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Using global databases to disentangle trait‐specific and environmental influences on forest drought sensitivity

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Abstract

Recent extreme climate events, such as droughts, have led to unprecedented forest dieback worldwide. Serra-Maluquer et al. (2022) used open access, global scale databases to link woody species' drought tolerance capabilities to their specific set of life-history traits. They found that species that produce denser woody tissues and can tolerate more negative leaf water potentials are more resilient to extreme drought events, regardless of local conditions. This is an invited commentary on Serra-Maluquer et al., https://doi.org/10.1111/gcb.16123.
Glob Change Biol. 2022;00:1–2. wileyonlinelibrary.com/journal/gcb
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1© 2022 John Wiley & Sons Ltd
Received: 9 February 2022 
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Accepted: 28 February 2022
DOI : 10.1111/gcb .16157
COMMENTARY
Using global databases to disentangle trait- specific and
environmental influences on forest drought sensitivity
William Marchand
Department of Forest Ecology, Faculty of Forestr y and Wood Sciences, Czech University of Life Sciences Prague, Suchdol, Czech Republic
Correspondence
William Marchand, Department of Forest Ecolog y, Faculty of Forestry and Wood Sciences, Czech University of Life Sciences Prague, Kamýcká 129, Praha
6— Suchdol 165 00, Czech Republic.
Email: william.marchand@uqat.ca
1 | COMMENTARY ON “WOOD DENSITY
AND HYDRAULIC TRAITS INFLUENCE
SPECIES’ GROWTH RESPONSE TO
DROUGHT ACROSS BIOMES”
As global warming increases the frequency and intensity of climate
extremes, the magnitude of their negative impacts on natural ecosys-
tems worldwide is a major concern. Forest biomes are among those
already experiencing drastic changes in their structure and functions,
with potentially significant impacts on the ecological services pro-
vided to human populations. The 2018– 2019 megadrought, which
resulted in unprecedented mortality of key tree species in Europe,
is a recent example of such impacts (Schuldt et al., 2020). The new
paper by Serra- Maluquer et al. (2022) is one of the most recent to
address the effects of dry extremes on forest biomes. This work,
which includes results from 65 tree species from six continents, sheds
new light on the relationship between a species’ ability to cope with
extreme drought episodes, its sensitivity to interannual variations in
moisture availability, and its morpho- physiological characteristics.
The response of a tree species to variations in environmental con-
ditions, including climate, is determined primarily by its ecological re-
quirements. The closer the environmental conditions are to a species’
ecological optimum, the better the growth performance of a tree,
and the higher the amount of fixed carbon. Conversely, when envi-
ronmental conditions move further away from the species’ ecological
thresholds, the capacity of a tree to acquire and use carbon from the
atmosphere is compromised. Thus, the growth rings that a tree pro-
du ces ea c h year do c ument an y change s in gr owing co n dition s. In terest
in using these natural archives to address numerous ecological ques-
tions has increase d in recent decades, lea ding to the internation al tree
ring data bank (ITRDB), an open- access repository providing annually
resolved tree- ring records for all forested continents (Grissino- Mayer
& Fritts, 1997). Tree- ring data are inherently noisy because they are
the result of a plethora of interacting environmental and internal
forces. Therefore, the global spatial coverage of the ITRDB network,
combined with an appropriate statistical framework such as that em-
ployed by Serra- Maluquer et al. (2022), is an invaluable resource for
accurately distinguishing the different processes involved in the sen-
sitivity of woody species to drought.
Climatically exceptional periods, such as the 20182019
European drought, push trees even further away from their eco-
logical thresholds, which can threaten their physiological integrity
through damage to photosynthetic organs and hydraulic pathways.
Drought- stressed trees thus face a “safety- efficiency” trade- off
(Grossiord et al., 2020). A more conservative behavior, through strict
adjustment of water- regulating physiological parameters, such as a
tight stomatal closure, makes it possible to avoid long- term damage
to hydraulic pathways at the expanse of carbon uptake. This strat-
egy makes trees more susceptible to rapid depletion of their carbon
reserves, which increases their propensity to be affected by pests
and diseases. Riskier behavior, on the other hand, allows for a more
efficient carbon uptake under dry conditions but increases the like-
lihood of permanently impaired conductive vessels and associated
death due to hydraulic failure (Sevanto et al., 2014). The ability of a
woody species to tolerate extreme drought is typically assessed by
comparing growth rates before, during, and after a drought event
using a set of normalized, simple, and efficient resilience indices
(Schwarz et al., 2020). The lower the initial impact on growth and the
faster and higher the post- stress recover y, the greater the chances
for a tree to sur vive an extreme drought event (DeSoto et al., 2020).
A relatively recent trend in forest ecology is to study the effects
of climate extremes using the set of morphological, physiological, and
chemical characteristics, also referred to as “functional traits,” that
have been evolutionarily selected to maximize a species’ fitness in a
given environment. In trees, functional traits play an important role
in the likelihood of surviving an extreme drought (Greenwood et al.,
This is an i nvited comment ary on Serra - Maluquer et al., ht tps://doi.or g/10.1111/
gcb .16123.
2 
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    COMMENTARY
2017). Serra- Maluquer et al. (2022) used open access trait datasets,
such as the TRY database (Kattge et al., 2020), to investigate the ef-
fects of “soft” and “hard” functional traits on drought tolerance of
woody species. “Soft” traits are those related to the leaf and wood
economic spectra. For example, wood density is the result of a trade-
off between the production of high- quality, drought- resistant vessels
and the time and carbon costs of these structures. “Hard,” or hydrau-
lic, traits determine water use and transport. For example, leaf min-
imum water potentials reflect a trade- off between the efficiency of
carbon assimilation under drought conditions and the risk of damage
to hydraulic pathways. Serra- Maluquer et al. (2022) observed that
species in arid areas are more sensitive to annual fluc tuations in mois-
ture availability and are less resistant but more resilient to extremely
dry years. In contrast to previous studies that attributed these rela-
tionships to regional climate gradients, Serra- Maluquer et al. (2022)
demonstrated that this pattern is explained to a significant extent by
the com binati on of high- densit y wood and the prope ns it y of a specie s
to tolerate more negative leaf water potentials.
Serra- Maluquer et al. (2022) provide a unique, global overview of
th e sens iti vit y of woody sp ecie s to dr ought. If imple ment ed in proce ss-
based models, these results have the potential to improve the predic-
tion of species distribution under future climate conditions. This could
also help in the selection of the most suitable candidates for assisted
migration of tree species. Their study poses the basis of a statistical
framework that allows trait- dependent effects to be separated from
those due solely to local environmental conditions. It demonstrates
the invaluable role of open data in better understanding geographic
patterns in species response to global warming. The current move to
open science will undoubtedly increase the number of publicly avail-
able records, which will help address some major challenges in forest
ecology. As discussed by Serra- Maluquer et al. (2022), the current
ITRDB network is highly biased in terms of geographic coverage and
species representativeness. A geographically expanded network that
would include a higher number of records from broadleaf tree spe-
cies would broaden the range of available values for environmental
conditions and functional traits. This would increase our ability to ac-
curately partition the variance in drought sensitivity among different
explanatory factors. Future research projects should focus on forests
in Africa and South America, which are currently understudied.
Another concern in forest ecology is the lack of clarity surround-
ing some important concepts and metrics. For example, the debate
over the use of raw versus detrended tree- ring data is ongoing.
Currently, a multiplicity of detrending methods is in use, all of which
have their own biases and uncertainties. More importantly, the cal-
culation of resilience indices is still debated (Schwarz et al., 2020),
with no universally accepted definition of “drought” years. Some
scientists recommend the use of normalized drought indices, while
others contend that absolute metrics would best capture local levels
of moisture availability (Slette et al., 2019 and related discussions).
The use of consistent metrics could improve our understanding of
the processes involved in forest sensitivity to climate extremes by
facilitating the comparison of results from different studies.
ORCID
William Marchand https://orcid.org/0000-0003-2114-5244
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