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Dwarf tundra shrubs growth as a proxy for late Holocene climate change


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The Arctic is the most sensitive zone to climate changes and the impacts are reflected in local ecosystems. In order to extract information of the past from proxy archives the detailed knowledge of such archive is crucial. The paper summarizes modern approaches of tundra dwarf shrub research for the purposes of paleoclimatology. Dwarf tundra shrubs as still relatively untapped archive are believed to contain valuable proxy data in their annual growth increments. Field sampling, and laboratory work are reviewed in detail. Constraints of dwarf tundra shrub research are discussed as well. The relationship between climate and growth is addressed to find a link between them depending on location and species. Majority of investigations found the strongest relationship between summer temperatures and ring widths, although exceptions are not rare. Dwarf tundra shrubs can fully serve as valuable proxy archive only if those are understood. Finally, the factors influencing the length of dwarf tundra shrub life are studied in order to sample the oldest living individuals in the field. Despite the field collection should aim to sample various sizes and ages of plants to make the dataset robust, the longest living individuals which are important to prolong chronologies are usually inhabiting rather nutrient poor and undisturbed sites close to their survival limits. The paper indicates the most suitable dwarf tundra shrub research designs for the purposes of paleoclimatology. As such it can help to harvest the benefits of dendrochronology from the vast and new territories.
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CZECH POLAR REPORTS 5 (2): 185-199, 2015
Received November 12, 2015, accepted January 20, 2016.
*Corresponding author: Jiří Lehejček <>
Acknowledgements: Support was provided by INTERACT (grant agreement N°262693) under the
European Union’s Seventh Framework Programme, Internal Grant Agency of Czech University of
Life Sciences Prague, Project No. 20154304, and by Internal Grant Agency of Faculty of Forestry
and Wood Sciences, Czech University of Life Sciences, Project No. B02/15). Many thanks belong
to Liam A. D. Harrap (Canada) for language editing and his advice that helped to improve the
Dwarf tundra shrubs growth as a proxy for late Holocene
climate change
Jiří Lehejček*
Department of Forest Ecology, Faculty of Forestry and Wood Science, Czech University
of Life Sciences in Prague, Kamýcká 1176, Prague 6 – Suchdol, Czech Republic
The Arctic is the most sensitive zone to climate changes and the impacts are reflected in
local ecosystems. In order to extract information of the past from proxy archives the
detailed knowledge of such archive is crucial. The paper summarizes modern approaches
of tundra dwarf shrub research for the purposes of paleoclimatology. Dwarf tundra
shrubs as still relatively untapped archive are believed to contain valuable proxy data in
their annual growth increments. Field sampling, and laboratory work are reviewed in
detail. Constraints of dwarf tundra shrub research are discussed as well. The relationship
between climate and growth is addressed to find a link between them depending on
location and species. Majority of investigations found the strongest relationship between
summer temperatures and ring widths, although exceptions are not rare. Dwarf tundra
shrubs can fully serve as valuable proxy archive only if those are understood. Finally, the
factors influencing the length of dwarf tundra shrub life are studied in order to sample
the oldest living individuals in the field. Despite the field collection should aim to
sample various sizes and ages of plants to make the dataset robust, the longest living
individuals which are important to prolong chronologies are usually inhabiting rather
nutrient poor and undisturbed sites close to their survival limits. The paper indicates the
most suitable dwarf tundra shrub research designs for the purposes of paleoclimatology.
As such it can help to harvest the benefits of dendrochronology from the vast and new
Key words: climate archive, lifespan, dendroclimatology, wood anatomy, the Arctic
DOI: 10.5817/CPR2015-2-16
Since tundra shrubs occur mainly in re-
mote and areas (such as the Arctic or high
mountains) where our understanding of
climate is biased due to scarcity of direct
observation (Atkinson et Gajewski 2002,
Rayback et Henry 2005, Bär et al. 2006)
the knowledge gained from such high-
resolution archives can extend our under-
stating of the climate in recent centuries
(Woodcock et Bradley 1994). The pres-
ence of annual rings in dwarf shrubs (also
called alpine/arctic dwarf shrubs) has been
first documented in the beginning of the
20th century (Rosenthal 1904, Kanngiesser
1906, 1909, 1914). One of the first authors
who recognized their potential for paleo-
ecological purposes was Warren-Wilson
(1964) who studied annual growth of Salix
arctica. Since then the significance of den-
droecological studies on nontree woody
life forms has been growing expressively.
However, many decades have passed be-
fore the first reconstructing studies (still
dealing with numerous problems) could be
obtained (Shaver 1986, Woodcock et
Bradley 1994). Tundra shrub potential to
learn more about impact of environmental/
climate change started to be discussed by
some authors (Büntgen et Schweingruber
2010, Hallinger et al. 2010). Studies of
dwarf tundra shrubs from a dendroclimatic
point of view have nevertheless received
increasing attention only recently (Schmidt
et al. 2006, Myers-Smith et al. 2011,
Buchwal et al. 2013, Myers-Smith et al.
Potential problem of shrub research is
on site identification of old individuals for
collection to extend the chronology. Life
span of dwarf tundra shrubs is species-
specific but in general they live between
ten to two hundred years maximally (for
details see Schweingruber et Poschlod
2005). Recently, the individuals of Juni-
perus communis reaching almost 700 yrs
were collected in the northern coast of
Kola peninsula (Lehejček, unpublished
work). The extremely short growing sea-
son, cold temperatures, and often also min-
eral nutrient stress in the arctic/alpine re-
gion result in extending individual life
span (Ward 1982, Körner 2003). It makes
those life forms convenient for paleo-
ecological reconstructions especially in the
Arctic and alpine environments where they
can fill observational gaps (AICA 2005).
Nonetheless, the study which evaluates the
findings from different environments in or-
der to detect the main general climate driv-
er of shrub growth is still missing.
The investigation of dwarf tundra
shrubs growth can in spite of limitations
expand the current dendrochronological
network into new extreme environments
beyond the survival limits of trees (Bünt-
gen et al. 2015). Therefore, the abilities of
shrubs climate proxies can together with
conventional tree ring research portray cli-
mate changes across most of the terrestrial
world (Schweingruber 1996). It is also not
without interest that shrub vegetation in
certain parts of the Arctic almost doubled
its extent over the last 50 years (Bunn et
al. 2007). This fact shows that arctic shrubs
are sensitive indicators of climate and
react abruptly to changes. Moreover, the
current rate of warming in the Arctic is
about 0.5°C per decade which is five time
faster than global average (Serreze et al.
2000, ACIA 2005). Thus, the Arctic is a
great environment for studying such
changes assuming that similar magnitude
of shifts occurred in the past as well.
The general objective of the first part of
the paper is to present comprehensive infor-
mation on field, laboratory, and analytical
techniques of dwarf tundra shrub research
for paleoecological, especially paleoclima-
tological purposes. Second part of this
review deals with the influence of climate
and site characteristics on length of plant’s
Review question of this paper state:
What are the main climate drivers of dwarf
shrub growth? Is it temperature, precipi-
tation, or season, or is it species and
Answering these questions can help to
understand signals given by growth of
dwarf tundra shrubs and can significantly
improve our knowledge of the past envi-
ronment in remote regions.
Material and Methods for dwarf tundra shrubs investigation
For the purposes of dendroclimatology
the specific sampling design is needed for
each study. Here, we provide several ad-
vices which should be followed in order to
receive the strongest climate signal from
our samples.
It is strongly recommended not to sam-
ple too early in the growing season, before
the leaves are developed, since it is then
difficult to distinguish dead individuals
from living (Zalatan et Gajewski 2006).
One should also keep in mind during
field sampling the potential effects of to-
pography, microclimatology, wind condi-
tions, snow cover (especially its relief re-
lationships as referred in Bär et al. 2006,
2007, 2008), soil properties, moisture avail-
ability, nutrition supply, mycorrhiza sym-
biosis, fungal diseases, insect defoliation,
browsing pressure and land-use/land-cover
as stressed by Büntgen et Schweingruber
(2010). They also suggest the broader spa-
tial scale of collected samples to overcome
local disturbance factors.
Concerning species selection it is im-
portant to work with the species that fre-
quently occur in the entire study area (Bär
et al. 2006) to be able to sample whole
variety of different plant ages for strong
master chronology development.
Sampled shrubs should be spatially dis-
tinct, and in larger cluster only one sample
should be taken to prevent collection of
the same genotype twice (Hallinger et al.
2010, Weijers et al. 2010). Before re-
moving the whole plant including braches,
stem, and roots from the soil it is appro-
priate to record position of the soil surface
by tape (Bär et al. 2006), slope aspect, and
GPS position. Photos of both the site as
well as the whole plant should be taken for
later reminder in case of later unexpected
findings (Bär et al. 2006).
Since alpine as well as arctic eco-
systems are fragile the investigator should
concern the degree of invasiveness. Stems
are examined in the great details and there-
fore a vast number of samples is not re-
quired. Nonetheless, one should be aware
of collecting too few samples since some-
times only about one third of samples can
be used for final chronology (Zalatan et
Gajewski 2006, Blok et al. 2011, Zongs-
han et al. 2013). Most studies work with
tens of samples (between 15 and 50) and
the authors often collect also the dead
material to extend the length of chro-
nology (e.g. Zalatan et Gajewski 2006,
Hallinger et al. 2010, Blok et al. 2011).
Kolishchuk (1990) resp. Schweingruber
(personal communication) believes that
not less than 10 (resp. 30) analysed indi-
viduals can sufficiently reflect and cover
the specific habitat. In general, annual
growth-ring patterns can differ among
shrub species (from uniform to variable
growth form). This growth variation could
therefore be taken into account in sam-
pling strategy (less number of individuals
with uniform growth are needed compared
to individuals with variable or irregular
growth form) to extract the similar
strength of climatic signal. To incorporate
this recommendation knowledge on eco-
logical and anatomical aspects of the spe-
cies is nevertheless required which does
not have to be always the case in the
Concerning the issue of sample trans-
portation Schweingruber et Poschlod
(2005) suggest plastic zip-lock bags con-
taining directly labelled compartments
while providing detailed records on site
and plant characteristics. An alternative
approach is to use paper bags which pre-
vent the material from moulding and fungi
Dendrochronology – lab work
Wedging rings, missing rings or piths,
frost rings, asymmetric growth including
lobes or “just” extremely narrow rings (see
Fig. 1.) are the results of the harsh envi-
ronment which dwarf shrubs inhabit often
also accompanied with one-sided (me-
chanical) stress or local death of cambium
(e.g. Kolishchuk 1990, Woodcook et
Bradley 1994, Schweingruber 2001, Bär et
al. 2006, 2007, 2008, Zalatan et Gajewski
2006, Hallinger et al. 2010). Due to this
difficulties the analysis of annual rings
must undergo a much more complicated
procedure in comparison with regular tree
ring research.
Additional steps of analysis are re-
quired to construct reliable master chro-
nology of tundra shrubs and thus recon-
struct the climate signal contained in vari-
ations of their growth (Kolishchuck 1990).
The method of serial sectioning first devel-
oped by Kolishchuck (1990) facilitates the
annually precise dating of each ring and it
is particularly important when annual rings
are poorly visible and hard to measure
(Schweingruber et Dietz 2001). Common-
ly, between 2 and 10 thin sections equally
distributed over the plant’s body is ob-
tained from the plant. The number depends
on the degree of expected cross-dating
constraints (Woodcock et Bradley 1994).
Stem section samples are made from
compartments by sledge microtome knife
and they are usually 10-30 μm thick
(Schweingruber et Poschlod 2005, Bär et
al. 2006). Gärtner et Schweingruber (2013)
suggest sticking the samples in glycerol to
prevent drying in case of long-term stor-
ing. There are also many ways of ring-
visibility improvement such as highlight-
ing by rubbing chalk, staining by Safranin
and Astrablue etc. (for more see Schwein-
gruber et Poschlod 2005 or Gärtner et
Schweingruber 2013). Subsequently, the
section is dehydrated and cleaned by
ethanol, permanent slide is created and
Some authors (e.g. Hallinger 2010,
Zongshan et al. 2013) exclude from further
dendrochronological analysis those discs
with stem wounds, rotten wood, or ex-
tremely eccentric growth.
Chronology development
a) Cross-dating
Cross-dating is a technique that ensures
each individual tree ring is assigned its
exact year of formation (Speer 2010).
Annual rings are measured using sensi-
tive digital encoder and softwares (e.g.
WinCell, Roxas) with a precision of 0.001
mm (Zalatan et Gajewski 2006, Hallinger
et al. 2010). Kolishchuk (1990) recom-
mends measuring the ring widths from the
periphery to the centre.
Fig. 1. Problematic areas within the microscopic sections (Juniperus communis). a) wedging ring
with arrow indicating the point of miss. b) extremely narrow ring consisted of two cell rows only;
and ring which can be easily omitted and considered as missing because only few and sporadic
latewood cells were formed - possibly distinguishable from incomplete rings or intra-annual
density fluctuations using serial sectioning method.
Visual cross-dating precedes verifying
by widely used computer programme
COFECHA (Holmes et al. 1986). Width of
annual rings should be measured on every
disc in several direction separated from
each other by at least 90° (Hallinger et al.
2010) while directions with minimum num-
ber of discontinuous rings or scars are
preferred (Bär et al. 2006). The following
operations in laboratories known as serial
sectioning (Kolishchuk 1990) are focused
on obtaining the complete record. It com-
bines findings about annual increments in
different directions within the incision (1st
order), in different parts of shrub (2nd or-
der), as well as between individuals (3rd
order) in order to detect missing rings and
prepare samples for cross-dating (Schwein-
gruber et al. 1990) to receive master chro-
nology (Bär et al. 2006). Cross-dated ring-
width series of samples from the same
stem height are averaged and compared to
those from other heights within the same
individual to check for wedging or missing
rings (Blok et al. 2011). In case of missing
ring detection in chronology such should
be artificially inserted with the lowest pos-
sible increment assigned (Bär et al. 2006).
To be considered as a missing ring Wood-
cock et Bradley (1994) suggest rather con-
servative criteria: 1) it has to be clearly
identified in at least two other samples; 2)
be narrow in the samples in which it
Hallinger et al. (2010) note some find-
ings gained during serial sectioning of
Junniperus nana. They state that the largest
diameter of the stem does not always con-
tain the most number of years. Often the
outermost ring at the stem base does not
refer to the last year of the growth, and
simultaneously, there is many missing
rings at the root collar. The possible reason
for this fact may be age-related move of
phytohormones higher up, resp. downward
in the stem (e.g. cytokinins, resp. auxins)
and therefore in particular years cambium
in the basal part of the stem might not
activated at all (Kolishchuk 1990, Wilm-
king et al. 2012). This could also explain
the fact that some dwarf shrub species at
high latitudes do not show any visible ra-
dial growth trend (Schmidt et al. 2006)
with wider stem near the surface. Limi-
tation of the stem base growth due to the
low close-to-ground temperatures might al-
so be the case of above mentioned if the
dwarf shrub is not in prostrate form or has
very low sizes which rules this effect out
since such conditions therefore influence
the whole plant body uniformly. The value
of the whole material for paleoclimato-
logical renconstruction is dependent upon
successfully cross-dating the samples
(Woodcock et Bradley 1994).
b) Removal of age trend - standardization
Variation in the width of annual growth
increments is not only caused by climatic
factors but also by long term trends which
can disturb the climatic fluctuations (e.g.
uneven growth over the plant’s lifetime,
Fritts 1976). Standardization is removing
of non-climatic growth trends from the an-
nual growth increments series. It allows
the resultant standardized values of par-
ticular plants to be averaged together into
master chronology (Cook et Kairiukstis
Such trends are generally removed by
dividing the individual tree ring series by
functional estimates of these trends (Wei-
jers et al. 2010). For standardization the
program ARSTAN is used most common-
ly (Cook 1985, Rayback et Henry 2005,
Zalatan et Gajewski 2006). Therefore,
Büntgen et Schweingruber (2010) warn of
averaging measurements series from differ-
ent stem sections without standardisation
the stem-height-curves first (done e.g. by
Bär et al. 2006, 2007 or Hallinger et al.
2010) otherwise it artificially increases the
relevance of juvenile growth phases be-
cause upper stem sections experience long-
er period of growth then those close to root
collar because the cambial activity starts
from the top (Kolishchuk 1990).
It is done by mean curves using a 32
year smoothing spline (Blok et al. 2011) or
linear regression lines (Zalatan et Gajew-
ski 2006, Zongshan et al. 2013) and nega-
tive exponential curves (Blok et al. 2011,
Zongshan et al. 2013) to remove intra-
plant variation and age related as well as
other long term growth trends. In addition,
the master chronology (using e.g. biweight
robust mean) of the aligned time series
from one disc is calculated to express the
overall average trend of the time series
(regional curve as used e.g. in the RCS
method; Esper et al. 2003). Detrending can
be performed by fitting a polynomial func-
tion (e.g. aka ‘Spline’) to the respective
master chronology of aligned series. For
each serie the residual to this average
spline trend can be used as detrended vari-
able. Such operations should lead to re-
moval of the most of the growth related
trends and low-frequency signals. Alterna-
tively, the ring-width data can be standard-
ized using a horizontal mean detrending to
preserve the low-frequency variability in
the data.
Age related trends of tundra shrubs ring
widths do not differ from trees if the cano-
py is closed. Such series are characterised
by early suppressed growth followed by a
relatively sharp growth increase and sub-
sequent decline (e.g. Forbes et al. 2010).
On contrary, the age trends are very dif-
ficult to generalize if canopy does not
close. Most commonly, they are declining
(e.g. Blok et al. 2011, Rixen et al. 2010,
Weijers et al. 2010), sometimes are not
evident (Buchwal et al. 2013) and often
also not consistent (Hallinger et al. 2010,
Tape et al. 2012). Selection of appropriate
detrending method for removing particular
age trends is therefore crucial for suc-
cessful paleoclimate reconstructions using
dwarf shrubs ring-width series.
The series which do not show appro-
priate correlation with the master chronol-
ogy can be excluded from the site chro-
nologies (Zongshan et al. 2013). Residual
standardised chronology retained without
the influence of the previous year growth
on the growth of next year (Cook et al.
1990) can be used for subsequent analysis
(Zalatan et Gajewski 2006).
c) Further data treatment
After cross-dating, standardization and
creating master chronology of each par-
ticular disc of all series are usually aver-
aged within the whole stem (Bär et al.
2006, 2007, Hallinger et al. 2010, Blok et
al. 2011) to obtain comparable growth es-
timates. Subsequently, also all individuals
of one site type are averaged in order to
receive master chronology.
The climate and growth relationship
There is lack of information what tem-
peratures present the limit for cambial
activity of dwarf shrubs. Very few relevant
studies have focused on critical tempera-
ture for cell division of conifers in cold
climates (e.g. Rossi et al. 2008). The au-
thors concluded that average limiting val-
ues are around 4-5°C for the daily mini-
mum temperature at 2 m height. Körner et
Paulsen (2004) defined the limit for grow-
ing season as the time period of ten days
air temperature means higher than 0°C.
Ten days interval is the minimum duration
for vessel development and lignification
(Suzuki et al. 1996). Such study for pros-
trate woody life forms, however, remains
for further investigations.
It is widely accepted (e.g. Fritts 1976,
Woodcock et Bradley 1994, Bär et al.
2008, Weijers et al. 2010) that climate
conditions preceding and during the grow-
ing season influence the growth. To find
which climate variables are the most im-
portant for plant growth it is crucial to
have climate data from nearby meteoro-
logical station which cover at least a part
of shrub’s lifespan or to use extrapolated
climate data e.g. from the Climatic Re-
search Unit. Subsequent statistical analyses
highly depend on particular meteorological
data availability. The more detailed the
observations are the finer scale can be
used for finding climate-growth relation-
ship. To generalize the common strength
of climate response in the tree rings of
dwarf shrubs is not simple since this is
influenced by factors like micro-climate,
species (diffuse vs. ring porous), or habi-
tats. Commonly, the correlation coeffi-
cients are lower than in similar studies
using tree material but significant, highly
correlating and over whole period stable
results have been reported. Following list
can give an impression on the wide variety
of results concerning both strength of
correlations and variables: Buchwal et al.
(2013) for Salix polaris and mean JJA
temperatures (r = 0.70, P < 0.01); Zhong-
shan et al. (2013) for Rhododendron
przewalskii and April, July temperatures
(r = 0.326, P < 0.05); Hallinger et al.
(2010) for Juniperus nana and June + July
temperatures (r = 0.4, P < 0.05); Blok et al.
(2011) for Salix pulchra and early summer
temperature (r = 0.73, P < 0.01); Zalatan et
Gajewski (2006) for Salix alaxensis and
December, March precipitation (r = 0.3,
P < 0.05).
Seasonal macro-climate and growth response relationship
a) Winter
Cold winters which are responsible for
root injuries are among the most limiting
factors of shrub growth (Pederson et al.
2004). They are often accompanied by
delayed snow melt resulting in shortened
vegetation season and possibly reducing
early wood formation (Vaganov et al.
1999, Schmidt et al. 2006, Pellizzari et al.
2014). Warm period within winter can also
cause significant damage when the plant
loses its frost resistance and is therefore
sensitive to upcoming extreme cold events
as discussed e.g. in Zongshan et al. (2013).
Also Sturm et al. (2001) believe that win-
ter snow cover plays crucial role in shrub
expansion due to enhanced nutrient supply
in the harsh Arctic environment. Zalatan et
Gajewski (2006) consider winter as crucial
season (for Salix alaxensis in western Ca-
nadian Arctic) as well but their findings
are different. They found a correlation be-
tween high winter precipitation associated
with enhanced soil moisture during the
growing season. Locality of their investi-
gation is, nevertheless very continental
(see Appendix 1.). Therefore, the effect of
precipitation is such an important factor of
growth compare to more oceanic areas
where soil moisture is not an issue (e.g.
Zongshan et al. 2013).
b) Spring
Climate in spring can influence the
growth as well. Early and warm spring can
extend the length of the growing season
and enhances earlywood formation (see
Schmidt et al. 2006, Zongshan et al. 2013).
Schmidt et al. (2006) are among a few
authors who consider early spring climate
conditions at their research site as the most
important factor for shrub growth. They
found out a correlation between late snow
melt and narrow growth rings indicating
deteriorated growth conditions. This was
observed especially from the 1960s on-
ward when the climate in northeast Green-
land has become more oceanic as a con-
sequence of diminishing sea ice. In this
case the changes are nevertheless locally
driven. This might be the reason why
Schmidt et al. (2006) presented growth
chronology signal with poor annual incre-
ments in recent decades in contrast to gen-
eral trend.
c) Summer
Many authors (e.g. Rayback et Henry
2005, Bär et al. 2008, Hallinger et al.
2010, Weijers et al. 2010) report summer
temperatures as the most important growth
influencing factor. Warm summers are the
most often described as crucial factors for
enhanced growth. Nonetheless, higher sum-
mer temperatures do not have to necessari-
ly lead to proportionally wider ring widths,
especially on south facing slopes which
may suffer from drought as stressed by
Bär et al. (2008).
Rayback et Henry (2005) also found a
negative correlation between growth and
Arctic Oscillation (AO) which brings sum-
mer cyclones to the Arctic accompanied
with bellow average temperatures and
above average precipitation to the influ-
enced regions. They reported higher values
of δ18O in Cassiopea tetragona samples
from western Canadian Arctic in years
with enhanced AO causing worse growth
of investigated plants. That indirectly cor-
responds to often reported findings that
high summer temperatures favour shrub
d) Autumn
Autumn as a post-vegetation period is
not believed to have a strong relationship
to shrub growth in any reviewed study.
e) Other non-seasonals effects
Sometimes it is believed that the pre-
vious growing season can have a stronger
impact on current year’s growth than the
actual season (Fritts 1976). The later stud-
ies generally disagree with this idea by
reporting a relatively strong correlation be-
tween current year’s growth and tempera-
tures (e.g. Buchwal et al. 2013). However,
such conclusions do not have to neces-
sarily be in contradiction with Fritts (1976)
while it is not difficult to find a correlation
between annual growth increments of two
subsequent years due to relatively gradual
climatic fluctuations. Recently, Weijers et
al. (2010) observed the effect of previous
year’s September precipitation on Casiope
tetragona growth explained by late-sum-
mer drought and snow protection against
frost damage by the end of the month.
Previous year effect should, however, be
only considered as trigger if proper stand-
ardisation is applied and autocorrelation is
Investigations of dwarf tundra shrubs
represent relatively untapped source for
climate reconstructions in the Arctic
(Schmidt et al. 2006). As it is possible to
see from Appendix 1. this archive con-
serves often different proxy information of
past climate. Only if such variations and
specifics are described and understood we
can have a full profit from this vast re-
We should therefore be aware of
generalized conclusions. Climate related
growth responses of every species at par-
ticular locality should be interpreted inde-
pendently concerning local climate with
consideration of site specifics both on mi-
cro and macro-spatial scale. There is also
lack of studies working with other growth
parameters than widths of growth rings
(e.g. lumen areas, lumen perimeters, cell
wall thicknesses). Recent progress in prep-
aration of permanent micro-sections (Gärt-
ner et Schweingruber 2013) enabled to
take into consideration the cell sizes, cell
wall thicknesses, vessel sizes, or cell or
ray density. It can expose new directions
of shrub research. In region where plant
growth has to overcome that many obsta-
cles such parameter can serve as a better
climate proxy and provide more reliable
information on paleoclimatic conditions.
Constraints of dwarf tundra chronology development
Büntgen et Schweingruber (2010) be-
lieve that dwarf shrub annual ring research
with accompanied serial sectioning has a
great potential in accurate dating of events
using extremely slow growing individuals.
They also believe that serial sectioning is
able to overcome often mentioned con-
straints of dwarf shrub research (missing
or wedging ring, lobes etc.). But they are
aware of averaging radii of different stem
heights from the same individuals due to
rather different development.
Nevertheless many authors found
strong correlation values between growth
and climate in their studies even when
“simply” averaging after removing age-
related growth trends (e.g. Bär et al. 2006,
2007, Hallinger et al. 2010). To obtain
higher levels of certainty and reliability
Büntgen et Schweingruber (2010) suggest
individual standardization of measurement
radii from different stem heights before
averaging measurements at the shrub level.
Kolishchuk (1990) offers an alternative
approach by averaging only three or four
neighbouring sections from the top to the
basal part of the trunk starting with the
disc where a ring occurs at first.
We suggest to focus on other growth
(cell) parameters which were documented
to serve as climate proxies in trees such as
cell radial diameter (Xu et al. 2013) or cell
wall thickness (Yasue et al. 2000). Dealing
with such parameters can help to over-
come the problem of relying on one infor-
mation only (ring width) which can be
rather problematic in the Arctic/alpine cli-
mate zone. In contrary, averaging many
individual cell parameters per annual ring
can deliver more robust results. The obsta-
cle of tapering of wood anatomical ele-
ments towards stem apex (for trees dis-
cussed e.g. in Carrer et al. 2015) can be
overcome by appropriate designed detrend-
ing which is described in Lehejček et al.
(in revision).
Micro-environmental conditions and growth response relationships
Soil moisture and nutrients
Soil moisture availability is by some
authors not believed to be a limiting
growth factor throughout the year even in
the continental areas (Löffler 2005). It is
assumed that the plants have adequate
supply with melt water especially during
the early growing season (Bär et al. 2007).
In contrary, Schweingruber et Poschlod
(2005) reported often discontinuous or
even false rings occurring in plants from
dry areas growing in shallow soils. They
suggested to sample plants from the sites
with intermediate soil moisture and poorer
nutrient availability which enables the
longest life span and does not present
stress factor for shrubs, yet.
Site conditions are imprinted in the
shrubs (e.g. occurrence of frost rings, early-
wood/latewood portions) and growth rates
(Pellizzari et al. 2014). Such site differ-
ences are often caused by resource parti-
tioning (water, nutrients, or photosyntha-
ses) which can easily modify the plant’s
growth response according to environmen-
tal conditions (Rayback et Henry 2005).
All variables should be therefore further
documented to extend our knowledge
about micro-climate and growth relation-
ship in order to separate climate driven
growth responses from locally disturbing
factors as well as to find the longest living
In general, shrubs achieve the maxi-
mum age in the deteriorated climate condi-
tions (Bär et al. 2006) close to their surviv-
al limits where their sensitivity to climate
variations is also enhanced. It is important
to stress that finding the oldest individual
has no relevance if it is not possible to
compile the chronology of its growth due
to extremely narrow rings, too many miss-
ing rings, or other factors. Woodcock et
Bradley (1994) reported increasing amount
of missing ring with the shrub age. It im-
plicates the need of collection of variously
old samples from a range of micro-
environment (Woodcock et Bradley 1994)
in order to obtain long and precise chrono-
logy. Although several hundred years old
individuals and consequent chronologies of
dwarf shrubs are not rare (e.g. Hantemirov
et al. 2000 or Ward 1982 for junipers) most
of the species are do not live longer than
one or two centuries at the most (Schwein-
gruber et Poschlod 2005). Chronologies
originating from such material therefor usu-
ally cover only tens of years or first cen-
tury. Nonetheless, in the conditions of the
Arctic with the scarcity of meteorological
stations containing only limited and short
records even such chronologies can help to
understand past climate and/or other varia-
bles connected to climate change (e.g. gla-
cier melt, period of snow cover etc.)
Research on dwarf tundra shrubs is im-
mature and has to surmount many diffi-
culties. Not only problems with anatomy
of plants growing in the harsh environ-
ments but often also the correlation of
master chronology with climate is weaker
than the one from nearby tree chronologies
(e.g. Zongshan et al. 2013). Such investi-
gations can nevertheless extend usage of
dendroecological studies into new climatic
zones and therefore might help to under-
stand future environmental changes, their
drivers and impacts in the arctic and alpine
According to reviewed literature higher
summer temperature are believed to be
the most positively influencing the shrub
growth. Ring width can be regarded as a
variable integrating temperature conditions
during the main growing season at particu-
lar localities. However, special cases where
local setting can eliminate climate effect of
growing season are reported. Therefore,
deep knowledge of local environment is
The longest life spans of tundra shrubs
are usually achieved at sites with an inter-
mediate level of soil moisture with rather
limited nutrient supply, and at undisturbed
sites (Schweingruber et Poschlod 2005).
That is why detailed reconnaissance of the
field is crucial to obtain the longest possi-
ble chronology. The field sampling should
not concentrate only on the oldest plants
since their growth becomes deteriorated
with age but should aim on population age
diversity to strengthen and make the cli-
mate signal more reliable over the whole
Despite many successes achieved with
tundra shrub ring width investigations the
further studies should focus also on other
growth parameter (lumen areas, lumen
perimeters, cell wall thicknesses etc.) to
strengthen the climate/growth correlation.
In spite of outlined constrains, dwarf
tundra shrubs present enormous and valua-
ble archive not only for paleoclimatology
but also for paleoecological reconstruc-
tions. Samples of dwarf tundra shrubs pro-
vide high-resolution year-to-year informa-
tion on climate variation based on annual
rings. Yet, such detailed record is not very
long, reaching only first centuries and it is
therefore crucial to involve in research oth-
er proxy archives giving longer but less
precise records such as lake sediment or
ice cores. This is nowadays one of the few
ways how to extend our knowledge of re-
mote regions paleoclimate as well as re-
cent climate development.
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Appendix 1.
Summary of the selected findings and observations hold on dwarf tundra shrubs. Mean monthly low and high temperatures indicate the mean
temperature of the coldest and the warmest month, respectively.
... Shrubs form annual growth rings like trees and many recent studies have shown that these annual rings in shrubs are sensitive to changes in the environment, especially climate variability, for example Danby and Hik (2007); Hallinger et al. (2010); Blok et al. (2011); Hallinger and Wilmking (2011). Consequently, ring-width data from shrubs can be useful for ecological reconstructions (Lehejček, 2015) like temperature (Liang and Eckstein, 2009;Weijers et al., 2010;Meinardus et al., 2011;Weijers et al., 2012;Weijers et al., 2013), humidity (Liang et al., 2012), glacier melting (Buras et al., 2012;Buras et al., 2017), permafrost degradation (Blok et al., 2010), sea surface temperatures (Beil et al., 2015) and fire history in a landscape (Oddi and Ghermandi, 2015). Therefore, increased use of shrubs in future dendrochronological research is very likely. ...
In recent years an increasing number of studies have shown shrubs to be reliable proxies of environmental conditions in regions where Trees − due to harsh climate conditions − are absent. Although many shrubs are monoecious, some are dioecious, which poses certain questions related to gender-specific growth as observed for trees in previous studies. Here, we address the questions whether dioecious shrubs, similar to trees, show growth differences between male and female plants, and − if so − whether this difference needs to be considered in terms of sample selection. We chose Juniperus communis. L., the most widely distributed woody plant, and a common and well-studied dioecious shrub species in the northern hemisphere, especially in the Boreal, Subarctic tundra and Alpine regions. Our samples were collected from four sites − three from the Ural Mountains and one site from Kirkenes in Norway. To see if there were differences in radial growth between sexes we performed four different analyses. First, we used multivariate explorative statistics to see if there were gender biased sub-populations and generally found no differences. Secondly, to compare growth over the lifetime of shrubs we computed cumulative annual increments of basal area which revealed no gender-specific growth patterns. Thirdly, to test if differences in radial growth between male and female shrubs affect the resulting site-chronology, we compared individual shrub chronologies with the site-chronology and found a significant differentiation between normalized correlations of gender-specific chronologies to the site-chronology. This significant difference was restricted to an overall comparison, but not evident at individual site-level. Lastly, we compared correlations of gender-specific chronologies and a mean site-chronology with monthly climate records to find only very few meaningful differences in their responses. In summary, we could not detect any clear gender-specific growth pattern in Juniperus communis but observed a trend towards more non-climatic signals in female junipers which may affect the resulting site-chronology.
... Alpine and arctic shrub sensitivity to contemporary climate change around the circumpolar arctic is well established (Sturm et al., 2001;Danby and Hik, 2007;Forbes et al., 2010;Hallinger et al., 2010;Hallinger and Wilmking, 2011;Myers-Smith et al., 2011, 2015a. Due to this sensitivity, many recent dendrochronological studies use growthring parameters from shrubs for reconstructions of environmental conditions in Arctic and alpine Tundra (Lehejček, 2015) ranging from temperature (Meinardus et al., 2011;Weijers et al., 2013), humidity (Liang and Eckstein, 2009;Liang et al., 2012), and sea surface temperatures (Beil et al., 2015) to understanding ecological processes like permafrost degradation (Blok et al., 2010;Blok et al., 2011), glacial retreat (Buras et al., 2012;Buras et al., 2017) and effects of reducing summer albedo and treeline advances and greening at high latitudes (Raynolds et al., 2006;Tape et al., 2006;Ropars and Boudreau, 2012;Tape et al., 2012;Juszak et al., 2014). This multi-faceted utility of shrubs makes them pivotal for future studies of climate change and vegetation dynamics in the arctic tundra. ...
Full-text available
Deciduous shrubs are expected to rapidly expand in the Arctic during the coming decades due to climate warming. A transition towards more shrub-dominated tundra may have large implications for the regional surface energy balance, permafrost stability and carbon storage capacity, with consequences for the global climate system. However, little information is available on the natural long-term shrub growth response to climatic variability. Our aim was to determine the climate factor and time period that are most important to annual shrub growth in our research site in NE-Siberia. Therefore, we determined annual radial growth rates in Salix pulchra and Betula nana shrubs by measuring ring widths. We constructed shrub ring width chronologies and compared growth rates to regional climate and remotely sensed greenness data. Early summer temperature was the most important factor influencing ring width of S. pulchra (Pearson's r =0.73, p <0.001) and B. nana (Pearson's r =0.46, p <0.001). No effect of winter precipitation on shrub growth was observed. In contrast, summer precipitation of the previous year correlated positively with B. nana ring width ( r =0.42, p <0.01), suggesting that wet summers facilitate shrub growth in the following growing season. S. pulchra ring width correlated positively with peak summer NDVI, despite the small coverage of S. pulchra shrubs (<5% surface cover) in our research area. We provide the first climate-growth study on shrubs for Northeast Siberia, the largest tundra region in the world. We show that two deciduous shrub species with markedly different growth forms have a similar growth response to changes in climate. The obtained shrub growth response to climate variability in the past increases our understanding of the mechanisms underlying current shrub expansion, which is required to predict future climate-driven tundra vegetation shifts.
Plants respond to the harsh alpine environment with a high degree of specialization, the structural and functional aspects of which this book aims to explore. Palaeorecords suggest that life on land started out in sheltered, warm and moist environments, and gradually expanded into more demanding habitats where water is rare, thermal energy is either low or overabundant or where mechanical disturbance is high. More than 100 million years ago, when the large, hot deserts of the Cretaceous period where formed, coping with drought became a matter of survival in higher plants. Why is this of relevance here? Because survival of both drought and freezing temperatures requires cell membranes which can tolerate dehydration. When plant tissues freeze, ice is first formed in gaps between cells, which draws water from protoplasts (see Chap. 8). A link between the ultrastructural and molecular basis for freezing tolerance and the evolution of dehydration tolerance of biomembranes has therefore been suggested (Larcher 1981). Plant survival in cold as well as hot “deserts” — the two thermal extremes on the globe — thus may have common evolutionary roots, although life under such contrasting thermal conditions requires many additional, rather different metabolic and developmental adaptations. The ability to survive low temperature extremes opened the highlands of the earth to plants. The Tertiary (and still ongoing) uplift of mountain ranges strongly accelerated the evolution of alpine taxa (Billings 1974;Agakhanyantz and Breckle 1995). Embedded in different floras of the world, high mountains became both highly fragmented refugia and corridors of cross-continental migration, and often bear plant diversities richer than those in their surrounding lowlands (Körner 1995a; Barthlott et al. 1996; Chap. 2 and color Plates 1–3 at the end of the book).
A study of the dry weights and ages of 57 Salix arctica plants growing in a higharctic region shows that their annual relative growth-rate, which decreases with age, is about one-fifth of the annual relative growth-rate of comparable plants under temperate conditions. Data on the thickness of annual rings indicate that there is a large and progressive increase in annual growth from high-arctic to sub-arctic regions.
Rapid climate warming in the tundra biome has been linked to increasing shrub dominance. Shrub expansion can modify climate by altering surface albedo, energy and water balance, and permafrost yet the drivers of shrub growth remain poorly understood. Dendroecological data consisting of multi-decadal time series of annual shrub growth provide an underused resource to explore climate–growth relationships. Here, we analyse circumpolar data from 37 Arctic and alpine sites in 9 countries, including 25 species, and ~42,000 annual growth records from 1,821 individuals. Our analyses demonstrate that the sensitivity of shrub growth to climate was: (1) heterogeneous, with European sites showing greater summer temperature sensitivity than North American sites, and (2) higher at sites with greater soil moisture and for taller shrubs (for example, alders and willows) growing at their northern or upper elevational range edges. Across latitude, climate sensitivity of growth was greatest at the boundary between the Low and High Arctic, where permafrost is thawing and most of the global permafrost soil carbon pool is stored. The observed variation in climate–shrub growth relationships should be incorporated into Earth system models to improve future projections of climate change impacts across the tundra biome.