Tree-ring reconstruction of past debris flows based on a small number of samples—possibilities and limitations

Tree-ring analyses have often been used in the past for the reconstruction of spatiotemporal patterns of previous debris-flow activity, often yielding very precise and extensive data for torrents where information on former activity was largely missing. Unless dendrogeomorphology is slated for multimillion Euro developments, the large sets of tree-ring series that are usually used in these studies render analysis time-consuming and not necessarily very cost-effective. In this study, we present results on past debris flows obtained with 35 Larix decidua Mill. trees growing on the cone of the Torrent de Pétérey (Zinal, Valais, Swiss Alps). It is concluded that studies based on a limited number of samples may yield valuable data on past events, but that the reconstructed frequency remains widely incomplete and indications on the spatial aspects of past events are only fragmentar

Rainfall characteristics for periglacial debris flows in the Swiss Alps: past incidences-potential future evolutions

Based on observational meteorological data since A.D. 1864 and tree-ring records of debris-flow activity, this paper assesses changes in rainfall characteristics and their impact on the triggering of geomorphic events in a high-elevation watershed of the Swiss Alps since the end of the Little Ice Age. No trends are visible in the frequency of heavy rainfall events, but we observe a reduced number of heavy, short-lived rainfalls in summer and a concentration of advective storms is recorded in late summer and early fall since the late 1980s. These changes in triggering meteorological conditions resulted in a cluster of debris flows in the early decades of the twentieth century and a lowering of debris-flow activity since the mid 1990s, and may be mirroring the observed changes in persistent high-pressure systems over the Alps. We also observe intra-seasonal differences in debris-flow system response reflecting the state of the permafrost body in the source area of debris flows, allowing for very small debris flows to be released by limited rainfall inputs (<20mm) in June and July. The same quantities of rain will not trigger debris flows in August or September, when a thick active layer of the permafrost body is capable of absorbing water. With the projected amplitude of climatic change, seasonality, return intervals and volumes of debris flows are likely to be altered. RCM projections based on the IPCC A2 scenario suggest a decrease in heavy summer rainfalls which will most likely result in a (further) reduction of the overall frequency of debris flows, leaving more time for sediment to accumulate in the channel. Such an increase of channel accumulation rates along with the projected destabilization of the steep rock-glacier body is likely, in turn, to exert control ultimately on sediment volumes released from the source areas during future events. Observations from adjacent catchments suggest that extremely large debris flows, beyond historical experience, could occur at the study site and in similar debris-flow systems of the Valais Alps originating from periglacial environment

Duration and extension of anatomical changes in wood structure after cambial injury

Cambial injury has been reported to alter wood structure in broad-leaved trees. However, the duration and extension of associated anatomical changes have rarely been analysed thoroughly. A total of 18 young European ash (Fraxinus excelsior L.) trees injured on the stem by a spring flood were sampled with the aim of comparing earlywood vessels and rays formed prior to and after the scarring event. Anatomical and hydraulic parameters were measured in five successive rings over one-quarter of the stem circumference. The results demonstrate that mechanical damage induces a decrease in vessel lumen size (up to 77%) and an increase in vessel number (up to 475%) and ray number (up to 115%). The presence of more earlywood vessels and rays was observed over at least three years after stem scarring. By contrast, abnormally narrow earlywood vessels mainly developed in the first ring formed after the event, increasing the thickness-to-span ratio of vessels by 94% and reducing both xylem relative conductivity and the index for xylem vulnerability to cavitation by 54% and 32%, respectively. These vessels accumulated in radial groups in a 30° sector immediately adjacent to the wound, raising the vessel grouping index by 28%. The wound-induced anatomical changes in wood structure express the functional need of trees to improve xylem hydraulic safety and mechanical strength at the expense of water transport. Xylem hydraulic efficiency was restored in one year, while xylem mechanical reinforcement and resistance to cavitation and decay lasted over several year

Quantifying the effect of forests on frequency and intensity of rockfalls

Forests serve as a natural means of protection against small rockfalls. Due to their barrier effect, they reduce the intensity and the propagation probability of falling rocks and thus reduce the occurrence frequency of a rockfall event for a given element at risk. However, despite established knowledge on the protective effect of forests, they are generally neglected in quantitative rockfall risk analyses. Their inclusion in quantitative rockfall risk assessment would, however, be necessary to express their efficiency in monetary terms and to allow comparison of forests with other protective measures, such as nets and dams. The goal of this study is to quantify the effect of forests on the occurrence frequency and intensity of rockfalls. We therefore defined an onset frequency of blocks based on a power-law magnitude–frequency distribution and determined their propagation probabilities on a virtual slope based on rockfall simulations. Simulations were run for different forest and non-forest scenarios under varying forest stand and terrain conditions. We analysed rockfall frequencies and intensities at five different distances from the release area. Based on two multivariate statistical prediction models, we investigated which of the terrain and forest characteristics predominantly drive the role of forest in reducing rockfall occurrence frequency and intensity and whether they are able to predict the effect of forest on rockfall risk. The rockfall occurrence frequency below forested slopes is reduced between approximately 10 and 90 % compared to non-forested slope conditions; whereas rockfall intensity is reduced by 10 to 70 %. This reduction increases with increasing slope length and decreases with decreasing tree density, tree diameter and increasing rock volume, as well as in cases of clustered or gappy forest structures. The statistical prediction models reveal that the cumulative basal area of trees, block volume and horizontal forest structure represent key variables for the prediction of the protective effect of forests. In order to validate these results, models have to be tested on real slopes with a wide variation of terrain and forest conditions

The days of plenty might soon be over in glacierized Central Asian catchments

Despite the fact that the fast-growing population of Central Asia strongly depends on glacial melt water for fresh water supply, irrigation and hydropower production, the impact of glacier shrinkage on water availability remains poorly understood. With an annual area loss of 0.36 to 0.76%, glaciers are retreating particularly fast in the northern Tien Shan, thus causing concern about future water security in the densely populated regions of Bishkek and Almaty. Here, we use exceptionally long in-situ data series to run and calibrate a distributed glacio-hydrological model, which we then force with downscaled data from phase five of the Climate Model Intercomparison Project CMIP5. We observe that even in the most glacier-friendly scenario, glaciers will lose up to two thirds (−60%) of their 1955 extent by the end of the 21st century. The range of climate scenarios translates into different changes in overall water availability, from peak water being reached in the 2020s over a gradual decrease to status quo until the end of the 21st century. The days of plenty, however, will not last much longer, as summer runoff is projected to decrease, independent of scenario uncertainty. These results highlight the need for immediate planning of mitigation measures in the agricultural and energy sectors to assure long-term water security in the densely populated forelands of the Tien Shan

Analysis and modelling of tree succession on a recent rockslide deposit

Dendroecological methods were combined with vegetation and soil mapping to study recolonisation of European larch (Larix decidua), silver birch (Betula pendula) and Norway spruce (Picea abies) on a recently formed rockslide cone (deposit of 30×106m3) in the Valais Alps (Switzerland). Tree density and tree height were predicted with regression models that we derived using an information-theoretic model selection approach. Results demonstrate that the deposits of the 1991 rockslide have been colonised relatively rapidly with larch (ecesis time 2years), birch (5years) and spruce (2years). Most of the colonisation occurred 5-11years following the rockslide. Clast size was the primary factor driving tree colonisation with the highest tree densities found on plots with mainly smaller (<30cm) clast sizes. Tree height was affected by a combination of different influences, with tree age and tree density showing the most obvious effects. This study demonstrates how dendroecological methods allow reconstruction of spatio-temporal patterns of tree succession on rockslides, which may ultimately facilitate a more accurate dating of similar landforms of unknown ag

Spatio-temporal variability in debris-flow activity: a tree-ring study at Geisstriftbach (Swiss Alps) extending back to AD 1736

Past debris-flow activity on the forested cone of the Geisstriftbach torrent (St. Niklaus, Valais, Swiss Alps) was assessed from growth disturbances in old conifer trees, providing a much improved record of past events. The study of 633 tree-ring sequences sampled from 252 European larch (Larix decidua Mill.), Norway spruce (Picea abies (L.) Karst.) and Silver birch (Betula pendula Roth.) trees allowed reconstruction of 53 debris-flow events since AD 1736. The spatial analysis of trees affected during particular events on the geomorphic map allowed for a spatial representation of individual events and a reconstruction of four flow patterns. Based on our results and Siegfried maps, we believe that before the formation of a dogleg near the cone apex in the late 1890s, debris flows preferentially used the channels located in the west-southwestern part of the Geisstriftbach cone. This study contributes to our understanding of debris-flow processes on cones and provides an example of how dendrogeomorphic techniques may help in the reconstruction and understanding of debris flows in Alpine area

Tmem27: A cleaved and shed plasma membrane protein that stimulates pancreatic β cell proliferation

SummaryThe signals and molecular mechanisms that regulate the replication of terminally differentiated β cells are unknown. Here, we report the identification and characterization of transmembrane protein 27 (Tmem27, collectrin) in pancreatic β cells. Expression of Tmem27 is reduced in Tcf1−/− mice and is increased in islets of mouse models with hypertrophy of the endocrine pancreas. Tmem27 forms dimers and its extracellular domain is glycosylated, cleaved and shed from the plasma membrane of β cells. This cleavage process is β cell specific and does not occur in other cell types. Overexpression of full-length Tmem27, but not the truncated or soluble protein, leads to increased thymidine incorporation, whereas silencing of Tmem27 using RNAi results in a reduction of cell replication. Furthermore, transgenic mice with increased expression of Tmem27 in pancreatic β cells exhibit increased β cell mass. Our results identify a pancreatic β cell transmembrane protein that regulates cell growth of pancreatic islets

Spatio-temporal variations of rockfall activity into forests results from tree-ring and tree analysis

Steinschlag stellt einen der am häufigsten in Gebirgsregionen auftretenden geomorphologischen Prozesse dar und wurde daher in der Vergangenheit ausgiebig studiert. Trotzdem bleiben detaillierte Kenntnisse zur Steinschlagfrequenz (wie häufig), zu Steinschlagvolumen (wie gross), zu räumlichen Mustern (wo) oder zur Jahreszeitlichkeit (wann) der Steinschlagaktivität die Ausnahme und Angaben zumeist nur sehr fragmentarisch. Ebenso wurden Jahrringanalysen bislang nur in wenigen Ausnahmen herangezogen, um die vergangene Steinschlagaktivität zu untersuchen. Es ist daher das Ziel dieser Arbeit, (i) saisonale Unterschiede in der Steinschlagaktivität festzulegen und (ii) räumlichzeitliche Variationen in der Steinschlagaktivität mit dendrochronologischen Untersuchungen zu durchleuchten. Daneben wurden die vertikale Verteilung und die Sichtbarkeit von Steinschlagverletzungen auf der Stammoberfläche betrachtet, bevor Stärken und Schwächen der Dendrochronologie im Bereich der Steinschlagforschung mit zahlreichen Datensätzen existierender Studien evaluiert wurden. In einer ersten Untersuchung wurden 270 Stammscheiben von 18 juvenilen Lärchen (Larix decidua) aufbereitet, um saisonale Unterschiede in der Steinschlagaktivität auf einem bewaldeten Hang im Täschgufer (Täsch, Schweizer Alpen) zu studieren. Basierend auf einem Ansatz, der bislang nur in der Waldbrandforschung zur Anwendung kam, wurde die Position von Kallusgewebe und traumatischer Harzkanalreihen innerhalb des Jahrrings bestimmt und daraus die Jahreszeitlichkeit der Ereignisse abgeleitet. Die Ergebnisse zeigen deutliche saisonale Unterschiede bezüglich der Steinschlagaktivität, wobei Verletzungen während der Vegetationsperiode (Anfang Juni bis Mitte Oktober) eindeutig die Ausnahme bilden (12%). Im Gegensatz dazu können 88% der Verletzungen der winterlichen Wachstumspause zugeordnet werden, welche lokal von Mitte Oktober bis Ende Mai andauert. Direkte Beobachtungen am Hang bestätigen diese Resultate und deuten überdies darauf hin, dass die Aktivität im April und Mai am ausgeprägtesten sein dürfte. Während dieser Periode nimmt die globale Sonneneinstrahlung am westexponierten Hang des Täschgufer kontinuierlich zu und vermag so die Eislinsen aufzutauen, die durch das Gefrieren von Schmelzwasser in den Spalten und Rissen der Anrisszonen gebildet werden konnten. Im Gegensatz dazu scheint die Steinschlagaktivität im Täschgufer weder durch Sommergewitter noch durch anhaltende Niederschläge im Herbst beeinflusst zu werden. In einer zweiten Untersuchung wurden im Täschgufer an 135 stark verletzten Lärchen 564 Bohrkerne gezogen, um langfristige Veränderungen in räumlichzeitlichen Steinschlagmustern festzustellen. Die Untersuchungen umfassten vier Jahrhunderte (1600-2002) und ermöglichten die jahrgenaue Rekonstruktion von 741 Wachstumsstörungen in der Form von Verletzungen, traumatischen Harzkanalreihen, Reaktionsholz oder abrupten Wachstumsänderungen. Räumliche Analysen zeigten klar auf, dass Steinschlag während der letzten 400 Jahre immer wieder auftrat, wobei die Bäume in der südlichen Hälfte des Hanges regelmässiger von Steinschlag beeinflusst wurden, teils sogar mehr als einmal pro Jahrzehnt. Im Gegensatz dazu wurden die Bäume in der nördlichen Hälfte des Untersuchungsgebiets weniger häufig durch Steinschlag gestört, was die Wiederkehrdauer zwischen zwei Steinschlagereignissen lokal auf mehr als 150 Jahre anwachsen liess. Des weiteren fällt auf, dass sich Steinschlag in der Vergangenheit vorab mittels kleinvolumiger aber hochfrequenter Ereignisse manifestierte. Die Ausnahme bildet das Jahr 1720, als durch ein grossvolumiges Ereignis der Waldrand im südlichen Bereich des Hanges umgeschlagen und der Waldbestand weitestgehend eliminiert wurde. Der aufwachsende Wald hat sich im Anschluss an das Ereignis jedoch wieder erholt und konnte dadurch seine Schutzfunktion je länger je besser wieder wahrnehmen, was sich unter anderem auch in einer Reduktion der rekonstruierten Steinschlagrate im Südsektor zwischen 1740 und 1990 um den Faktor 13 auswirkt. Danach wurden im Altdorfer Bannwald (Altdorf, Schweizer Voralpen) drei ausgewachsene Bäume (Abies alba, Fagus sylvatica, Picea abies) gefällt und die vertikale Verteilung sowie die Sichtbarkeit von Steinschlagschäden am Baum untersucht. Für die Analyse vergangener Schäden wurden zwischen dem Stammanlauf und der Krone alle paar Zentimeter Stammscheiben gesägt, insgesamt 307. Aus den Resultaten der Untersuchung geht hervor, dass die Höhenverteilung der Schäden in Zusammenhang stehen dürfte mit der Grösse der Steine und Blöcke, der Hangneigung und dem Waldbestand. Infolgedessen lassen sich Schäden sowohl in Bodennähe wie auch auf über neun Metern Höhe erkennen. Schliesslich wurden – basierend auf den Resultaten aus dem Altdorfer Bannwald – Schwachpunkte, Möglichkeiten und Grenzen der Rekonstruktion von Steinschlagereignissen mit Jahrringanalysen unter die Lupe genommen. Die Untersuchung ging davon aus, dass Steinschlagschäden sehr viel zufälliger auf verschiedene Bäume verteilt sind und in sehr viel stärker variierenden Höhen auftreten als etwa Wunden, die durch das Auftreten von Fliessprozessen wie Überschwemmung, Nassschneelawine oder Murgang entstehen können. Nebst der vertikalen Verteilung der Schäden wurde anhand bestehender Daten aus den Schweizer Voralpen und Alpen unter anderem auch der Anteil der von aussen sichtbaren Schäden mit der Gesamtzahl der im Jahrringbild rekonstruierbaren Wunden verglichen. In ähnlicher Weise wurde danach versucht, den Prozentsatz der Verletzungen festzulegen, der erfasst werden kann, falls anstelle des ganzen Baumes nur eine einzelne Stammscheibe untersucht wird. Die Resultate der Untersuchungen deuten darauf hin, dass der Prozentsatz ‘unsichtbarer’ Schäden im wesentlichen von artspezifischen Borkeneigenschaften, jährlichen Zuwachsraten, dem Baumalter und dem Baumdurchmesser sowie von der Grösse des Steinschlags abhängen. Dies hat zur Folge, dass in einzelnen Baumarten nur 15% der Schäden von aussen nicht mehr sichtbar sind, während in anderen 90% nur mehr durch Jahrringanalysen festgestellt werden können. Das Studium einzelner Scheiben auf einer durch das äussere Erscheinungsbild des Stammes vorgegebenen ‘Testhöhe’ zeigt zudem auf, dass mit nur einer Scheibe pro Baum im besten Fall 13 bis 35% aller Wunden erfasst werden können. Eine wesentliche Verbesserung der Resultate kann jedoch erreicht werden, wenn weitere Indikatoren für die Präsenz von vergangener Steinschlagaktivität herangezogen werden, wie etwa Reaktionsholz, abrupte Wachstumseinbrüche oder – falls vorhanden – traumatische Harzkanalreihen. Zusammenfassend kann festgehalten werden, dass die vorliegende Arbeit Einblicke zu Möglichkeiten und Grenzen der Jahrring-Steinschlag-Forschung schuf und umfassende Daten zur Jahreszeitlickeit, Frequenz und Magnitude der Steinschlagaktivität sowie der Sichtbarkeit und vertikalen Verteilung von Steinschlagschäden auf der Stammoberfläche lieferte. Die im Rahmen dieser Arbeit entwickelten Methoden konnten seither auf einem anderen Standort erfolgreich angewandt. Trotzdem sind weitere Untersuchungen zur Jahrring-Steinschlag-Forschung im Allgemeinen und zur vergangenen Aktivität im Täschgufer im Speziellen unerlässlich, da die gewählten Analysemethoden weiter ausgebaut und nicht zuletzt auch der Einfluss von Erdbeben oder Klimafluktuationen auf die Steinschlagaktivität im Detail untersucht werden sollten.Rockfall represents one of the most common geomorphological processes in mountain regions and has extensively been studied in the past. Nonetheless, detailed data on frequencies (how often), volumes (how large), spatial distributions (where) or the seasonality (when) of rockfall activity remain scarce and most of the time fragmentary. Similarly, tree-ring analysis has only exceptionally been used to investigate past rockfall activity. It was therefore the aim of this study to (i) assess intra-annual differences in rockfall activity and (ii) to investigate spatial and decadal fluctuations of rockfall activity on forested slopes with dendrochronological analysis. Furthermore, the vertical distribution of scars was investigated and the visibility of scars determined, before other strengths and weaknesses of dendrochronology in rockfall research were evaluated with extensive datasets from existing studies. Firstly, 270 stem discs from 18 juvenile Larix decidua trees were prepared to study 25 years of intraseasonal differences in rockfall activity on a forested slope at Täschgufer (Täsch, Swiss Alps). Based on approaches used to date past forest fires, the position of callous tissue and resin ducts has been assessed within the tree ring as to determine the intra-seasonal timing of events. Results show distinct differences in rockfall activity, indicating that rockfall is scarce during the vegetation period (12%), which locally lasts from early June through mid October. In contrast, 88% of the impacts occur during the winter dormancy of trees between mid October and the end of May. Direct observations on the slope confirm the results, indicating that rockfall activity would be highest around April and May, when global insulation on the westfacing slope gradually rises and ice lenses formed from meltwater slowly disappear in the joints and fissures of the rockfall source areas. In contrast, rockfall seems to be neither influenced by thunderstorms in summer nor abundant rainfall in autumn. Secondly, 564 increment cores from 135 severely injured Larix decidua trees have been sampled at Täschgufer to investigate long-term spatial and temporal variations of rockfall activity. The analysis covers four centuries (1600–2002) and allowed reconstruction of 741 growth disturbances such as scars, traumatic rows of resin ducts, reaction wood and abrupt growth changes. Spatial analysis clearly shows that evidence from past rockfall events can commonly be found in trees located in the southern part of the slope, where they recurred more than once per decade. In contrast, trees in the northern part were less frequently disturbed by rockfall and locally define recurrence intervals of more than 150 years. Throughout the last four centuries, rockfall has caused growth disturbances to the trees sampled on the slope, most frequently in the form of low magnitude-high frequency events. In addition, analysis allowed identification of one high magnitude-low frequency event in 1720, which displaced the forest fringe of the northern sector a considerable distance downslope and eliminated an entire forest stand. Data further show that the forest recolonizing the southern sector after the 1720 event gradually improved its protective function, reducing the rate of reconstructed rockfall activity by a factor of 13 between the 1740s and the 1990s. Thirdly, three adult trees (Abies alba, Fagus sylvatica, Picea abies) have been felled in the Altdorfer Bannwald (Altdorf, Swiss Prealps) as to investigate the height distribution and visibility of impacts (scars) on trees. Past impacts on trees were analyzed with 307 cross-sections taken every few centimeters between the stem’s base and its crown. Results demonstrate that impact heights of rockfall fragments on trees largely vary depending on the diameter of rockfall fragments, the slope gradient as well as the forest cover. As a consequence, scars were identified at heights ranging from almost 0 to more than 900 cm above ground. Lastly and based on the results obtained from the three adult trees gathered at Altdorfer Bannwald, methodological difficulties, possibilities and limits of tree-ring analysis in rockfall research have been investigated. The study has been based on the idea that – in contrast to other hazardous processes such as debris flows, floods or wet snow avalanches – scars caused by rockfall activity are more randomly distributed on trees and may occur at largely varying heights. As a consequence, data gathered from nine different datasets in the Swiss Alps and Prealps have been analyzed in order to determine inter alia the percentage of scars remaining visible on the stem surface. Similarly, the number of scars reconstructed on entire trees has been compared to the events identified on only one cross-section or a series of increment cores taken at the height with a maximum number of wounds (i.e. test height) visible on the stem. Data indicate that the amount of overgrown scars would much depend on the bark properties of the species, yearly increment rates, the age of the tree, tree diameter as well as the size of rockfall fragments, resulting in an amount of ‘blurred evidence’ ranging from 15 to 90%. Analysis of single cross-sections at a given ‘test height’ indicates that, at best, 13 to 35% of the scars occurring on the entire tree can be detected. Data further suggest that the amount of events reconstructed at this ‘test height’ can be considerably improved as soon as other growth disturbances such as reaction wood, abrupt growth reductions or – if existing – traumatic rows of resin ducts are considered as well. In conclusion, new insights about possibilities and limitations of tree ring-rockfall research have been gained in this thesis and comprehensive data obtained on the seasonality, frequency or magnitude of rockfall activity as well as the vertical distribution and visibility of scars on stem surfaces. Replicate studies have since proved the general applicability of the approaches developed. Nonetheless, further research is needed in general and at Täschgufer in particular, as there is potential for improvements and further research, namely on earthquake-rockfall or climate-rockfall interactions

Kontextualisierung und Positionierung von Generationenprojekten. Ergebnisse des ersten trinationalen Workshops Generationenprojekte vom 30./31. Mai 2013 in Zürich

Zahlreiche Forschungsarbeiten im In- und Ausland sowie vielfältige, private, zivilgesellschaftliche und staatliche Initiativen zeigen, dass der Generationenfrage und mit ihr dem Verhältnis zwischen den Generationen gegenwärtig hohe Aufmerksamkeit zuteil wird. Die Gründe für dieses Interesse sind bekannt: Aufgrund des demographischen Wandels wird das Zusammenleben von drei bis vier Generationen zur Regel. In dieser neuen Konstellation werden Risiken und Chancen erkannt: So wird einerseits die Sorge geäussert, dass sich die verschiedenen Generationen wegen des beschleunigten sozialen Wandels auseinanderleben und unterschiedliche Interessenslagen im politischen Bereich in Generationenkonflikte münden könnten. Andererseits wird auf die Chancen und Potenziale hingewiesen, die sich aus dem Zusamenleben mehrerer Generationen ergeben und die sich mittels generationenübergreifender Projekte nutzen liessen