The EuroDendro project -Snow-avalanche frequency and magnitude in European Middle Mountain unravelled by dendrogeomorphological analyses

International audienceThe EuroDendro project aims to detect the signal of recent snow-avalanche activity within the analyses of tree rings. In mountain areas, even middle mountains, snow avalanches are a common process & pose serious hazards & risks in recently occupied sectors, either for residence or recreation purposes. Where (i) geomorphological evidence of snow-avalanche occurrence are scarce, (ii) historical records are poor & (iii) trees are colonizing part of the slopes, tree-ring analyses appear as a good alternative to reveal the recent snow-avalanche history. The dendromorphological approach consists in a survey of the investigated areas to inventory damages that affect the morphology of the tree: tilting of the trunk, presence of scars on the trunk, topped trees. Such a survey enables mapping the most frequent snow-avalanche paths, helping to: locate the maximum potential damages estimate the runout distances appreciate the lateral dispersion of the events The EuroDendro project is very interested in: (i) broad-leaved trees, which attracted seldom attention in dendrogeomorphology related to snow-avalanche activity: birch (Betula pubescens) in N Iceland birch (Betula pubescens) & alder (Alnus) in W Norway beech (Fagus sylvatica) in NE Cantal-France (ii) coniferous trees in little explored areas where broad-leaved trees are unavailable: spruce (Picea abies) in Central Romania Core extractions in c & d directions, i.e. parallel to the main flux lines, on both the uphill & the downhill sides of the tree Core analyses on a Lintab measurement table, enabling tree-rings counting, location & dating of growth disturbances (narrow & large rings) & reaction wood formation. The EuroDendro project investigates four main areas, some of them in connection with other scientific frameworks. The investigated areas are located in: xx (A) Northern Iceland, in the Dalsmynni valley, the Ljósavatn-skarð pass & the southern Fnjóskadalur valley, (B) Western Norway, at the bottom of Nordfjord, in Erdalen & Bødalen, (C) Central Romania, in the Southern Carpathians, in the Bâlea valley, Făgărăş massif, (D) Central France, in the Northeastern valleys of the Cantal massif. xx All these areas are characterised by steep slopes, harsh wintry conditions, known frequent snow-avalanche activity, a relative remotness & a tree-cover on talus & cones. A B C D The Fjnóskadalur site, Iceland © Armelle Decaulne The Bâlea site, Romania © Armelle Decaulne The Erdalen site, Norway © Armelle Decaulne The Cantal site, France © Emmanuelle Defive © GoogleEarth Study sites location Recurrent snow avalanches strongly influence the growth of the birches that develop typical tilted trunks (Ljósavatnskarð, Iceland). © Armelle Decaulne Slow growth Samples from the lower track & upper runout zones Broad-leaved trees => tension wood on the strained side of the trunk => asymmetrical growth Reference growth from outside the snow-avalanche influence area isolate the role of climatic accident & insect attacks Location of the reaction wood date recurrent events over the last 80 years, with a different distribution Young trees could indicate: (i) recent snow-avalanche activity drop at valley level (ii) change in grazing zones Well-defined areas of felled trees = recent major snow-avalanche events Reference growth = average of all cores extracted in c-d directions Samples from the lower runout zones Bødalen: sudden drop of tree growth in 2000 = major avalanche felled the tree Erdalen: recurrent avalanchy winters post-1995, severe from 2000 & especially in 2002 & 2006 Reference curve built from straight spruces sampled below the path Narrow path with poor tree cover, even in the distal runout zone Most of the trees are very young (+/-30 years) Reaction wood formation on the downhill side = recurrent stress due to snow-avalanche activity. Oldest trees signal several avalanchy periods, especially from 1920s to early 1950s again during the 1980s. Spatial distribution of major impact areas affecting trees in the Bødalen valley, Norway. Background image is © Norwegian land survey Main dendrogeomorphological investigated paths in the Bødalen valley, Norway. © Katja Laute Map of the tree morphology, reflecting damages due to snow-avalanche occurrence on a colluvial cone in the Dalsmynni valley, Iceland. Background images are © skipulag.is Birches felled by a snow avalanche in the Bødalen valley, Norway. © Armelle Decaulne Topped & tilted spruce in the distal part of the path, Romania. © Armelle Decaulne Asymetric branch distribution on a spruce in Romania. © Armelle Decaulne Scars on the trunk & main branches in the snow-avalanche path. © Armelle Decaulne Tilting, scars & topping Tilting & scars Main areas of felled trees Main areas of damaged shrubs Tilting 200