Alejandro Casteller

An Evaluation of Dendroecological Indicators of Snow Avalanches in the Swiss Alps

ABSTRACT Avalanche records are important for land-use planning and risk management in mountainous areas. Written records on dates, disturbed area, and pressure patterns of past snow avalanches are scarce. Tree rings can be used to complement written records on past avalanche activity. We conducted two case studies at documented avalanche tracks in the Swiss Alps to determine the most valuable dendroecological indicators for reconstructing past avalanche events. Both tracks were impacted by avalanche events in 1951 and 1999, the two most exceptional avalanche years in recent Swiss history. The difference in tree age among the track, border, run-out, and control sectors was a valuable indicator of the area impacted by past avalanches, but not a useful tool to establish dates of avalanche occurrences. Comparisons of reaction wood formation, changes in stem eccentricity, and presence of traumatic resin canals among sectors showed significantly larger values of these indicators after the 1999 event in the track areas, and in minor degree in the border areas. Abrupt growth changes following the 1999 event were identified in most disturbed trees. Growth releases were commonly accompanied by the formation of reaction wood. Tree-ring evidences for the 1951 avalanche event were limited mainly due to the removal of debris by local communities. The intensity of avalanche events was difficult to determine using tree-ring indicators. Although dates and areas impacted by documented avalanche events were sometimes assessed using single indicators, more precise reconstructions result from the combined use of several dendroecological indicators.

Spatiotemporal analysis of channel wall erosion in ephemeral torrents using tree roots—An example from the Patagonian Andes

This paper presents absolute ages for fl ash fl oods and related sediment entrainment in headwater catchments to construct a spatiotemporal framework of process dynamics and locations of major areas of channel wall erosion. The most reliable method for dating erosion is through dendrogeomorphic studies of exposed tree roots. Based on the analysis of erosion signals in root-ring records we documented a time series of channel wall erosion and successfully dated 21 erosive fl ash fl ood events since A.D. 1870 in an ephemeral gully in the Patagonian Andes. The study was performed with roots from Austrocedrus chilensis, Nothofagus dombeyi, and Pseudotsuga menziesii. Results demonstrate the potential of root analyses for the determination of major areas of sediment entrainment. In addition, we show that the position of damage within individual root rings allows inferences about the seasonal timing of fl ash fl ood effects and thus an assessment of possible meteorological triggers of erosive events, short intense storms occurring primarily in austral fall and late winter in this case. The approach presented adds signifi cantly to the documentation of sediment entrainment and facilitates identifi cation of areas of rapid erosion in small, remote headwater catchments with ephemeral fl ash fl ood activity.

Reconstrucción espacial y temporal de la ocurrencia de avalanchas de nieve en los Andes patagónicos utilizando técnicas dendrocronológicas

Las avalanchas de nieve representan un importante riesgo natural en diversas regiones montanosas alrededor del mundo. Danos a infraestructura y perdidas de vidas humanas son reportados frecuentemente en relacion a eventos catastroficos de avalanchas. En los Andes, numerosas obras de infraestructura se ubican en zonas vecinas a senderos de avalanchas, de las que poco se conoce sobre sus alcances maximos, periodos de retorno y presiones de impacto. A traves de la implementacion de tecnicas dendrocronologicas hemos reconstruido las fechas de ocurrencia y areas de influencia de eventos pasados de avalanchas de nieve. Ejemplares de Nothofagus pumilio con perturbaciones visibles de avalanchas fueron muestreados en sectores del canal, bordes y zonas de frenado de 11 senderos de avalanchas ubicados en Loma de las Pizarras, proximo a El Chalten, Santa Cruz, Argentina. Ademas, areas de control fueron muestreadas para determinar las condiciones de crecimiento en arboles no afectados por avalanchas. Nuestros analisis indican que las cicatrices, las variaciones de excentricidad en el leno, los cambios abruptos de crecimiento y la presencia de leno de tension son los principales indicadores dendrocronologicos en N. pumilio asociados a la ocurrencia de avalanchas. Basados en una ponderacion cuantitativa de los indicadores y en la profundidad de muestreo, calculamos un indice de ocurrencia de eventos, el cual nos permitio determinar para cada sendero los anos con ocurrencia de avalanchas. Considerando de manera integral la actividad de avalanchas en los 11 senderos muestreados, los anos con mayor ocurrencia de eventos fueron 1936, 1966, 1978 y 1995. Complementariamente, registros climaticos fueron analizados con el objetivo de determinar las relaciones entre los anos con ocurrencia de avalanchas y las variaciones mensuales de precipitacion y temperatura. Se observa que los anos con gran frecuencia de avalanchas estan significativamente correlacionados con la ocurrencia de abundantes precipitaciones invernales; no se detectaron relaciones significativas con las variaciones de temperatura. Los resultados alcanzados en esta investigacion prueban el potencial dendrocronologico de N. pumilio para reconstruir patrones espaciales y temporales de avalanchas de nieve en los Andes patagonicos.

Using Dendrochronology to Validate Numerical Simulations of Snow Avalanches in the Patagonian Andes

Snow avalanches pose a considerable risk in the Patagonian Andes. Despite this, avalanche hazard maps are rare for this region. One way to reduce this risk uncertainty is to apply numerical models that predict avalanche runout. These models require that the avalanche release areas must be precisely determined (Maggioni and Gruber 2003) and the friction parameters used by the models must be adjusted to local terrain and snow conditions (Furdada et al. 2002). These parameters can be estimated by simulating avalanche events for which the extent of runout is known. Dendrochronological methods become a valuable tool for precisely defining the spatial extent of past events in the Andes where directly observed evidence is not available. Tree-ring studies are also important for reconstructing years of avalanche occurrences in such cases (Casteller et al. 2008).