NEWS & VIEWS RESEARCH
configuration in their target alloys. This
revealed that the more thermo­dynamically
stable the configuration, the stiffer the resulting material.
Many researchers use computation to identify atomic structures of a given material that
have desirable properties, often with little
regard for whether those structures are feasible to make (thermodynamically stable).
Maisel and co-workers’ approach shows that,
at least in the case of elastic stiffness, hunting for metastable structures that have better
properties than stable structures — whether
known or predicted — is essentially futile, and
that researchers should focus on other mater­
ials instead. That said, being able to predict
both the stability and another target property
of a material will allow scientists to efficiently
scan through sets of hypothetical materials
and ‘see’ promising candidates, lending vision
to an established computational approach.
It is in this discovery mode that Maisel and
colleagues’ work could contribute greatly to
efforts such as the Materials Genome Initiative.
It remains to be seen how many other
materials’ properties will be studied using the
new approach. In principle, any property that
directly depends on atomic configuration is
within reach, but many properties of relevance
to engineering are still difficult to compute
in practice. Furthermore, some of the most
important properties of materials depend not
only on the atomic configuration of a fixed
lattice, but also on microstructural elements
— such as boundaries between microscopic
crystals (grains), grain sizes and extended crystal defects. These remain beyond the reach of
quantum-mechanical calculations.
Still, high-throughput approaches4,5 for
sifting through thousands, or tens of thousands, of candidate materials are poised to
make a substantial contribution to society’s needs by generating large databases of
information that will be of use to researchers6,7. These databases will be more effective if the information they contain about
physical properties is used to build computational models that, in turn, could search for
P L AN T ECO LO GY
Forests on the brink
An analysis of the physiological vulnerability of different trees to drought shows
that forests around the globe are at equally high risk of succumbing to increases
in drought conditions. See Letter p.752
BETTINA M. J. ENGELBRECHT
W
ater is the most limiting factor for
ecosystem diversity and productivity worldwide. But the global
climate is changing, and both warming and
shifts in rainfall patterns are projected, which
will leave large areas of the planet with less rain
and a higher likelihood of extreme drought
events1,2. These changes will almost certainly
affect forests, which cover more than 30% of
the world’s land surface. Understanding these
effects is imperative: forests play an integral
part in carbon and water cycles, they provide
timber and other products, and they are home
to a vast diversity of plants, animals and microorganisms. But forests occur in a wide range of
climatic conditions, so it is a challenge to predict how the vulnerability of trees to changes in
water availability compares between different
biomes. In this issue, Choat et al.3 (page 752)
use a combination of physiological measurements of the vulnerability of trees to drought
and of the drought stress they actually experience in their natural habitats to show that forests worldwide are at high risk*.
We might expect that trees in forests
*This article and the paper under discussion3 were
published online on 21 November 2012.
currently exposed to seasonal or multi-annual
droughts, such as in ‘Mediterranean-type’
systems, are already well adapted and will
therefore suffer less from an increase in
drought conditions than trees in wet forests.
Conversely, but equally reasonably, we could
predict that trees in dry areas are already at
their physiological limits and would therefore
be more vulnerable to increased drought than
trees in wet forests. To investigate these questions, Choat and colleagues compared the vulnerability of the tree water-transport system to
drought in different species worldwide.
In plants, water is transported through a
tubing system, a tissue called xylem that is
made up of a multitude of conduits. Loss of
water vapour (transpiration) through stomata
(pores) in the plants’ leaves generates suction
that pulls water in the xylem from the soil
through the roots and stem to the leaves —
much like sucking water through a straw. On
its way, the water provides crucial services
to the plant: it is the medium for metabolic
reactions, it transports nutrients and other
substances, and it provides stability. However,
the powerful suction that pulls water through
the xylem brings with it the risk of pulling air
through small holes, called pit pores, in the
sides of the conduits. These air bubbles can
thermodynamically stable materials to meet a
particular need8. Maisel and colleagues’ work,
coupled with automated model-building
methods9, might help us to achieve that goal. ■
Gus L. W. Hart is in the Department of
Physics and Astronomy, Brigham Young
University, Provo, Utah 84602, USA.
e-mail: [email protected]
1. Sass, S. L. The Substance of Civilization (Arcade, 1998).
2. Maisel, S. B., Höfler, M. & Müller, S. Nature 491,
740–743 (2012).
3.www.whitehouse.gov/sites/default/files/microsites/
ostp/materials_genome_initiative-final.pdf
4. Curtarolo, S., Morgan, D., Persson, K., Rodgers, J. &
Ceder, G. Phys. Rev. Lett. 91, 135503 (2003).
5. Curtarolo, S. et al. Comput. Mater. Sci. 58, 218-–226
(2012).
6. Curtarolo, S. et al. Comput. Mater. Sci. 58, 227–235
(2012).
7. Jain, A. et al. Comput. Mater. Sci. 50, 2295–2310
(2011).
8. Yang, K., Setyawan, W., Wang, S., Buongiorno
Nardelli, M. & Curtarolo, S. Nature Mater. 11,
614–619 (2012).
9. Nelson, L. J., Zhou, F., Hart, G. L. W. & Ozoliņš, V.
preprint at http://arxiv.org/abs/1208.0030 (2012).
block the xylem and impair water transport,
just like sucking air into a broken straw. This
process is called xylem embolism, and the
higher the suction in the conduit, the more
embolism occurs.
The link between this physiology and
drought conditions comes from the fact that
suction increases with increasing transpiration
and/or decreasing water availability in the soil.
Plants can regulate their stomata to delay the
increase in suction, but if water is not replenished, more and more conduits will become
clogged, leading to hydraulic failure and the
eventual death of the plant. However, different
plant species have different xylem structures,
so the vulnerability of a plant’s xylem conduits to embolism, and therefore its ability to
tolerate drought, are variable.
The authors compiled data on the xylem
vulnerability of 480 tree species from 183 sites
worldwide, covering the broad range of
climatic conditions in which forests occur.
They included both angiosperms (flowering
trees, such as oak and maple) and gymnosperms (such as pine and cedar), which vary
substantially in their xylem structure. Wherever the data were available, they also included
the maximum suction occurring in the trees in
their natural habitats. Combining these data
enabled Choat et al. to explore how the suction
that induces hydraulic failure in a given species
compares with the suction that it actually experiences. If these values are close together, this
represents a small ‘safety margin’ with respect
to hydraulic failure and indicates that the
species is at risk; if they are far apart, the species
is likely to be able to withstand more intense
drought conditions.
The data show that, as expected, trees growing in more arid conditions around the globe
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© 2012 Macmillan Publishers Limited. All rights reserved
B. WERMELINGER, WSL
RESEARCH NEWS & VIEWS
50 Years Ago
It seems to be generally agreed that
the standard of self-expression in
spoken and written English among
sixth-form and undergraduate
scientists and technologists is low.
Various causes have been blamed
… but in all the diagnoses and cures
I have seen so far, all the emphasis
has been on past failures by English
experts and future remedies to
be administered by other English
experts. It is not my intention to
dissociate English teachers from
the problem altogether … but I
want to suggest that scientists and
technologists themselves must take
most of the responsibility for the low
standards of self-expression in their
professions, and that a major change
of outlook on their part is the only
thing that can bring a substantial
improvement in the situation.
From Nature 1 December 1962
100 Years Ago
Biologische und morphologische
Untersuchungen über Wasser- und
Sumpfgewächse. By Prof. H. Glück
— Prof. Glück has produced a
portentous volume on the riparian
flora, forming the third instalment
of his work on water and swamp
plants. Frankly, we do not find
justification for the 600 or more
pages of his book, and we fancy
most readers who have been in
the habit of using their eyes when
observing or collecting plants will
find but little to reward them for
the trouble of its perusal … No
doubt a work of this kind possesses
some value, but, as it appears to us,
it excellently illustrates the truth
of the saying that the secret of
dullness lies in the attempt to write
all one knows. Prof. Glück gives
the impression (perhaps unjustly)
that he has written all he knows
about his subject, and certainly he
has jotted down a good deal that is
already very familiar to others.
From Nature 28 November 1912
Figure 1 | Thirsty trees. Reports of drought-induced forest die-off 4, such as that in Switzerland in 1999
shown here, have increased in recent decades, suggesting that climate change is already having an impact
on tree health in many locations. Choat and colleagues’ study3 of trees across the globe suggests that they
are at high risk from even small increases in drought intensity.
are better at withstanding xylem embolism.
The exciting finding, however, is that angiosperm trees in all forest biomes have converged
on a risky strategy, operating at extremely
narrow safety margins. This implies that
these trees are already, under current conditions, on the verge of injurious levels of water
availability, and that even a minor increase in
drought intensity will induce levels of xylem
embolism that will impair growth and lead
to tree death.
The suggestion that all forests are on the
brink of succumbing to drought, and may
already be responding to climate change,
is supported by observations of increased
drought-induced forest die-offs and tree
mortality in many ecosystems4 (Fig. 1). For
gymnosperms, Choat et al. found wider safety
margins, suggesting that these trees may
have a higher tolerance to increased drought.
However, even these trees are threatened by
hydraulic failure, as recent regional die-offs
of pines show4. Taken together, these studies
sound a warning bell that we can expect to
see forest diebacks become more widespread,
more frequent and more severe — and that no
forests are immune. The ramifications of this
scenario are diverse and, in many respects,
dire: forest mortality will be accompanied by
changes in species composition, changes in
ecosystem function and losses of services and
biodiversity4.
Advancing our knowledge of organismal
responses to factors such as drought and
temperature is essential to improving predictions of the consequences of climate change5,6.
6 7 6 | NAT U R E | VO L 4 9 1 | 2 9 NOV E M B E R 2 0 1 2
© 2012 Macmillan Publishers Limited. All rights reserved
Through their meta-analysis of the global distribution of xylem vulnerability, Choat et al.
have dramatically increased our understanding of the comparative vulnerability of forests.
Nevertheless, the mechanisms that actually
lead to drought-induced tree mortality still
remain elusive; in fact, it is known that some
species can survive complete hydraulic failure
for extended periods of time7. Although many
studies have assessed the response of plants
to experimentally manipulated precipitation
and/or temperature8, the results of these studies do not lend themselves to comparisons of
drought responses across biomes, because of
differences in treatments and in the resulting
drought intensities. A coordinated network
of standardized experiments is needed to further advance understanding of climate-change
responses in ecosystems worldwide.
Our ability to forecast the consequences
of drought for forests is also limited by the
high regional uncertainty in current models
for rainfall and drought prediction, for both
long-term trends and extreme events1,2. A
fundamental lesson from Choat and colleagues’ study is that even small changes in
drought intensity can be expected to lead
to mortality in forests all over the world.
This only highlights the urgent need for
climate models that return more-confident
predictions. ■
Bettina M. J. Engelbrecht is at the Bayreuth
Center of Ecology and Environmental Science,
Department of Plant Ecology, University of
Bayreuth, 95440 Bayreuth, Germany, and at
NEWS & VIEWS RESEARCH
the Smithsonian Tropical Research Institute,
Panama.
e-mail: [email protected]
1. Parry, M. L. et al. (eds) Climate Change 2007:
Impacts, Adaptation and Vulnerability. Contribution
of Working Group II to the Fourth Assessment Report
of the Intergovernmental Panel on Climate Change
(Cambridge Univ. Press, 2007).
2. Field, C. B. et al. (eds) Managing the Risks of Extreme
Events and Disasters to Advance Climate Change
Adaptation. A Special Report of Working Groups I and
II of the Intergovernmental Panel on Climate Change
(Cambridge Univ. Press, 2012).
3. Choat, B. et al. Nature 491, 752–755 (2012).
4. Allen, C. D. et al. Forest Ecol. Mgmt 259, 660–684
(2010).
5. Svenning, J.-C. & Condit, R. Science 322, 206–207
(2008).
6. Craine, J. M. et al. Nature Clim. Change http://dx.doi.
org/10.1038/nclimate1634 (2012).
7. McDowell, N. G. Plant Physiol. 155, 1051–1059
(2011).
8. Wu, Z., Dijkstra, P., Koch, G. W., Penuelas, J. &
Hungate, B. A. Glob. Change Biol. 17, 927–942 (2011).
E ARTH SCIENCE
Magma chambers
on a slow burner
An assessment of crystallization processes occurring in magma chambers in the
ocean floor finds an unexpected enrichment in trace elements, reviving an old
theory of the cycling of magma in these chambers. See Article p.698
A L B R E C H T W. H O F M A N N
T
he world’s ocean basins are constantly
being regenerated by an 80,000kilo­m etre-long volcanic system of
mid-ocean ridges, where Earth’s mantle melts
to form magma that eventually produces the
basaltic floor of the oceans. The composition of ocean-floor basalts is one of the main
sources of information about Earth’s deeper
interior. On page 698 of this issue, O’Neill and
Jenner1 re-examine the chemical compositions of basaltic lavas from this global magmatic system. They find new, and remarkably
systematic, chemical relationships between
the concentrations of ‘incompatible’ trace
elements (so named because they are largely
excluded from magmatic crystals) and that of
magnesium oxide (MgO).
As expected, the content of incompatible
elements increases in the basaltic-liquid
component (the melt) of magmas, because
MgO-bearing crystals precipitate in suboceanic magma chambers (reservoirs),
causing the MgO content of the liquid to
decrease. But O’Neill and Jenner show that
the observed incompatible-element increase
is much greater than conventional crystallization processes can explain. Their proposed
solution to this dilemma would require a
revision in the way geochemists calculate
the composition of parental magmas entering magma chambers, and therefore also the
composition of the mantle rocks from which
these magmas are derived.
When basaltic lava comes into contact with
cold sea water, it is chilled into glass. Geochemists like to analyse such glasses because
they preserve the chemical composition of the
lava particularly well, and they can thus tell the
researchers much about the composition of
the underlying mantle in which the melt forms.
However, this view of the mantle is blurred
because there are several intervening stages
between melt formation and the eruption of
lava. These are: partial melting of the mantle
at depth (greater than about 30 km); extraction
of the melt from the partially molten mush; its
emplacement in shallow magma chambers; the
formation and settling out of magmatic crystals in these chambers; and, finally, eruption of
the remaining liquid on the ocean floor.
Two fundamentally opposing views of
the mantle composition inferred from these
glasses have prevailed over the past 40 years.
One holds that the mantle has an essentially
uniform composition, and that the compositional variability of the erupted basaltic lavas is
produced primarily by processes occurring in
the shallow magma chambers. The other view
holds that magma-chamber processes have
only minor effects on the erupted lavas that can
be easily corrected for, and that the variations
in lava composition mainly reflect differences
in the composition of the mantle source and in
the specifics of the melting process.
This latter view has gradually gained the
upper hand, because much of the observed
chemical variability of the lavas correlates
with variations in the isotopic composition
of the elements strontium, neodymium,
hafnium and lead. These elements are the
products of very slow radio­active decay, and
therefore accumulate only during long residence times in the mantle. The observed differences in isotopic composition can therefore
not be produced in short-lived magma chambers, but require long-term differences in
parent–daughter ratios in the (mantle) source
of the melts.
A crucial requirement when going backward from observed compositions of erupted
basalts to their mantle sources is to evaluate
the effects of partial crystallization and loss
of the crystals in magma chambers. This is
widely assumed to involve ‘fractional crystallization’, whereby newly formed crystals are
immediately removed from chemical inter­
action with the liquid. Laboratory experiments
have shown that the crystallization process
in ocean-ridge magma chambers invariably
involves the magnesium-bearing mineral
olivine. The net effect of this is that the MgO
content of the liquid progressively decreases
as freshly crystallized olivine is removed from
the liquid, whereas there is an increase in the
contents of incompatible trace elements (such
as barium, thorium and neodymium) because
they are excluded from the crystals.
This was thought to be well understood —
until O’Neill and Jenner plotted the incompatible-element contents against MgO for
two recently assembled global data sets2,3.
They found excellent linear correlations
(with the expected negative slopes) between
incompatible-element and MgO contents.
However, they were startled to find that these
slopes are consistently greater than the maximum allowed from fractional-crystallization
theory.
If fractional crystallization does not explain
this effect, what process does? One possibility
is that lavas that have higher incompatibleelement contents start out with systematically lower parental MgO contents. But that
would mean that the sources of these magmas
could not contain olivine, even though this is
the most common of all upper-mantle minerals. Nevertheless, it has been proposed4 that
some mid-ocean-ridge basalts are mixtures of
liquids formed from peridotite, the ‘standard’
olivine-bearing mantle rock, and other liquids
formed from eclogite or pyroxenite, which are
olivine-free rocks that form from subducted,
recycled oceanic basalts. Melts from such
recycled basalts should also have a higherthan-normal content of incompatible elements
and a lower-than-normal MgO content. Such
recycled basalts should also have different
isotopic compositions of neodymium, for
example. However, the expected correlations
between neodymium isotopes and MgO have
not been documented for any global set of
ocean-ridge basalts.
As a way out of the dilemma, O’Neill and
Jenner revive and generalize a model that
was originally proposed by O’Hara 5 and
later modified by Albarède6, but which has
been mostly forgotten. This model envisages a magma chamber that is periodically
refilled with fresh parental liquid from below.
The fresh liquid mixes with the pre-existing
liquid, and the mixture is tapped by a volcano,
whereupon crystallization resumes. This ‘trick’
of replenishment with fresh parental magma
keeps the MgO content of the liquid from
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© 2012 Macmillan Publishers Limited. All rights reserved