Correlation between climate, man and debris flow events — A palynological approach H. Schneider ⁎, D. Höfer, R. Irmler, G. Daut, R. Mäusbacher Friedrich-Schiller-Universität Jena, Physische Geographie, Institut für Geographie, Löbdergraben32, D-07743 Jena, Germany abstract article info Available online 23 September 2009 Keywords: Palynology Debris flows Alps Climate change Debris flows occur frequently in the high-alpine catchment area of lake Lago di Braies (1492m a.s.l.). The sediments of the lake are used for detecting and dating past debris flow events during the last 4000 years and their driving factors. Comparing palynological and sedimentological data the results suggest, that mainly heavy precipitation in winter and summer as well as snow melt events in spring enhance the release of debris flow events. Therefore especially longer periods with humid and cool climate conditions show a clearly higher debris flow activity. The pollen diagram documents human impact on the catchment area since the late Neolithic, but the anthropogenic activities influenced the event activity only indirectly. Grazing and deforestation reduced the retention of debris by vegetation and results in thicker debris flow layers in the lake sediments. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Studies discussing the relationship between debris flow activity and climate increased during the last years. An overview is given in a special issue of Palaeoclimate Research (Matthews et al., 1997), where Innes (1997), van Asch (1997) and Kotarba and Baumgart-Kotarba (1997) presented studies from North and East Europe. Data sets from the French and Suisse Alps were evaluated by Zimmermann et al. (1997), Baumann and Kaiser (1999), Helsen et al. (2002) and Jomelli et al. (2004). A good correlation between debris flow frequency and climate is visible in the papers of van Asch (1997) and Kotarba and Baumgart- Kotarba (1997), whereas Innes (1997); Zimmermann et al. (1997) and Jomelli et al. (2004) missed a clear signal between a change in debris flow activity and any climatical trend. The problem of a possible human influence on the event activity is not discussed in most papers. Only Innes (1997) outlined possible anthropogenic influences on debris flow activity in a study in Scotland. According to these papers the relationship between debris flow activity, climate and man is still unclear. In the catchment of Lago di Braies Irmler (2003) tried to compile a debris flow calendar by counting varves of the annual laminated lake sediment. In addition palynological sequences of the same sediment core were analysed. In this paper results of the palynological investiga- tions are presented. The chronological frame for the palynological data set is not derived from the investigations on varves and debris flows in order to get an independent temporal archive. The classification relies on the correlation with palynological data sets from other lakes and mires of the Alps (e.g. Kompatscher and Oeggl, 2000; Kofler, 2000; van der Knaap et al., 2005). According to the different aims and archives the results of the several authors differ in temporal resolution. 2. Lake Lago di Braies and its catchment Lake Lago di Braies is situated in the Northeast of Italy (Fig. 1), close to the Austrian border at an altitude of 1492 m a.s.l. The lake area is 0.36 km² and the catchment area covers 30 km 2 . A natural barrier is causal for the origin of the lake, which was formed by an extensive alluvial fan in connection with several rock falls (Fig. 1B). Except in the north, the lake is surrounded by mountains, whereas mount Croda del Becco (2810 m a.s.l.) in the south of the lake is the highest one. Several debris flow cones are located on the flanks, which are fed by steep chutes and cirques. These debris flow cones reach the southern shore of lake Lago di Braies. The catchment area is located in the North of the Italian Dolomite Alps (Fig. 1). The dominant geological formations are carbonates of Triassic age (Pia, 1937). Quaternary sediments primarily consist of alluvial or debris flow fans. Subordinately, glacial material and landforms can be found. The lake is ice-covered from November to May (Engelen, 1972) because of the high elevation. The present local climate is characterised by a mean annual temperature of 5.7 °C, with mean summer and winter temperatures of 14.2 °C and -3.3 °C, and a mean annual precipitation of 856 mm. During the last 50 years nearly 70% of the precipitation were registered in summer (328 mm) and autumn (228 mm) (data provided by Ufficio Idrografico — Provincia Autonomia di Bolzano, 1992). The present-day vegetation is characterised by forests of Picea abies and Larix decidua. In addition, Pinus mugo is dominant in areas with debris flows and avalanches (Rösner, 2002). 3. Methods A pollen record from a 7.25 m sediment core (PRKL 01/01) was analysed in intervals of 25–30 varve years (cf. Irmler et al. 2005). 0.3 cm³ material was prepared as mentioned in Moore et al. (1997) Geomorphology 120 (2010) 48–55 ⁎ Corresponding author. Tel.: +49 3641 948808. E-mail address: heike.schneider@uni-jena.de (H. Schneider). 0169-555X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2009.09.014 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph