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Updating historical tree-ring records for climate reconstruction
Quaternary Science Reviews 29 (2010) 1957e1959
Contents lists available at ScienceDirect
Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev
Viewpoint
Updating historical tree-ring records for climate reconstruction Willy Tegel a, Jan Vanmoerkerke b, Ulf Büntgen c, d, * a
Institute for Forest Growth (IWW), University of Freiburg, 79106 Freiburg, Germany Regional Archaeological Service (DRAC/S.R.A.), 51037 Châlons-en-Champagne, France c Swiss Federal Research Institute (WSL), 8903 Birmensdorf, Switzerland d Oeschger Centre for Climate Change Research (OCCR), University of Bern, 3012 Bern, Switzerland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 January 2010 Received in revised form 13 May 2010 Accepted 13 May 2010
Over the past three decades, numerous Late Holocene-long tree-ring (TR) chronologies have been developed for different parts of Europe that allow archaeological, historical and cultural wood remains to be dated with annual precision. Ironically, palaeoclimatic evidence inherent in such composites is limited as modern updates essential for calibration/verification with instrumental measurements are often inappropriate, incomplete or even missing. Here we proposes a novel approach to updating historical TR records while preventing statistical over-fit with the target data and advocate ‘horizontal’ splitting between historical (early) and recent (modern) TR samples prior to their standardization (detrending). This split-technique will help to overcoming unprecedented effects of increased atmospheric greenhouse-gas, biospheric fertilization, forest management, sample replication, age-structure and chronology development associated with modern proxy updates. Ó 2010 Elsevier Ltd. All rights reserved.
Supra-long chronologies of annually resolved TR measurements that span time-scales from a millennium to the Holocene are restricted to Central Europe and describe a unique dating tool, not only for archaeological artefacts and historical construction wood, but also for antique artwork, instruments and furniture (see Haneca et al., 2009 for a review). Such records are mainly composed of oak (Quercus spec.) wood e Quercus robur L. and Quercus petraea (Matt.) Liebl. are not anatomically distinguishable. The material contains trees from archaeological, sub-fossil and historical surveys originating from lower elevation temperate forests north of the Alps and south of the Baltic. Spatially explicit boundaries remain difficult to define as sample sources are often scattered over wide geographic areas and site control appears to be generally low (Laurelut et al., 2009). Roughly a dozen Late Holocene records have been developed over the past 30 years or so (e.g., Becker and Delorme, 1978), with the ‘Southern German Oak Chronology’ continuously extending back to BC8480 (Friedrich et al., 2004). Sample size fluctuates between hundreds and thousands of series in Roman, Medieval and Modern times, but drops to a few individual series during the so-called transition periods shortly before and after Roman times. Data extension towards the present, if at all given,
* Corresponding author. Swiss Federal Research Institute (WSL), 8903 Birmensdorf, Switzerland. Tel.: þ41 44 739 2679; fax: þ41 44 739 2215. E-mail address: [email protected] (U. Büntgen). 0277-3791/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2010.05.018
relies upon a handful of tree cores necessary to ensure calendar dating of the historical material. At the same time such composites can be of exceptional palaeoclimatic value: various studies have proven chronologies of living (Friedrichs et al., 2009a,b), as well as living and historical oaks (Kelly et al., 1989, 2002; Cufar et al., 2008) to significantly correlate with variations in summer precipitation/drought. Assessment of their ‘true’ climatic signal, i.e., high- to low-frequency variability, however depends on sufficient overlap with instrumental measurements allowing robust calibration/verification trials to be performed (Frank et al., 2007). Büntgen et al. (2010) demonstrated the potential of a millennium-long German oak network to separate inter-annual to multi-decadal climatic from non-climatic information, but simultaneously stressed methodological constraints in preserving longer-term trends from TR composites (e.g., Esper et al., 2003; Büntgen et al., 2005, 2006, 2008a; Helama et al., 2005). Additional bias towards the most recent end of historical proxy records can emerge from exceptional changes in concentrations of atmospheric greenhouse-gas, levels of biospheric fertilization, the amount of forest management and degree of habitat opening (Kaplan et al., 2009), as well as chronology replication and tree-age structure (Cook and Peters, 1997). Such conditions associated with the 20th century would directly affect tree growth (Büntgen et al., 2008b) and subsequently complicate any long-term comparison with the backdrop of natural variability (e.g., Salzer et al., 2009).
1958
W. Tegel et al. / Quaternary Science Reviews 29 (2010) 1957e1959
To make use of the enormous potential of supra-long European TR records for climate reconstruction, a new strategy to solve the ‘update desideratum’ of modern site bias e adaptation of the recent to the historical data e is herein introduced. We present a unique compilation of 2813 archaeological, subfossil and historical oak samples, collected at approximately 200 sites across Northeast France (Champagne, Lorraine, Vosges) during the past 15 years (Fig. 1). The annual-resolved chronology reaches back into the Iron Age. Sample replication considerably decreases before c. BC400 and after c. 1900AD, hampering any robust conclusions to be drawn prior to the Roman period and for the 20th century, during which instrumental station measurements are most reliable. To allow growth/climate relationships to be analyzed, we added 67 samples of modern oak timbers following the new update convention. Oak beams and timbers were randomly sampled at 10 different sawmills and lumberyards scattered over Northeast France from where the historical data derived. Due to this artificial signal-degradation, the amount of site control and ecological understanding of the modern samples is designed to be as low as it is for the historical subset; the chronology internal signal-to-noise ratio remains equal throughout time. A total of 2880 TR series represents various wood archives, covers the BC450e2009AD period, and shares a reasonable fraction of common variance over most of the last 2500 years (Fig. 1). The oldest samples from the Iron Age and Roman era mainly originate from archaeological excavations of buildings, strongholds, water supplies and other infrastructures; whereas younger samples following the collapse of the Roman Empire generally represent sub-fossil trees from rivers, gravel pits and sinkholes. Construction timbers are the major source not only for medieval to modern samples, but also for the novel update. The resulting composite chronology is characterized by an even distribution of series start and end dates (Fig. 1). The mean segment length is 91 years with a minimum of 6 years and a maximum of 314 years. The average increment is 1.76 mm, with small differences between the historical
and recent samples (1.76 and 1.71 mm). The inter-series correlation (RBAR) and the Expressed Population Signal (EPS; Wigley et al., 1984) display similar degrees of internal coherency for the historical and recent subsets (Fig. 1). EPS values constantly range above the frequently applied quality threshold of 0.85 over most of the past 2500 years, whereas RBAR values fluctuate around 0.3. Exceptionally low EPS and high RBAR values in the 4th, 7th and 10th centuries refer to periods of reduced sample size, which are most likely related to the collapse of the Roman Empire and the change from the Merovingian to the Carolingian dynasties. These transition periods, for which almost no wooden archaeological findings exist, were characterized by economic and political instability. Synthetic RBAR inflation known to arise from the insertion of multiple radii from the same few samples can be excluded. Correlation with gridded summer precipitation/drought indices (CRUTS3v; Mitchell and Jones, 2005; van der Schrier et al., 2006) computed over the 1901e2002 period and 5e7 E and 48e50 N region is 0.38/0.46. Correlation with precipitation totals measured at the Nancy station (48.70 N, 6.20 E, 217 m asl, 7180 WMO-code) slightly increases to 0.50. These correlations have been obtained from an artificially degraded random sample collection not optimized to reveal highest relationships with climate forcing. The resulting response patterns, however, imply that regional variations in summer precipitation control radial oak growth. Accordingly, our approach e adaptation of the recent to the historical data e prevents from statistical over-fitting during the proxy/target calibration interval. The observed relationship between 20th century oak growth and summer precipitation/ drought variability should further hold during the record’s historical portion, because internal chronology characteristics have been found to be similar for the modern and the historical subsets. Nevertheless, uncertainties remain. These include a diminished climate signal, imprecise knowledge of sample location at the tree level and site control at the network level, as well as significant changes in sample replication. Complex climate forcing of temperate
Fig. 1. Summary plot of the French oak data: Spatial and temporal sample distribution, examples of archaeological, historical (red) and recent (green) wood sources and 30 y moving EPS and RBAR statistics.
W. Tegel et al. / Quaternary Science Reviews 29 (2010) 1957e1959
forest growth (Nemani et al., 2003) is amplified by the random update strategy in comparison to traditional ‘dendroclimatic’ approaches of careful site selection, making it obvious that much more data is needed. Pith and the outermost rings are not always preserved and the position where the sample has been extracted from the tree is mostly unknown. Germination and tree age however, can be estimated. Knowledge about the origin of construction timbers generally ceases back in time, although some scattered historical documents can contain useful information. Nevertheless, the mountainous landscape and small river system of the study area limited wood transportation and floating over longer distances, and the oak net-weight hinders any floating activity. Changing forest management strategies, which ought to cause nonclimatic noise (Haneca et al., 2009), appear to be of minor importance as sample size is quite large over most of the past 2500 years or so, limiting local-scale effects of forest management and population differences in the chronology. Regional-scale wood provenance of our oak compilation thus constrains the ecological boundary of past forest sites. Besides past intervals of low sample size, mainly associated with cultural transition periods, there remains an unnecessary replication drop at the transition from historical to recent data. Despite of gathering information on building history, sampling of relevant construction timbers could easily be performed and would subsequently fall between recent and historical material. Additional uncertainty emerges from the fact that modern conditions of Late Holocene-long TR chronologies differ substantially from any other part of the historical record. This concern is based on increased concentrations of atmospheric greenhouse-gas, amplified levels of biosphere fertilization, intensified forest management, opened growth habitats (Kaplan et al., 2009), and methodological end-effect problems in chronology development (Cook and Peters, 1997; Büntgen et al., 2008b; Melvin and Briffa, 2008), thus obliging horizontal data splitting. Areas of research have been necessarily focused upon to exploit the huge palaeoclimatic potential of historical TR data and conclude to: i) update via a random sampling that is most representative of the historical data, ii) consider construction timbers from the 19th and 20th century to bridge the modern gap and iii) correct recent biases. The new update strategy is of relevance not only for TR records, but also for other proxy archives that rely upon calibration. Acknowledgements We are thankful to J. Esper, D. Frank and U. Heussner for discussion. Supported by the European Union Project MILLENNIUM (#017008-GOCE) and the SNSF (NCCR-Climate). References Becker, B., Delorme, A., 1978. Oak chronologies for Central Europe: their extension from medieval to prehistoric times. BAR International Series 51, 59e64. Büntgen, U., Esper, J., Frank, D.C., Nicolussi, K., Schmidhalter, M., 2005. A 1052-year tree-ring proxy for Alpine summer temperatures. Climate Dynamics 25, 141e153.
1959
Büntgen, U., Frank, D.C., Nievergelt, D., Esper, J., 2006. Summer temperature variations in the European Alps, AD 755e2004. Journal of Climate 19/2, 5606e5623. Büntgen, U., Frank, D.C., Grudd, H., Esper, J., 2008a. Long-term summer temperature variations in the Pyrenees. Climate Dynamics 31, 615e631. Büntgen, U., Frank, D.C., Wilson, R., Career, M., Urbinati, C., Esper, J., 2008b. Testing for tree-ring divergence in the European Alps. Global Change Biology 14, 2443e2453. Büntgen, U., Trouet, V., Frank, D., Leuschner, H.H., Friedrichs, D., Luterbacher, J., Esper, J., 2010. Tree-ring indicators of German summer drought over the last millennium. Quaternary Science Reviews 29, 1005e1016. Cook, E.R., Peters, K., 1997. Calculating unbiased tree-ring indices for the study of climatic and environmental change. The Holocene 7, 361e370. Cufar, K., De Luis, M., Eckstein, D., Kajfez-Bogataj, L., 2008. Reconstructing dry and wet summers in SE Slovenia from oak tree-ring series. International Journal of Biometeorology 52, 607e615. 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 Research 59, 81e98. Frank, D., Büntgen, U., Böhm, R., Maugeri, M., Esper, J., 2007. Warmer early instrumental measurements versus colder reconstructed temperatures: shooting at a moving target. Quaternary Science Reviews 26, 3298e3310. Friedrich, M., Remmele, S., Kromer, B., Hofmann, J., Spurk, M., Felix, K.K., Orcel, C., Küppers, M., 2004. The 12.460-year Hohenheim oak and pine tree-ring chronology from Central Europe e a unique annual record for radiocarbon calibration and palaeoenvironment reconstruction. Radiocarbon 46, 1111e1122. Friedrichs, D., Büntgen, U., Esper, J., Frank, D., Neuwirth, B., Löffler, J., 2009a. Complex climate controls on 20th century oak growth in Central-West Germany. Tree Physiology 29, 39e51. Friedrichs, D., Trouet, V., Büntgen, U., Frank, D.C., Esper, J., Neuwirth, B., Löffler, J., 2009b. Twentieth century climate sensitivity of Central European tree species. Trees, Structure and Function 23, 729e739. Haneca, K., Cufar, K., Beeckman, H., 2009. Oaks, tree-rings and wooden cultural heritage: a review of the main characteristics and applications of oak dendrochronology in Europe. Journal of Archaeological Science 36, 1e11. Helama, S., Timonen, M., Lindholm, M., Merilainen, J., Eronen, M., 2005. Extracting long-period climate fluctuations from tree-ring chronologies over timescales of centuries to millennia. International Journal of Climatology 25, 1767e1779. Kaplan, J.O., Krumhardt, K.M., Zimmermann, N., 2009. The prehistoric and preindustrial deforestation of Europe. Quaternary Science Reviews 28, 3016e3034. Kelly, P.M., Munro, M.A.R., Hughes, M.K., Goodess, C.M., 1989. Climate and signature years in west European oaks. Nature 340, 57e60. Kelly, P.M., Leuschner, H.H., Briffa, K.R., Harris, I.C., 2002. The climatic interpretation of pan-European signature years in oak ring-width series. The Holocene 12, 689e694. Laurelut, C., Tegel, W., Vanmoerkerke, J., 2009. Neue Aspekte zur Architekturgeschichte von der späten Eisenzeit bis zum Beginn der Römerzeit in Ostfrankreich. In: Architektur, Interpretation und Rekonstruktion. Beiträge zur Sitzung der AG Eisenzeit während des 6. Deutschen Archäologie-Kongresses in Mannheim 2008 (55), 79e95. Melvin, T.M., Briffa, K.R., 2008. A “Signal-Free” approach to Dendroclimatic Standardization. Dendrochronologia 26, 71e86. Mitchell, T.D., Jones, P.D., 2005. An improved method of constructing a database of monthly climate observations and associated high-resolution grids. International Journal of Climatology 25, 693e712. Nemani, R.R., Keeling, C.D., Hashimoto, H., Jolly, W.M., Piper, S.C., Tucker, C.J., Myneni, R.B., Running, S.W., 2003. Climate-driven increases in global terrestrial net primary production from 1982 to 1999. Science 300, 1560e1563. Salzer, M.W., Hughes, M.K., Bunn, A.G., Kipfmueller, K.F., 2009. Recent unprecedented tree-ring growth in bristlecone pine at the highest elevations and possible causes. Proceedings of the National Academy of Sciences of the United States of America 106, 20348e20353. van der Schrier, G., Briffa, K.R., Jones, P.D., Osborn, T.J., 2006. Summer moisture variability across Europe. Journal of Climate 19, 2818e2834. Wigley, T.M.L., Briffa, K.R., Jones, P.D., 1984. On the average of value of correlated time series, with applications in dendroclimatology and hydrometeorology. Journal of Climatology and Applied Meteorology 23, 201e213.
Contents lists available at ScienceDirect
Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev
Viewpoint
Updating historical tree-ring records for climate reconstruction Willy Tegel a, Jan Vanmoerkerke b, Ulf Büntgen c, d, * a
Institute for Forest Growth (IWW), University of Freiburg, 79106 Freiburg, Germany Regional Archaeological Service (DRAC/S.R.A.), 51037 Châlons-en-Champagne, France c Swiss Federal Research Institute (WSL), 8903 Birmensdorf, Switzerland d Oeschger Centre for Climate Change Research (OCCR), University of Bern, 3012 Bern, Switzerland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 January 2010 Received in revised form 13 May 2010 Accepted 13 May 2010
Over the past three decades, numerous Late Holocene-long tree-ring (TR) chronologies have been developed for different parts of Europe that allow archaeological, historical and cultural wood remains to be dated with annual precision. Ironically, palaeoclimatic evidence inherent in such composites is limited as modern updates essential for calibration/verification with instrumental measurements are often inappropriate, incomplete or even missing. Here we proposes a novel approach to updating historical TR records while preventing statistical over-fit with the target data and advocate ‘horizontal’ splitting between historical (early) and recent (modern) TR samples prior to their standardization (detrending). This split-technique will help to overcoming unprecedented effects of increased atmospheric greenhouse-gas, biospheric fertilization, forest management, sample replication, age-structure and chronology development associated with modern proxy updates. Ó 2010 Elsevier Ltd. All rights reserved.
Supra-long chronologies of annually resolved TR measurements that span time-scales from a millennium to the Holocene are restricted to Central Europe and describe a unique dating tool, not only for archaeological artefacts and historical construction wood, but also for antique artwork, instruments and furniture (see Haneca et al., 2009 for a review). Such records are mainly composed of oak (Quercus spec.) wood e Quercus robur L. and Quercus petraea (Matt.) Liebl. are not anatomically distinguishable. The material contains trees from archaeological, sub-fossil and historical surveys originating from lower elevation temperate forests north of the Alps and south of the Baltic. Spatially explicit boundaries remain difficult to define as sample sources are often scattered over wide geographic areas and site control appears to be generally low (Laurelut et al., 2009). Roughly a dozen Late Holocene records have been developed over the past 30 years or so (e.g., Becker and Delorme, 1978), with the ‘Southern German Oak Chronology’ continuously extending back to BC8480 (Friedrich et al., 2004). Sample size fluctuates between hundreds and thousands of series in Roman, Medieval and Modern times, but drops to a few individual series during the so-called transition periods shortly before and after Roman times. Data extension towards the present, if at all given,
* Corresponding author. Swiss Federal Research Institute (WSL), 8903 Birmensdorf, Switzerland. Tel.: þ41 44 739 2679; fax: þ41 44 739 2215. E-mail address: [email protected] (U. Büntgen). 0277-3791/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2010.05.018
relies upon a handful of tree cores necessary to ensure calendar dating of the historical material. At the same time such composites can be of exceptional palaeoclimatic value: various studies have proven chronologies of living (Friedrichs et al., 2009a,b), as well as living and historical oaks (Kelly et al., 1989, 2002; Cufar et al., 2008) to significantly correlate with variations in summer precipitation/drought. Assessment of their ‘true’ climatic signal, i.e., high- to low-frequency variability, however depends on sufficient overlap with instrumental measurements allowing robust calibration/verification trials to be performed (Frank et al., 2007). Büntgen et al. (2010) demonstrated the potential of a millennium-long German oak network to separate inter-annual to multi-decadal climatic from non-climatic information, but simultaneously stressed methodological constraints in preserving longer-term trends from TR composites (e.g., Esper et al., 2003; Büntgen et al., 2005, 2006, 2008a; Helama et al., 2005). Additional bias towards the most recent end of historical proxy records can emerge from exceptional changes in concentrations of atmospheric greenhouse-gas, levels of biospheric fertilization, the amount of forest management and degree of habitat opening (Kaplan et al., 2009), as well as chronology replication and tree-age structure (Cook and Peters, 1997). Such conditions associated with the 20th century would directly affect tree growth (Büntgen et al., 2008b) and subsequently complicate any long-term comparison with the backdrop of natural variability (e.g., Salzer et al., 2009).
1958
W. Tegel et al. / Quaternary Science Reviews 29 (2010) 1957e1959
To make use of the enormous potential of supra-long European TR records for climate reconstruction, a new strategy to solve the ‘update desideratum’ of modern site bias e adaptation of the recent to the historical data e is herein introduced. We present a unique compilation of 2813 archaeological, subfossil and historical oak samples, collected at approximately 200 sites across Northeast France (Champagne, Lorraine, Vosges) during the past 15 years (Fig. 1). The annual-resolved chronology reaches back into the Iron Age. Sample replication considerably decreases before c. BC400 and after c. 1900AD, hampering any robust conclusions to be drawn prior to the Roman period and for the 20th century, during which instrumental station measurements are most reliable. To allow growth/climate relationships to be analyzed, we added 67 samples of modern oak timbers following the new update convention. Oak beams and timbers were randomly sampled at 10 different sawmills and lumberyards scattered over Northeast France from where the historical data derived. Due to this artificial signal-degradation, the amount of site control and ecological understanding of the modern samples is designed to be as low as it is for the historical subset; the chronology internal signal-to-noise ratio remains equal throughout time. A total of 2880 TR series represents various wood archives, covers the BC450e2009AD period, and shares a reasonable fraction of common variance over most of the last 2500 years (Fig. 1). The oldest samples from the Iron Age and Roman era mainly originate from archaeological excavations of buildings, strongholds, water supplies and other infrastructures; whereas younger samples following the collapse of the Roman Empire generally represent sub-fossil trees from rivers, gravel pits and sinkholes. Construction timbers are the major source not only for medieval to modern samples, but also for the novel update. The resulting composite chronology is characterized by an even distribution of series start and end dates (Fig. 1). The mean segment length is 91 years with a minimum of 6 years and a maximum of 314 years. The average increment is 1.76 mm, with small differences between the historical
and recent samples (1.76 and 1.71 mm). The inter-series correlation (RBAR) and the Expressed Population Signal (EPS; Wigley et al., 1984) display similar degrees of internal coherency for the historical and recent subsets (Fig. 1). EPS values constantly range above the frequently applied quality threshold of 0.85 over most of the past 2500 years, whereas RBAR values fluctuate around 0.3. Exceptionally low EPS and high RBAR values in the 4th, 7th and 10th centuries refer to periods of reduced sample size, which are most likely related to the collapse of the Roman Empire and the change from the Merovingian to the Carolingian dynasties. These transition periods, for which almost no wooden archaeological findings exist, were characterized by economic and political instability. Synthetic RBAR inflation known to arise from the insertion of multiple radii from the same few samples can be excluded. Correlation with gridded summer precipitation/drought indices (CRUTS3v; Mitchell and Jones, 2005; van der Schrier et al., 2006) computed over the 1901e2002 period and 5e7 E and 48e50 N region is 0.38/0.46. Correlation with precipitation totals measured at the Nancy station (48.70 N, 6.20 E, 217 m asl, 7180 WMO-code) slightly increases to 0.50. These correlations have been obtained from an artificially degraded random sample collection not optimized to reveal highest relationships with climate forcing. The resulting response patterns, however, imply that regional variations in summer precipitation control radial oak growth. Accordingly, our approach e adaptation of the recent to the historical data e prevents from statistical over-fitting during the proxy/target calibration interval. The observed relationship between 20th century oak growth and summer precipitation/ drought variability should further hold during the record’s historical portion, because internal chronology characteristics have been found to be similar for the modern and the historical subsets. Nevertheless, uncertainties remain. These include a diminished climate signal, imprecise knowledge of sample location at the tree level and site control at the network level, as well as significant changes in sample replication. Complex climate forcing of temperate
Fig. 1. Summary plot of the French oak data: Spatial and temporal sample distribution, examples of archaeological, historical (red) and recent (green) wood sources and 30 y moving EPS and RBAR statistics.
W. Tegel et al. / Quaternary Science Reviews 29 (2010) 1957e1959
forest growth (Nemani et al., 2003) is amplified by the random update strategy in comparison to traditional ‘dendroclimatic’ approaches of careful site selection, making it obvious that much more data is needed. Pith and the outermost rings are not always preserved and the position where the sample has been extracted from the tree is mostly unknown. Germination and tree age however, can be estimated. Knowledge about the origin of construction timbers generally ceases back in time, although some scattered historical documents can contain useful information. Nevertheless, the mountainous landscape and small river system of the study area limited wood transportation and floating over longer distances, and the oak net-weight hinders any floating activity. Changing forest management strategies, which ought to cause nonclimatic noise (Haneca et al., 2009), appear to be of minor importance as sample size is quite large over most of the past 2500 years or so, limiting local-scale effects of forest management and population differences in the chronology. Regional-scale wood provenance of our oak compilation thus constrains the ecological boundary of past forest sites. Besides past intervals of low sample size, mainly associated with cultural transition periods, there remains an unnecessary replication drop at the transition from historical to recent data. Despite of gathering information on building history, sampling of relevant construction timbers could easily be performed and would subsequently fall between recent and historical material. Additional uncertainty emerges from the fact that modern conditions of Late Holocene-long TR chronologies differ substantially from any other part of the historical record. This concern is based on increased concentrations of atmospheric greenhouse-gas, amplified levels of biosphere fertilization, intensified forest management, opened growth habitats (Kaplan et al., 2009), and methodological end-effect problems in chronology development (Cook and Peters, 1997; Büntgen et al., 2008b; Melvin and Briffa, 2008), thus obliging horizontal data splitting. Areas of research have been necessarily focused upon to exploit the huge palaeoclimatic potential of historical TR data and conclude to: i) update via a random sampling that is most representative of the historical data, ii) consider construction timbers from the 19th and 20th century to bridge the modern gap and iii) correct recent biases. The new update strategy is of relevance not only for TR records, but also for other proxy archives that rely upon calibration. Acknowledgements We are thankful to J. Esper, D. Frank and U. Heussner for discussion. Supported by the European Union Project MILLENNIUM (#017008-GOCE) and the SNSF (NCCR-Climate). References Becker, B., Delorme, A., 1978. Oak chronologies for Central Europe: their extension from medieval to prehistoric times. BAR International Series 51, 59e64. Büntgen, U., Esper, J., Frank, D.C., Nicolussi, K., Schmidhalter, M., 2005. A 1052-year tree-ring proxy for Alpine summer temperatures. Climate Dynamics 25, 141e153.
1959
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