A first dendrogeomorphologic approach of snow avalanche magnitude–frequency in Northern Iceland
Highlights
►Dendrogeomorphic analyses deliver a chronology of snow avalanches in North Iceland. ►Determination of the most reliable growth disturbance in birch. ►52 avalanche winters are highlighted. ►5 major events with activity index over 40%. ►Return period ranging from 4.2 to 19 years on the cone.
Introduction
Annual increment rings in tree growth reflect and record local environmental conditions (Wiles et al., 1996). Therefore, all changes in climate, weather and other external factors are manifested in the wood reaction occurring in trunk morphologies and tree-ring sequences (Shroder, 1980, Shroder and Butler, 1987, Schweingruber, 1996, Strunk, 1997, Stoffel, 2005). Dendrogeomorphology has been used effectively in snow-avalanche research (i.e. Ives et al., 1976, Carrara, 1979, Butler and Malanson, 1985a, Butler and Malanson, 1985b, Jenkins and Hebertson, 1994, Patten and Knight, 1994, Rayback, 1998, Larocque et al., 2001, Hebertson and Jenkins, 2003, Dubé et al., 2004), mostly with the aim of dating events, using softwood from diverse conifer species. Few studies have investigated hardwood species (Bryant et al., 1989, Mundo et al., 2007, Casteller et al., 2008, Szymczak et al., 2010) with similar efficiency.
Recently, tree-ring analyses have been conducted to reconstruct the frequency and extent of snow avalanches in several places, e.g. in the Spanish Pyrenees (Muntán et al., 2004, Muntán et al., 2009), the Swiss (Stoffel et al., 2006, Casteller et al., 2007) and French Alps (Corona et al., 2010). To date, snow-avalanche chronologies based on proxy data with a one-year resolution do not exist for Iceland. In this country, during the 20th century alone, 166 lives were lost due to snow avalanches (Sæmundsson et al., 2003), and economic losses were worsening in the country (Jóhannesson and Arnalds, 2001). The devastating year 1995 (Decaulne, 2007) reinforced the need to gather critical information on historical events, such as extent and occurrence, to further develop mitigative measures. Therefore recent snow-avalanche documentation mainly concentrates on areas where population density and the number of transportation corridors have increased over the past decades (Decaulne, 2007). Historical records are scarce or lacking in newly occupied areas, and do not cover the return period of major events, i.e. +/− 200 years (McClung and Schaerer, 1993). During recent years, studies based on geomorphological evidence of snow-avalanche activity have been conducted in remote areas (Decaulne and Sæmundsson, 2006, Decaulne and Sæmundsson, 2007), determining runout distances for major avalanches in selected paths and transferring results to paths located above fjord communities (Decaulne et al., 2008). However, occurrence of snow-avalanche deposits far downslope was determined only by the mean of relative dating (Decaulne and Sæmundsson, 2010).
The purpose of this study is to supplement previous researches on snow-avalanche occurrence, based on a geomorphologic approach, with a high resolution chronology of snow-avalanche winter events at cone scale using dendrogeomorphic methods on European White Birches (Betula pubescens Ehrh.), inferring snow-avalanche frequency, return period and spreading.
Section snippets
Geographic setting
The study was conducted in Northern Iceland, in the southern Fnjóskadalur valley, on one of the large colluvial cones on the eastern side of the valley (65°35′30″N, 17°44′17″W). The topography is characterized by steep slopes ranging from 200 m a.s.l. to 820 m a.s.l., talus and colluvial cones form below 350–300 m a.s.l. The cone itself, exposed south-westwards, exhibits a tree and shrub European White Birch cover (Betula pubescens Ehrh.), as previously described in Decaulne and Sæmundsson (2008) (
Sampling strategy
Trees impacted by snow-avalanche display characteristic external evidence: deformation or tilting of the main stem; more or less deep scars where debris (snow, rocks or vegetal) have impacted the bark (uphill face of the trunk); broken branches; topping; uprooting (Akifyeva et al., 1978, Luckman, 2010). This leads trees to develop various kinds of morphologies (Khapayev, 1978, Schweingruber, 1987), the most famous being the sabre and candelabra forms. Some of the previously mentioned features
Age structure of the stand/stand characteristics
The 39 Betula pubescens Ehrh. growing on the cone are 53 years old on average, as indicated by data on the pith age. The oldest tree was 103 years at the time it was sampled, and the youngest was 12 years old (Fig. 4). Age structure in the cone is in accordance with the first birch seedlings in Vaglaskógur, about 20 km northwards, in the beginning of the 20th century (Skogrækt ríkisins, 2012). On the cone, birches are issued from natural seed spreading, developing on slopes instead of at valley
Discussion and conclusion
This paper aims to test the potential of dendrogeomorphology to determine the magnitude and frequency of snow-avalanche activity on a single cone in Northern Iceland.
Twenty two cross sections and seventeen cores sampled from trees all over the cone reconstruct the chronology of 52 winters with clear signal of snow-avalanche activity. The chronology goes back to 1927. Temporal reconstruction is satisfactory in an area where only few snow-avalanche winters has been officially recorded at the
Acknowledgements
Financial support was received through the “EuroDendro” project funded by Maison des Sciences de l'Homme in Clermont-Ferrand, from the French Arctic Research Group CNRSGDR3062, Besançon, France, and from the Natural Research Centre of Northwest Iceland, Sauðárkrókur, Iceland. The Iceland Forest Service, Research Branch in Mógilsá supported the dendrochronological analyses and helped with great facilities during the time consuming tree-ring counting process. We thank the reviewers for their
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