Quaternary Science Reviews xxx (2010) 1e3
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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).
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).
0277-3791/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.quascirev.2010.05.018
Please cite this article in press as: Tegel, W., et al., Updating historical tree-ring records for climate reconstruction, Quaternary Science Reviews
(2010), doi:10.1016/j.quascirev.2010.05.018
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W. Tegel et al. / Quaternary Science Reviews xxx (2010) 1e3
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.
Please cite this article in press as: Tegel, W., et al., Updating historical tree-ring records for climate reconstruction, Quaternary Science Reviews
(2010), doi:10.1016/j.quascirev.2010.05.018
W. Tegel et al. / Quaternary Science Reviews xxx (2010) 1e3
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).
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Please cite this article in press as: Tegel, W., et al., Updating historical tree-ring records for climate reconstruction, Quaternary Science Reviews
(2010), doi:10.1016/j.quascirev.2010.05.018