon. These relatively stable features have recently been shaped by secondary rockfall, toppling, sliding, or slumping. On afforested slopes, such processes can be efficiently analysed and dated by dendrogeomorphic methods. We performed detailed analyses of 355 increment cores from 81 Picea abies (L.) Karst. trees growing on two dormant rockslides in northeastern Czechia to reconstruct the activity of rock block movements and rockfalls. For the event determination we used standard event-response (I t) index and a semi-quantitative approach involving logical spatial position of disturbed trees during a three-year period. Furthermore, climate preparatory and triggering factors were analysed to investigate possible main drivers of recent secondary processes. Overall, four periods of certain block reactivations at the Prudký site since 1940 and seven periods of certain block reactivations at the Rudohorský site since 1834 were reconstructed. Most of the events can be dated to the period 1960–2000, but our data do not indicate any high-magnitude activity. This paper also demonstrates the results of tree eccentric growth not only in the main supposed direction of stem tilting but also in the direction perpendicular to the main direction when ca. 40% of all trees growing on rock blocks recorded the movements in both analysed axes of stem tilting, suggesting possible complex deformation and different directions of block movements over time. It also appeared that the periods with greatest activity of secondary movements were characterised by a significantly higher rain-on-snow factor (p = 0.007 and 0.026 at the Prudký and the Rudohorský site, respectively) thus indicating block detachments during periods of rapid snowmelt. • Dating of rock block movements and rockfalls using dendrogeomorphic techniques. • First chronologies of rockslide body disintegration in Variscan mid-mountains. • Spatial patterns of disturbed trees during three-year periods were examined. • Tree-ring data suggests complex movements of rock blocks in multiple directions. • Activity is related to years with significantly higher rain-on-snow precipitation. [ABSTRACT FROM AUTHOR]

• Impacts of contaminated debris flow deposits on forest degradation are demonstrated. • The first combined use of dendrogeomorphic dating, wood anatomy and dendrochemistry. • Growth decline of P. abies is accompanied by TRDs and decreased area of ew cells. • Higher concentrations of metals in trees buried by debris flow deposits since 1995. • Increased uptake of several elements matches with the debris flow activity. The degradation of forest ecosystems represents a severe environmental issue in anthropogenically modified landscapes worldwide. We demonstrated the negative impact of contaminated debris flow deposits on riparian forest in the Romanian Carpathians through the innovative combination of several dendroecological approaches. Reconstruction of debris flow activity using 95 individuals of Picea abies (L.) Karst. was performed to identify 11 events during the period 1952–2018. Recent events (e.g., 2007, 2014, 2016–2017) filled the valley floor after the stream was dammed in the 1980s, which deteriorated tree growth. This state was worsened by site acidity (pH = 3.8), low organic carbon, high sulfur content, and high concentrations of several metals in debris flow deposits. Wood anatomical analyses confirmed that trees producing tangential rows of traumatic resin ducts as a possible response to burial by toxic debris material subsequently produced smaller and flatter earlywood tracheids. In particular, the most obvious changes (average decrease in tracheid lumen area by 62.9% over two years) occurred after the 2016 debris flow event. Using inductively coupled plasma optical emission and/or mass spectrometry, we identified peak or increased concentrations of thallium (max. 0.7 mg kg−1), caesium (max. 0.4 mg kg−1), and aluminium (max. 434 mg kg−1) in tree biomass that could be related to debris flow activity in 1996, 2007, and 2016. Regarding the individual-specific responses of sampled trees, we conclude that the forest decline is caused by complex interaction between the depth of tree burial by debris flow deposits, the mechanical weight and chemistry of deposits, and the rate of nutrient and toxic element uptake. [ABSTRACT FROM AUTHOR]

• Comparison of tree ring-based chronologies with instrumental landslide monitoring • Recommendation of separate tree ring analysis of individual landslide zones • Relationship between seasonality of events and tree ring record • Reaction wood creation as a response to movements of even mm magnitude • Intensification of tree ring records when using extra cores per tree 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. [ABSTRACT FROM AUTHOR]