Updating the Tyrol tree-ring dataset

Transcription

1 Updating the Tyrol tree-ring dataset J. Esper 1, U. Büntgen 1, D. Frank 1, T. Pichler 2, K. Nicolussi 2 1 Swiss Federal Research Institute WSL, 8903 Birmensdorf, Switzerland 2 Institute of Geography, University of Innsbruck, 6020 Innsbruck, Austria esper@wsl.ch Tyrol collection and use in palaeoclimatology The Tyrol dataset is a collection of 71 Picea abies ring width measurement series from the Stubaital and Oetztal in Austria. It was sampled in the 1970s (Lamprecht 1978, Siebenlist- Kerner 1984) the outermost ring is 1975 and extended back into the 14th century. Despite this limited length, the collection and derived chronologies have been used in recent publications dealing with millennium-long, hemispheric scale temperature variations (D'Arrigo et al. 2006, Esper et al. 2002, Osborn & Briffa 2006), indicating the relevance of these data in reconstructing large-scale climate variability. The data were considered despite the early end date (1975) limiting the calibration period by about 30 years, i.e. recent decades of instrumental data could not be used for climate signal analysis. Figure 1: Map showing the study area in Tyrol in the central Alps. We here describe efforts of updating this relevant dataset. These include extending the collection back in time and updating it during recent decades. We review the common variance of the various sub-samples combined in the Tyrol compilation, and describe some initial assessment of the climate signal. Interestingly, past efforts to reconstruct larger scale temperature variations considered only the tree-ring width (TRW) data fraction of the Tyrol collection. The original and now updated datasets contain both TRW and density measurements, however. So, here we focus on the maximum latewood density (MXD) measurements and compare their climate signal with that found in TRW.

2 Table 1: Characteristics of the sub-samples combined in the Tyrol dataset. Sub-sample Oetztal (rec.) Stubaital (rec.) Oetztal (hist.1) Oetztal (hist.2) Number Period Period (>4 series) MSL [yrs.] MTRW [mm] MMXD [g/cm 3 ] Rbar (TRW) Rbar (MXD) MSL is the mean segment length, MTRW the mean tree-ring width, MMXD the mean maximum density, and Rbar the inter-series correlation calculated using Cofecha. Extending and updating the existing record Samples combined in the Tyrol dataset are from the Oetztal and Stubaital, two neighbouring valleys south of Innsbruck in western Austria (Fig. 1). The original dataset as developed in the 1970s was composed of Picea abies samples from living trees spanning the period and material from relict wood spanning the period (Fig. 2). After truncation at a replication of 5 samples the collection covered (Lamprecht 1978). We now updated this record by integrating 46 measurement series from living trees in the Oetztal and 110 measurement series from historic material from the Oetztal/Stubai Alps and adjacent areas (Nicolussi 2002), thereby tripled the size of the original collection. Inclusion of data resulted in an extension back to 1028 (1053 after truncation at n>4) and an update through Relevant characteristics, including mean segment lengths, growth rates, and densities of the original and new sub-samples are listed in Tab. 1. Figure 2: Replication of the original 'Stubaital recent' and 'Oetztal historic 1', and newly measured 'Oetztal recent' and 'Oetztal historic 2' sub-samples integrated in the Tyrol dataset. Each bar represents one measurement series, and 1-2 of these series represent individual trees.

3 To assess the common variance of the various sup-samples and provide a visual impression of the newly composed Tyrol collection, we power transformed (Cook and Peters 1997) the TRW and MXD measurement series, standardized the data by calculating residuals from 300-year splines, and developed mean chronologies of the sub-samples using the robust mean (Fig. 3). Cross-correlations between these sub-sample chronologies ranged for TRW and for MXD (see Tab. 1 for overlap between the truncated chronologies), indicating a much stronger common signal in the density data. Lowest correlations for both TRW and MXD were recorded between the original Stubaital recent and Oetztal historic 1, and highest correlations between the Oetztal recent and Oetztal historic 1 sub-samples. The substantial range in correlation between the TRW and MXD chronologies is consistent with the Rbar results obtained for the various sub-samples (Tab. 1), even though differences were smaller on the level of individual measurement series. Figure 3: 300-year spline detrended MXD chronologies separated by sub-sample (a) and plotted together (b). Oetz. rec. vs. Stub. rec. TRW/MXD chronologies correlate at 0.54/0.87 ( period), Oetz. rec. vs. Oetz hist. 1 at 0.28/0.71 ( ), Stub. rec. vs. Oetz hist. 1 at 0.22/0.65 ( ), and Oetz hist. 1 vs. Oetz. hist. 2 at 0.40/0.68 ( ).

4 Climate signals Comparison with high elevation temperature data (Auer et al. 2005) revealed a stronger climate signal in the MXD chronology (Fig. 4). Maximum response was found to Aug-Sep mean temperatures (r = 0.66, 189 years). The correlation pattern of the TRW data is less clear and reached maximum values with Jun-Jul mean temperatures (r = 0.43). These results and the significances of the updated Tyrol collection for palaeoclimatic studies are supported by the fit between the linearly modelled and target instrumental data (Fig. 4b). Figure 4: (a) Correlation of the TRW and MXD chronologies with previous to current year monthly and seasonal high elevation temperature data. (b) Instrumental and modelled Aug-Sep and Jun-Jul mean temperatures over the period Discussion A significant update of a widely used tree-ring dataset from Tyrol in Austria is presented. Inclusion of 156 new measurement series allowed extending the original record by about 300 years back to 1053 AD. Investigation of the common variance in TRW and MXD data demonstrated significantly higher coherence for the density parameter. While these results suggested a stronger climate signal in MXD, this assumption was confirmed by comparison with high elevation instrumental records from the region.

5 Our results suggest that future efforts in reconstructing past temperature variability should consider the MXD data from the Tyrol collection. To analyze the full spectrum of climate variations including long-term temperature changes, age-related standardization methods such as RCS (Esper et al. 2003) could be evaluated. Also the combination of TRW and MXD data (Luckman and Wilson 2005) might help to better estimate past temperature variations and particularly the absolute amplitude of such variations (Esper et al. 2005a, 2005b). Future efforts should also re-consider the inclusion of the Stubaital sub-sample, which could possibly be removed and substituted with the newly measured samples from the Oetztal. Acknowledgements We are grateful to Danni Nievergelt and Anne Verstege for performing density measurements. Supported by the National Science Foundation (NCCR Climate), the Austrian Science Fund (P15828-N06) and the European Community (Grant EVK-CT , BBW # , Alp-Imp). References Auer, I., and 24 co-authors (2005) A new instrumental precipitation dataset for the greater alpine region for the period Int. J. Climatol. 25: Cook, E.R., Peters, K. (1997) Calculating unbiased tree-ring indices for the study of climatic and environmental change. The Holocene 7: D Arrigo, R.D., Wilson, R.J.S., Jacoby, G. (2006) On the long-term context for late twentieth century warming. J. Geophys. Res. 111: doi /2005JD Esper, J., Cook, E.R., Schweingruber, F.H. (2002) Low-frequency signals in long tree-ring chronologies and the reconstruction of past temperature variability. Science 295: Esper, J., Cook, E.R., Krusic, P.J., Peters, K., Schweingruber, F.H. (2003) Tests of the RCS method for preserving low-frequency variability in long tree-ring chronologies. Tree-Ring Res. 59: Esper, J., Frank, D.C., Wilson, R.J.S., Briffa, K.R. (2005a) Effect of scaling and regression on reconstructed temperature amplitude for the past millennium. Geophys. Res. Lett. 31: doi /2004GL Esper, J., Wilson, R.J.S., Frank, D.C., Moberg, A., Wanner, H., Luterbacher, J. (2005b) Climate past ranges and future changes. Quat. Sci. Rev. 24: Lamprecht, A. (1978) Die Beziehung zwischen Holdichtewerten von Fichten aus subalpinen Lagen des Tirols und Witterungsdaten aus Chroniken im Zeitraum von AD. Diploma Thesis. Univ. Zurich. Luckman, B. H., Wilson, R.J.S. (2005) Summer temperatures in the Canadian Rockies during the last millennium: a revised record. Clim. Dyn. 24:

6 Nicolussi, K. (2002) Zur Verwendung von Holz als Baumaterial im Bereich von Tirol - Ergebnisse dendrochronologischer Untersuchungen. In: Hausbau im Alpenraum: Bohlenstuben und Innenräume. Jahrbuch für Hausforschung 51: Osborn, T.J., Briffa, K.R. (2006) The spatial extent of 20th-century warmth in the context of the past 1200 years. Science 311: Siebenlist-Kerner, V. (1984) Der Aufbau von Jahrringchronologien für Zirbelkiefer, Lärche und Fichte eines alpinen Hochgebirgsstandortes. Dendrochronologia 2: 9-29.