Download Beyond tree-ring widths: stable isotopes sharpen

Tree Physiology 35, 1–3
doi:10.1093/treephys/tpu115
Commentary
Beyond tree-ring widths: stable isotopes sharpen the focus on
climate responses of temperate forest trees
Lucas A. Cernusak1,3 and Nathan B. English2
1College
of Marine and Environmental Sciences, James Cook University, Queensland, Australia; 2College of Science, Technology and Engineering, James Cook University,
Queensland, Australia; 3Corresponding author ([email protected])
Received November 8, 2014; accepted December 4, 2014; handling Editor Danielle Way
Tree rings provide an indispensable tool for assessing a tree's
response to variability in its environment, oftentimes also providing a means of reconstructing that variability beyond instrumental records. The wood that trees produce is laid down
sequentially, creating an archive of temporally ordered material
that is rich in physiological and environmental information. This
is made all the more useful because trees are globally distributed, can live for thousands of years and in some cases remain
intact long after they die. Tree-ring archives are used in a wide
variety of studies, including, but not limited to, climate reconstructions (Cook et al. 2010), archaeological dating of ruins
(Čufar 2007), reconstructions of fire history and recurrence
intervals (Swetnam et al. 1999), and assessments of tree
physiological responses to drought (McDowell et al. 2010).
The number of applications for tree rings has grown steadily
in recent decades, and the variety of measurements that can
be made on tree rings has grown as well (Speer 2010). The
simplest measurement that can be made is in-series wholering widths. From this starting point, additional information can
be gained from measuring early-wood and late-wood widths,
wood density, elemental abundances, microfibril angles, radial
diameters of xylem conduits and stable-isotope ratios of the
organic material in tree rings. A tree-ring record based on a
single sample from a tree is referred to as a series, whereas a
composite of many series is referred to as a chronology.
To be useful, a measured tree-ring record must be both dateable and responsive. Being dateable means the rings must be
shown to represent specific years at annual increments, with
as few missing or false rings as possible. Being responsive
means the rings or their characteristics must yield significant
variations in response to physiological or climatic drivers, and
these variations must be strong enough to stand out from the
background noise. Tree-ring series can be described as ‘sensitive’ or ‘complacent’, depending on whether or not the measured parameter is responsive to the physiological or climatic
drivers of interest (Stokes and Smiley 1968). Recent work has
progressed towards more accurately dating tree rings (Hua
2009), including dating rings with non-annual increments
(Loader et al. 2011), and in a new paper published in this
issue, Hartl-Meier et al. (2015) have expanded the definition of
just what constitutes a sensitive tree-ring series and the type
of forest in which one can expect to find such a series.
As the science of dendrochronology advanced from the
early 20th century, it became evident that there were geographic sweet spots for developing ring-width chronologies.
The trick is to sample trees that are growing away from their
optimal climatic conditions. Individuals that are growing near
the edge of the species' geographical range, either horizontally or elevationally, are more likely to be sensitive to a limiting environmental factor, for example, rainfall or temperature
(Figure 1). At the same time, however, the target trees must
not be so far towards the edge of the species' range that missing rings resulting from multiple poor growth years prevent the
development of an accurate and reliable chronology (Figure 1).
This sampling paradigm mostly restricts ring-width based chronologies to the inward portion of the periphery of most large
temperate forests. With careful site selection, or by substituting elevation for horizontal distance, many interior forests can
be used to develop useful chronologies. This includes stableisotope series (e.g., Marshall and Monserud 1996, Duquesnay
et al. 1998, Tognetti et al. 2014), but these have seldom been
recognized as useful from a climate reconstruction perspective.
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2 Cernusak and English
Figure 1. A schematic of ring-width (RW) and stable-isotope (δ) characteristics in tree-ring series across the climatic range of a hypothetical forest. The top panel shows mean ring widths (black), correlations
(red) of ring width and isotope series among trees and with climate,
the variability (dark blue) of the ring width and isotope series as measured by mean sensitivity and standard deviation, the percentage
of absent or partial rings (grey), and the dominance of the canopy
(green). The bottom diagram illustrates where ring-width and stableisotope-based chronologies are best able to act as proxies for climatic
or physiological variability. Figure modified from Fritts (1976).
In general, regions or sites within the optimal growth habitat
of a species will just not yield ring-width chronologies that
are both dateable and sensitive. Stable-isotope studies have
tended to follow the inner periphery sampling paradigm in subpolar, temperate and semi-arid forests (e.g., Breshears et al.
2009, Loader et al. 2013a, Heres et al. 2014), due to accessibility of existing samples and sites, and to a continued emphasis on picking sites on the basis that they are the most likely to
harbour climate-sensitive ring-width chronologies, in addition
to, one hopes, stable-isotope chronologies.
Hartl-Meier et al. (2015) constructed tree-ring chronologies
for ring widths and for the stable-isotope ratios of carbon and
oxygen for three mid-latitude tree species: Norway spruce,
European larch and beech. Stable-isotope ratios of carbon and
oxygen have previously been measured in tree rings and are
a commonly applied tool in dendrochronology (McCarroll and
Loader 2004). The real novelty in the Hartl-Meier et al. (2015)
analysis lies not so much in what they did, but rather in where
they did it. The climatic conditions at their study site in the
Tree Physiology Volume 35, 2015
Austrian pre-Alps are characterized by relatively high annual
precipitation (1370 mm year−1) and by relatively optimal growing conditions for the three focal tree species. Three different
topographic positions were sampled at the site, with European
larch not present on the driest, south-exposed position. The
plateau position, in particular, had deep soils and productivity high enough that it was judged appropriate for establishment of a plantation early in the 20th century. Thus, this does
not represent the periphery of the geographical range of these
three tree species; rather, it is more characteristic of the interior of the climatic space over which these temperate forest
trees are distributed.
Based purely on the ring-width chronologies, one would
indeed be forced to agree that the site is not ideal, as the
ring widths were only weakly correlated with climatic indices,
and there was little coherence among species in ring-width
responses to climate. Unlike the ring widths, however, stableisotope ratios in tree-ring cellulose showed a strong coherence
among species and strong correlations with climatic indices,
including growing season cloud cover, temperature and moisture. These strong correlations were maintained across the
positions within the site, from the dry, south-facing slope to the
productive plateau. These results of Hartl-Meier et al. (2015)
suggest that there is a strong potential for developing stableisotope based tree-ring chronologies throughout the geographical ranges of temperate tree species, rather than just on
the periphery of their distributions. Recent results also suggest
that the same may be true for tree-ring chronologies based
on tracheid radial diameter and microfibril angle (Allen et al.
2013, Drew et al. 2013). The ability to sample a single species of tree, or multiple species of different functional types,
across the full range of their geographical distributions could
provide a significant advantage for understanding species-level
responses to climate change.
In addition to providing a tool for climate reconstructions
based on tree-ring chronologies, stable isotopes also provide
insights into the physiological responses of trees to extreme
climate events. For example, Hartl-Meier et al. (2015) conducted a superposed epoch analysis (SEA) on the responses
of the three tree species to drought events that occurred in
1983, 1992, 1994 and 2003. The SEA technique creates a
composite response by overlaying multiple events, thereby
allowing a signal to emerge from noise associated with other
drivers that work on similar time scales. By applying SEA,
Hartl-Meier et al. (2015) resolved intriguing differences among
their three study species on how they responded to drought.
Norway spruce, which has been shown to employ an isohydric strategy with respect to its stomatal regulation (McDowell
et al. 2008), suffered the largest growth reductions during and
following drought. This species also had the highest δ 18O in its
wood, consistent with having shallower roots and not having
access to deep soil moisture; and it also had the highest δ 13C
Beyond tree-ring widths 3
in its wood, consistent with a greater stomatal limitation on
photosynthesis than in European larch or beech.
We do not mean to suggest that there is not real value in
obtaining ring-width chronologies; the ring widths are, after all,
a whole-tree integration of the tree's growth response. And
developing isotopic chronologies is not without its drawbacks,
requiring a great deal more money, effort and equipment than
ring-width chronologies (Gaudinski et al. 2005), and recent
work suggests that greater sample numbers are required
than have been used in past studies (Loader et al. 2013b).
However, it is clear that much complementary information can
be gained by analysing stable-isotope ratios, and other wood
properties, in addition to ring widths. Hartl-Meier et al. (2015)
have demonstrated this nicely. The combination of ring width
and stable-isotope analyses appears especially well suited to
providing information on how forest tree species are likely to
respond to climate stress (Allen et al. 2010). This information
could be of critical importance as we seek to manage forest
ecosystems in the current era of rapid global climate change.
Conflict of interest
None declared.
Funding
L.A.C. was supported by an Australian Research Council Future
Fellowship (FT100100329). N.B.E. was supported by an
Australian Research Council Discovery Early Career Research
Award (DE130100295).
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