Establishing a Holocene Pinus cembra tree-ring chronology as a basis for investigations of glacial and climatic history in the central Eastern Alps

The knowledge of the Holocene climatic history is an important basis for assessing present and possible climatic variations. High mountain regions with their specific number of enviromnental indicators, like glaciers or the subalpine timber line, are particularly suited for palaeoclimatic studies, because variations of these indicators can be explained by relatively straightforward climate-proxy-relationships. Although the Alps are one of the best investigated high mountain regions in the world, there still exist remarkable gaps in the knowledge of the Holocene climatic history in this area. The proposed project on establishing a Holocene Pinus cembra chronology should provide new results concerning the date, the temporal duration and intensity of climatic variations in the Alps. Although c. 50 years ago first dendrochronological studies on typical Alpine tree speccies (Pinus cembra, Larix decidua) were carried out, there is as yet no corresponding continous Alpine chronology for the Holocene. Studies on subfossil tree trunks, most of which were carried out in Switzerland, have produced only a number of "floating" tree-ring series mainly for Larix decidua. For the Eastern Alps the proposed project on establishing a Pinus cembra chronology should fill this gap. Ihe basis for the establishment of the Holocene chronology is the existing c. 950 year long Pinus cembra chronology, which has been acquired at the Institut für Hochgebirgsforschung during the last years. For the extension of the chronology samples from subfossil tree trunks from peat bogs and small lakes from the central Eastern Alps shall be used. Only samples from sites at or near the timber line - above c. 2000 m a.s.1. - shall be collected. Due to the high altitude the tree-ring growth at this sites is essentially climatically determined. Subfossil Pinus cembra trunks from sites near the timber line mostly have several hundred years long tree-ring series, which is a good basis for successfully establishing the Pinus cembra chronology. Also subfossil trunks, which have been found in gletschervorfeld areas in die Eastern Alps and have been investigated at the Institut für Hochgebirgsforschung, shall be included into the porposed project. Tree-ring width measurments of the samples shall be made using the equipment available at the Institut für Hochgebirgsforschung. For a part of die subfossil samples radiocarbon datings for a first age determination are intended. With the Pinus cembra chronology of the Eastern Alps on the one hand climatically relevant events as glacier advances and maximum extents shall be determined by absolute dating trunks from gletschervorfeld areas, and on the other hand the climatic conditions during the Holocene shall be reconstructed by a direct analysis of the tree- ring series. The results will improve our knowledge of the climatic variations during the Holocene.

Within the project P13065-GEO one of the longest tree-ring chronologies in the world was established. This multi- millennial chronology was used as a basis for investigations on glacier and climate history in the central Eastern Alps. Although dendrochronological studies on Alpine tree species were carried out for decades there was no corresponding multi-millennial tree-ring chronology in the Alps when project P13065-GEO was started in 1998. The main aim of this project was the establishment of such an ultra-long chronology which should be based on subfossil Pinus cembra samples from the timberline ecotone of the central Eastern Alps. With this tree-ring chronology as a basis on the one hand glacier fluctuations should be calendar dated within the last millennia, and on the other hand extended climatic reconstructions should be carried out. Within the project, fieldwork has been carried out for the exploration and collection of subfossil logs at high- elevation sites of the central Eastern Alps. This work was mainly concentrated on sites in the western part of Tyrol. Altogether 700 samples of subfossil logs have been collected at 34 high-elevation sites (between approx. 1900 to 2500 m a.s.l.). The collected samples belong to three different species: Pinus cembra: 93 %; Larix decidua: 6 %, Picea abies: 1 %. Based on radiocarbon dates, the oldest samples found are approx. 10.500 years old. With the samples collected in this project, the first calendar dated continuous tree-ring chronology covering several thousand years (back to 5125 BC) has been established in the Alps. This ca. 7100-year-long tree-ring chronology (Fig. 2) is based on about 730 samples of living trees and subfossil and subrecent logs, respectively. At the moment the ca. 7100-year-long Eastern Alpine Pinus cembra Chronology is one of the 10 longest tree-ring chronologies worldwide. For the early Holocene, some additional tree-ring chronologies, one of which is approx. 1900 years long, are available now for the first time in the Alps. Presently they are fixed by 14C-data. The main use of the established chronologies is in the field of Holocene environmental and climate change. With the ultra-long Pinus cembra chronology as a basis, an absolute dated glacier advance record was carried out for the glacier Gepatschferner for the last four millennia. A summer temperature (June to August) reconstruction has been established for the last two millennia using the Pinus cembra chronology. The comparison of this reconstruction with a published millennial temperature reconstruction for the northern hemisphere shows agreement. This result indicates a more than only regional value of this tree-ring chronology from the central Eastern Alps. The Eastern Alpine Pinus cembra chronology can also be used as a basis for dendrochronological dating of archaeological timber: the logboat from "Obersee", 2016 m a.s.l., Eastern Tyrol, dates into the 11th century AD.