To: Keith Briffa <firstname.lastname@example.org>
Subject: Ph.D. in Sweden
Date: Mon, 19 Jan 1998 06:15:55 +0100
Reply-to: gerner <email@example.com>
I contacted Hakan Grudd last week. He is also positive about a Ph.D. for me
I have tried to make a formulation of a project. Please, read it and let me
know what you think. Maybe the project is overlapping with that of Grudd or
maybe you have better ideas. It could also be that I have misunderstood
I have sent the project formulation to Schweingruber, Grudd and Kalen. I
send it to Schweingruber because I already contacted him last week (before
I got the message from you). He is also interested in the project and
anyway he will get involved if I am going to train in Birmensdorf.
Best regards from:
Description of project
Dendroclimatology can be defined as the use of tree rings to study and
reconstruct past and present climate (Kaennel & Schweingruber, 1995).
Global average surface temperatures have risen by 0.3-0.6 �C since the
middle of the 19th century (Folland et al., 1990). Climatologists seek to
establish the extent to which this rise may be attributable to an enhanced
greenhouse effect and so need to distinguish anthropogenic from 'natural'
climate fluctuations (those that would occur without anthropogenic
influences) to help them make predictions of future climate changes (Briffa
et al., 1996a). Clearly the century-long instrumental record is not long
enough to accomplish this. Paleoclimatic fluctuations older than
meteorological measurements can be inferred from a variety of data sources,
including tree rings, records of vegetation processes (e.g. pollen in lake
sediments), records of ice layer in ice cores, historical records, etc.
(Eddy, 1992). However, within a time frame of the last two millennia
dendroclimatology has shown to be the most powerful tool available to
provide globally distributed, annually resolved paleoenvironmental records
(Luckman, 1996). The growing influence of dendroclimatology in
paleoenvironmental studies can be seen in the fact that almost a third of
Bradley and Jones' volume Climate since AD 1500 (Bradley & Jones, 1992)
deals with dendrochronology and dendroclimatic reconstruction.
Near the polar and altitudinal tree lines, tree growth is mainly dependent
on summer temperature. As northern latitudes are regarded as being strongly
affected by global climate changes, a network of chronologies is
established along the polar tree-line in Eurasia (Briffa et al., 1996b). At
specific locations in these northern high-latitude regions it is possible
to extend the tree-growth record back beyond the life span of living trees
by amalgamating the measurements from overlapping, absolutely-dated series
of measurements made on dead wood from historical or archeological
provenances or naturally surviving above ground, in peat or alluvial
sediments, or preserved in lakes. The first pair of (ring-width and
density) chronologies, made up from samples of Scots pine (Pinus sylvestris
L.) at several locations adjacent to Lake Tornetr�sk, northern Sweden, have
been used to reconstruct summer (April-August) temperatures representing a
large region of northern Fennoscandia from AD 500 to 1980 (Briffa et al.,
1990, 1992). The Fennoscandian temperature records show that marked
high-frequency (interannual-to-century) timescale variability together with
marked long-timescale (multicentury) variations in summer temperatures have
been a characteristic feature in this region during the last millennium.
Similar data from samples of larch (Larix sibirica) on the eastern slopes
of the northern Urals have been used to reconstruct regional summer
(May-September) temperatures representing a region of north-western Siberia
for the period 914 to 1990 (Briffa et al., 1995b). As a part of developing
the north Eurasian chronology network, two projects currently underway aim
to build continuous multimillennial pine ring-width chronologies in
northern Sweden and Finland, spanning 7000-8000 years (Briffa et al.,
1995a). In Russia a similar project underway aim to build larch ring-width
chronologies in Yamal Peninsula, also spanning 7000-8000 years (Shiyatov,
The application of radiodensitometry in the analysis of conifer rings
throughout Europe (Schweingruber, 1985) show the considerable amount of
additional information lying in density, as compared with total ring width.
Obviously, external factors have a more uniform influence on cell wall
growth in latewood (density) than on cambial activity (ring-widths). In
trees of the northern and subalpine timberlines, maximum latewood density
is essentially a measure of mean summer temperature (ibid.).
2. Purpose of this study
2.1. Main objective
The main objective of this study is to provide additional information for a
more precise climate reconstruction based on the already existing
Tornetr�sk-chronology in northern Sweden (AD 500 to 1980) and a future
supra-long chronology (BC 7000 to 1996), based on ring-widths and maximum
latewood density of Scots pine (Pinus sylvestris L.) from the same area.
2.2. Elaboration of the main objective
One of the most fundamental underlying principles in dendroclimatology is
the assumption of uniformitarianism in the response of data to climate
forcing. The uniformitarian principle implies that "the physical and
biological processes which link today's climate with today's variations in
tree growth must have been in operation in the past" (Fritts, 1976).
However, it is a moot point whether the assumption of uniformitarianism
holds when past climate variations are inferred from long chronologies. The
problem arises because the extrapolation always is based on a regression
model calibrated on very short meteorological records. Long chronologies,
as those seen in northern Scandinavia and Siberia, are made up from trees
of different ages growing under more or less uniform conditions. In such
chronologies there must always be uncertainty regarding the long-term
stability of (non-climate) environmental influences or differing climate
sensivity due to the inhomogeneity in the sampled material (Briffa, 1995a,
Briffa et al., 1996a). The climate signals in chronologies may, to some
extent, be affected by:
Inhomogeneity in the site characteristics of the samples (soil
fertility, water holding capacity of the soil, altitude, exposure of slope,
Inhomogeneity in series length of samples (tree age)
Inhomogeneity in tree growth form and population density of samples
Anthropogenic influence (nitrogen deposition, raise in CO2 level)
producing enhanced tree growth in the recent part of the chronology
Series replication in the chronology
The technique used to remove the non-climatic, age-related bias in
individual series (a technique known as standardization in
This study will focus on the influence of point 1-3 on the climate signal
seen in densities of Scots pine from the area of Tornetr�sk in northern
Sweden. It is well-known that the Tornetr�sk-chronology is subject to the
inhomogenity in samples described in point 1-3, but it is not clear to what
extension these inhomogenities affect the climate signal in the chronology.
Thus, a study of the influence of inhomogenity in the samples will provide
valuable additional information for a more precise interpretation of the
summer-temperature record inferred from the already existing
Tornetr�sk-chronology. In the same way it will highly increase the value
and confidence of climate reconstructions from future supra-long
pine-chronologies in this region. The growth parameter under investigation
is maximum latewood density. In this way the study will complement an
ongoing similar study on ring-widths of Scots pine from the same region
2.3. Partial objectives of the study and publications
Methodologically, the project can be divided into three, but overlapping
Building of density pine-chronologies around Tornetr�sk from different
sites. Various site conditions (mainly soil fertility, water holding
capacity of the soil, altitude, and tree population density) and different
age classes must be taken into consideration. No less than 10-12
chronologies must be estimated.
Analysis of climate-growth relationships of the pine-chronologies,
focusing on differences between high-frequency and low-frequency
variability in the climate date. The results are compared and conclusions
are drawn about the diversity of climate signal seen in
density-chronologies from Scots pine growing under various conditions in
the area around Tornetr�sk.
Re-interpretation of the already existing Tornetr�sk-chronology on the
basis of the new information provided by the study in case and the ongoing
similar study of ring-widths from the same region (Grudd, 1998)
The results are published in three articles with the following provisional
"Site-induced differences in climate-growth response of Pinus sylvestris
L." (The article focuses on differences in climate-growth response for
trees growing on different soil types and for trees from stands with
different population density)
"Altitude and age as parameters of climate-growth response in Pinus
sylvestris L." (The article focuses on differences in climate-growth
response for trees growing at different altitudes and trees in different
"Possible site-induced changes in the climate-growth response of the
1,400 year tree-ring chronology from northern Fennoscandia" (A
re-interpretation of the existing Tornetr�sk-chronology is made on the
basis of the new information)
3.1. Sampling strategy
3.1.1. Selection of sites and stands
As already pointed out, various site conditions and different age classes
must be taken into consideration. Site homogeneity largely determines the
quality of the chronology. That is, the factor under investigation which is
assumed to affect the climate-growth response must be constant all over the
site, and other possible affecting factors are minimised. It is important
that the stand have not been similarly damaged by fires, wind, or other
catastrophic factors to extract reliable climatic information. Site
characteristics will be noted (typography/geomorphology, soil conditions,
vegetation description, signs of human impact, etc.).
3.1.2. Selection of trees
Trees should be in a dominant position (with the possible exception of
stand density studies), without irregular growth which probably disturb the
climate signal in the tree-rings. Individual variability in the final
chronology decreases with an increasing number of samples. Consequently,
two cores from at least 12 living trees are necessary to obtain a
site-chronology of sufficient quality. It is best to sample a few more
trees than necessary so that anomalous cores may be discarded. Trees of
different age classes will be cored to allow for systematical studies on
age-related bias in the climate-growth response.
Samples are taken at breast height with an increment borer. The cores are
stored in air-dry conditions after labelling with a pencil. Growth
irregularities (compression wood, wound tissue, etc.) are excluded by
avoiding sampling in the vicinity of wound and of upslope and downslope
sides of trees growing on sloping ground. Cores are taken as nearly
perpendicular to the fibre orientation as possible. This can greatly reduce
the variability owing to technical processing in densitometric studies
(Schweingruber et al., 1990). Core characteristics will be noted (tree
height, stem diameter at breast height, crown size and condition, injuries
and irregular growth, coring direction and height, etc.). Sites and trees
will be documented photographically.
3.2.Sample preparation, measurement, and chronology building
Resins and heartwood substances must be chemically removed as they will
influence on the X-ray absorption (Schweingruber, 1990). This is done
through distillation in Soxhlett device; resins are extracted with alcohol,
heartwood substances with water. After removal of resins and heartwood
substances, laths of equal thickness have to be cut from the round cores.
The Birmensdorf system may be used where the core is glued to a wooden
support with the radial surface uppermost and a 1.25-mm-thick lath cut out
with a small twin-bladed circular saw. To obtain comparable density values,
the moisture content of the wood must be kept constant.
3.2.2. Measurement of density
The irradiation of film can be done with different methods. Two methods,
which have proved to be useful are:
Irradiation of a film (Kodak, Type R, single-coated industrial X-ray film)
resting on the moving stage. The film is transported at five cm/min under
the radiation source, which is 31 cm above, and irradiated at 20kVh and 2mA
Irradiation of a film (Kodak, Type X-Omat TL, double coated medical X-ray
film) resting on a stationary stage at 11 kVh and 20 mA for 90 min. The
source is 250 cm above the film (Nancy system)
The film is developed and the different gray levels produced on the
radiograph by the wood samples are converted to wood density values. The
basic instrument used is the densitometer (ibid). Analog or digital
processing of the actual measurements produces a density profile from which
the desired parameter (maximum density) is registered.
3.2.3. Dating and chronology building
For dating, chronology building and quality control, the program COFECHA
(Holmes et al., 1986) may be used. In addition a manual dating control has
to be done at the light table or monitor, comparing each curve with an
existing master chronology. The procedure ensures precise dating of every
3.3. Data processing
3.3.1. Standardization of tree-ring data
Before averaging tree-ring curves to mean chronologies which shall be used
for dendroclimatological purposes, the raw values must be standardized to
index values. In the same process, one has to remove the natural age trend
of trees and eventual density variations caused by stand dynamics, and not
representing climate. Also in this process, it is crucial to control the
effect of detrending at the light table or on the monitor, comparing the
original with the detrended curve. Much depends from this process, as the
dendrochronologist here decides which portion of low frequency variation
that is removed from the series. This in turn affects climate information
inferred from the chronology. Therefore, several detrending methods have to
be tested in this study.
3.3.2. Computing climate-growth response
Climate-growth models will be computed for all individual chronologies. The
period selected for climate-growth modelling, is the period for which
climate data are available (the earliest series start in AD ??). Different
techniques are existing for estimation of the climate-growth response. For
example, simple correlation analysis may be used or a regression-technique
based on principal component analysis. It may be relevant to detect
non-linear relationships between climate variables and ring growth, as well
as to study single years with special tree-ring (pointer years) and climate
events. To detect changes in climate-response over time the Kalman filter
can be used.
4. Time schedule
The project will be performed during three years (June 1998 to June 2001).
The Ph.D. student will follow courses corresponding to 40 weeks of studies.
>From earlier working, the following assumptions regarding time consume for
field work and measuring can be made: It can take a number of days to
become familiar with the localities and to find the most suitable pine
stands. At each site, one to two days are needed for sampling and site
description, provided that the pines do not stand too scattered, and long
walking distances can be avoided. Time for measuring and chronology
building should be estimated rather high (2-3 weeks per site).
Preparing of a detailed sampling strategy for the whole project (2 weeks)
and field work (6 weeks). The field work will focus on sampling of trees
from about six sites with varying conditions (soil fertility and water
Training in use of densitometry equipment at the institute of Forest, Snow
and Landscape in Birmensdorf, Switzerland. Measurement of samples collected
in the summer.
Continued measuring of samples at the university in Stockholm. Systematical
analysis of standardization methods and construction of six site
chronologies. Start of analysing climate-growth response in chronologies.
Field work (6 weeks) which will put focus on sampling trees from about six
sites in different altitudes and with different stand densities.
Measuring of the summer's material at the university in Stockholm.
Systematical analysis of standardization methods and construction of six
new site chronologies. Analysing climate-growth response in chronologies.
Analysing climate-growth response in all chronologies. Preparation of
Analysing age-related climate-response. Preparation of publication (b).
Comparison of results with similar study on ring-widths (Grudd, 1998).
Last statistics, preparation of publication (c), preparation of
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Schweingruber, F.H. & Zetterberg, P. (1990). A 1,400-year tree-ring record
of summer temperatures in Fennoscandia. Nature. 346: 434-439.
Briffa, K.R., Jones, P.D., Bartholin, T.S., Eckstein, D., Schweingruber,
F.H., Karl�n, W., Zetterberg, P. & Eronen, M. (1992). Fennoscandian summers
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Marathonvej 21, 1. door 5
2300 Copenhagen S
Tel: (+45) 3159 6095
Fax: (+45) 3155 9409