Tree-ring growth of Pinus cembra at the timberline in the central Eastern Alps: preliminary results *

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1 Tree-ring growth of Pinus cembra at the timberline in the central Eastern Alps: preliminary results * Kurt Nicolussi & Gerhard Lumassegger Indroduction and material Over the past few years some studies with tree-ring material from the central Eastern Alps have been made at our Institute and the Institute of Botany. These studies have been focussed on the possible effects of anthropogenic air pollution on the forests near the timberline, on a reconstruction of the alpine glacier history and on the history of a forest at the timberline (Nicolussi 1994, Nicolussi et al. 1995, Lumassegger 1996). All of the treering material for these studies came from locations at or near the timberline. These treering series are now summarized for a first regional and temporal overview, extending back to Medieval Period. The presented results are preliminary, further investigations are planned. Tree rings from arctic or alpine timberline locations are climatic indicators, mainly influenced by summer temperature (Schweingruber 1987). The aim of this summary is to investigate the recent tree-ring width values in contrast to the normal values of the last centuries. Therefore, with one exception all tree-ring values and series will be presented in the form of non-indexed values. This avoidance of indexations should help to prevent the loss of long term trends, with all advantages and disadvantages. The sample locations, from where the tree-ring material has been collected, are situated in the central Eastern Alps, more precisely in the Ötztal and Stubay Alps. The four locations of the Radurscheltal, Kaunertal, Vent and Gurgl, both in the Ötztal, are on the northern side of the main alpine ridge, (Nicolussi 1994, Nicolussi et al. 1995, Lumassegger 1996). The northernmost location is Kühtai, the sixth location in the Matscher Tal lies in the south of the main alpine ridge ( Nicolussi 1994, Nicolussi et al. 1995). The timberline in the central Eastern Alps is formed by Pinus cembra. In the northern Stubay Alps, e. g. in Kühtai, the timberline reaches an elevation of about 23 m a.s.l., south of the main alpine ridge the elevation of the timberline is higher, at about 24 m a.s.l.. Under natural conditions, the timberline is characterized by solitary standing trees or small groups of trees. In the Ötztal and Stubay Alps caused by human activity there are not so many sites, where the actual timberline is at its climatic limit. It is also rare to find subfossil wood in the central Eastern Alps. An exception is the Radurschel valley, where there are such dead trees at the timberline, which in part have lain or stood there for some hundred years. At the other five locations, the tree-ring material comes from living trees. The elevation of all sampled trees is between 213 and 23 m a.s.l.. Altogether 323 Pinus cembra trees were investigated for this study. * Dieser Beitrag stellt einen Auszug aus dem bei der Tagung Euro-Dendro 97 in Savonlinna, Finnland, gehaltenen Vortrag dar. Institut für Hochgebirgsforschung - Jahresbericht 1997 Seite 48

2 Results Dendroclimatological investigations using tree-ring raw data, that means non-indexed tree ring-values, have to consider the so-called age-trend, because the endogenous factor age can determine the tree-ring width growth of trees. Pinus cembra is a tree species with a pronounced age trend. On average the maximum tree-ring growth will be reached at a cambial age of about 4 years a with a mean width of about 1.5 mm (Fig. 1). This maximum is followed by decreasing widths up to a cambial age of around 25 years. Pinus cembra trees, which are older than 25 years, indicate no clear age trend. This old trees show mean tree-ring width of about.4 mm. Fig. 1: The age determined tree-ring width trend of Pinus cembra. To avoid or at least to reduce the effect of the cambial age of the tree rings values on the results the tree-ring material has been divided into three age groups: an age class with treering widths from very old trees, this means trees rings with a cambial age of more than 25 years, a second age class with tree-ring widths with a cambial age between 15 and 25 years, and an additional age class with very young trees, this means of an age up to 1 years. In the last two age classes we had to consider a possible influence of the age trend for the interpretation. A comparison of the three age classes together with the density, showing the number of investigated trees, for the last eight centuries is given in Fig. 2. For the age class of the very old trees (cambial age > 25 years) the maximum density, about 3 trees, occurs in the 18 th and in the 2 th century. Before the year 14 we only have one tree. With the exception of the 2 th century, there is a similar range of tree-ring widths. Minima occur e.g. around the years 16, 17 or 182. The diagram also shows periods where very small tree rings are absent. The years around 157 are an example for such a period. At the same time there are also relatively wide tree rings. The years around 187 are a second example for such a period with relatively wide tree rings. The age class of the very old trees show a similar density of tree-ring series in the late 2 th century as in the 18 th century, but the tree rings are clearly wider in our century than 2 years ago. The age class between 15 and 25 years shows a similar picture (Fig. 2). The replication of trees is quite different, the maximum with over fifty trees is in the 2 th century, a second maximum occurs around 16, but before approximately the year 135, the tree-ring Institut für Hochgebirgsforschung - Jahresbericht 1997 Seite 49

3 values are based only on one tree. This age class indicates a similar range of tree-ring widths for the five centuries between about 14 and about 19. In this time period there are again some phases with small tree rings and missing bright rings: for example around the years 147, 16 and 182. In the five centuries between 14 and 19 again periods can be found, during which there are no very small tree rings and at the same time an TREES [n] age class: TREE-RING WIDTH [mm] 5 age class: TREES [n] 5 4 age class: TREE-RING WIDTH [mm] age class: TREES [n] 4 age class: > TREE-RING WIDTH [mm] age class: > YEAR Fig. 2: Tree-ring width values and number of trees investigated for the three age classes. Institut für Hochgebirgsforschung - Jahresbericht 1997 Seite 5

4 accumulation of wide tree rings. The first half of the 15 th century is such a period, through this time we only have a small number of trees. There is a higher density in the years around 157 and 179. In the 2 th century the age class as the age class of the very old trees indicates good tree-ring growth conditions at the timberline in the central Eastern Alps. The last age class includes tree rings from young trees only: tree rings with a cambial age between 1 and 1 years. In the 2 th century this age class has a density up to about 14 trees, around 18 the density is of about 7, around 14 the number of trees is approximately 2 (Fig. 2). Low tree-ring widths can be observed again around the years 147, 16 and 182. It should be noted that during the last time period Fig. 2 shows no wide tree rings, even when the density is high. Relatively wide tree rings can be observed, besides the 2 th century, again around the years 178 and 157 and also in the first half of the 15 th century. For the evaluation of the middle and long term trends the tree-ring values of each of the three age classes have been averaged for each calendar year and and than smoothed with an 15-year running mean (Fig. 3). In general the three curves show a similar trend. A first depression phase is before 13, but in this time the material comes only from a small number of Pinus cembra trees. Similar minima of the smoothed curves can be observed around 147, 16, shortly after 17 and 182. Periods with wide rings can be found in the first half of the 15 th century, around 156/7, or before 18. The curves of all three age classes reach their maxima for the investigated time period at present. Fig. 3: Comparison of the averaged and smoothed tree-ring widths of the three age classes. The picture becomes clearer after averaging these smoothed curves of the three age classes (Fig.4). To avoid a dominant influence of the age class of the young trees the three curves have been transformed into z-scores before averaging. The period before the year 13, Institut für Hochgebirgsforschung - Jahresbericht 1997 Seite 51

5 indicated by a very low replication, has been excluded from this averaging. Again, the result shows a sequence of depressions and periods with wide tree rings. For the time period from 13 up to 192 there is a similar range. The maximum of the averaged curve is reached at the present day. Fig. 4: The tree-ring growth trend of Pinus cembra for the last seven centuries after averaging the standardized mean curves of the three age classes. Comparison with the glacier history Since the middle of the last century the glaciers of the Alps have been retreating with some short re-advances. The glacier extent of the present is the smallest at least since the 13 th century (e.g. Zumbühl & Holzhauser 1988). This agrees well with the observed tree-ring width trends and levels for the last centuries. Also the pronounced tree-ring depressions in the 14 th century, around c. 147 and c. 16, shortly after 17 and around c. 182 match well with periods of large glacier extents and/or glacier advances in the Eastern Alps (e.g. Patzelt and Bortenschlager 1977, Patzelt et al. 199, Nicolussi 1994, Nicolussi and Patzelt 1996). Besides the time after the middle of the last century the Alpine glaciers retreated during the first half of the 15 th century and around 157 for example (e.g. Holzhauser 1984). This would roughly correspond with the maxima of the tree-ring values at these times. Summary The tree-ring record shows a similar range of width variations over the last seven to eight centuries, with the exception of the decades after 192. For these last decades the tree-ring Institut für Hochgebirgsforschung - Jahresbericht 1997 Seite 52

6 values are reflecting favourable growth conditions. In general, the mentioned variations agree well with the Alpine glacier history. References Holzhauser H. (1984): Zur Geschichte der Aletschgletscher und des Fieschergletschers. - Physische Geographie 13, Zürich, 448 p. Holzhauser H., Zumbühl H.J. (1996): To the history of the Lower Grindelwald Glacier during the last 28 years - palaeosols, fossil wood and historical pictorial records - new results. - Zeitschrift für Geomorphologie N.F., Suppl.-Bd. 14: Lumassegger G. (1996): Dendrochronologische Untersuchungen an Zirben (Pinus cembra) im Waldgrenzbereich des Radurscheltales, Ötztaler Alpen. - Diplomarbeit, Univ. Innsbruck, 9 pp. Nicolussi, K. (1994): Jahrringe und Massenbilanz. Dendroklimatologische Rekonstruktion der Massenbilanzreihe des Hintereisferners bis zum Jahr 14 mittels Pinus cembra-reihen aus den Ötztaler Alpen, Tirol. - Zeitschrift für Gletscherkunde und Glazialgeologie 3: Nicolussi K., Bortenschlager S., Körner Ch. (1995): A possibly CO 2 -related increase in tree-ring width in subalpine Pinus cembra of the central alps. - Trees, 9: Nicolussi K., Patzelt G. (1996): Reconstructing glacier history in Tyrol by means of tree-ring investigations. - Zeitschrift für Gletscherkunde und Glazialgeologie 32: Patzelt G., Bortenschlager S. (1973): Die postglazialen Gletscher- und Klimaschwankungen in der Venedigergruppe (Hohe Tauern, Ostalpen). - Zeitschrift für Geomorphologie N.F., Suppl.Bd. 16: Patzelt G., Bortenschlager S., Nicolussi K., Poscher G. (199): Neue Ergebnisse zur Holozänforschung in Tirol. Führer zur Exkursionstagung vom Schweingruber F.H. (1987): Flächenhafte dendroklimatische Temperaturrekonstruktionen für Europa. - Naturwissenschaften 74: Zumbühl H.J. & Holzhauser H. (1988): Alpengletscher in der Kleinen Eiszeit. - In: Sonderheft zum 125jährigen Jubiläum des SAC, Die Alpen 64/3: Institut für Hochgebirgsforschung - Jahresbericht 1997 Seite 53