Abstract
Extreme drought conditions across the globe are impacting biodiversity
with serious implications for the persistence of native species.
However, quantitative data on drought tolerance is not available for
diverse flora to inform conservation management. We quantified
physiological drought tolerance in the diverse Hakea genus (Proteaceae)
to test predictions based on climatic-origin, life history and
functional traits. We sampled terminal branches of replicate plants of
16 species in a common garden. Xylem cavitation was induced in branches
under varying water potential (tension) in a centrifuge and the tension
generating 50% loss of conductivity (stem P50) was characterized as a
metric for drought tolerance. The same branches were used to estimate
plant functional traits, including wood density, specific leaf area, and
Huber value (sap flow area to leaf area ratio). There was significant
variation in stem P50 among species, which was negatively associated
with the species climate-origin (rainfall and aridity). Drought
tolerance did not differ among life histories; however, a drought
avoidance strategy with terete leaf form and greater Huber value may be
important for species to colonize and persist in the arid biome. Our
findings will contribute to future prediction of species vulnerability
to drought and adaptive management under climate change.
Keywords: Aridity, climate change, drought tolerance,
life-history, functional traits, Proteaceae.
Acknowledgement : We appreciate the support of Benedict Lyte
from The Royal Botanic Garden, Sydney for granting us excess to their
living collections. Nzie Peter for support in data collection and Rosana
Lopez for technical insights into the hydraulic techniques. Australian
Postgraduate Award (Western Sydney University), Ecological Society of
Australia-Holsworth Wildlife Research Endowment grant to O.O.O. This
project has been supported by the New South Wales Government’s
Department for Planning, Industry and Environment, Saving Our Species
grant to P.D.R and D.T.
Introduction
The impacts of drought on diverse biomes across the globe are
substantial, with prolonged drought resulting in forest dieback and
plant mortality, changes in species distribution, local extinction and
decline in ecosystem function and resilience (Allen et al., 2010;
Goulden and Bales, 2019; Powers et al., 2020). Predicting the impacts of
drought on biomes and plant lineages remains a challenging task for
scientists, as most predictions relying on species distribution models
(SDM) and climatic niche data lack the species physiological tolerance
(Fitzpatrick et al., 2008; McDowell et al., 2008; Razgour et al., 2019;
Urban, 2015). Hence, quantifying species physiological thresholds is key
to understanding how plants will cope with extreme climatic-induced
events such as drought in the future (Allen et al., 2010).
One promising strategy to quantify physiological tolerance to drought is
by characterizing hydraulic traits in relation to water limitation
(Choat et al., 2012; Martin-StPaul et al., 2017). This is particularly
important, as studies have shown that most flowering plants
(angiosperms) function close to their hydraulic safety margin (minimum
xylem pressure experienced in the field - water potential causing 50%
loss of conductivity (P50)), and are vulnerable to
climate change (Choat et al., 2012). Under prolonged drought conditions,
stomatal closure is unable to prevent the continuous decline of the
xylem pressure, leading to cavitation, a phase change from liquid water
to gas, and the formation of gas emboli (Choat et al., 2012). This
results in loss of xylem hydraulic conductivity, and in severe cases,
hydraulic failure and subsequent mortality (Anderegg et al., 2016;
Brodribb and Cochard, 2009; Cochard, 2014; McDowell et al., 2008;
Pockman et al., 1995; Urli et al., 2013). A large body of evidence has
shown species drought tolerance is quantitatively linked with resistance
to cavitation in woody species (Adams et al., 2017; Brodribb and
Cochard, 2009; Choat et al., 2012; Kursar et al., 2009; McDowell et al.,
2008; Pockman et al., 1995). The xylem tensions associated with
irreversible damage (hydraulic failure) are approximated by
P50 in gymnosperms and by P88 (i.e.
water potential at 88% loss of conductivity) in angiosperms (Anderegg
et al., 2016; Urli et al., 2013), possibly reflecting structural and
functional differences in water transport systems (Choat et al. 2018).
Plants have adapted to water deficit through a wide range of life
history and functional traits, with underlying anatomical and
physiological mechanisms enabling them to colonise and persist in
variable climate. P50 is known to be correlated with
life history (Pratt et al., 2007), structure and function (Brodribb and
Holbrook, 2004; Jacobsen et al., 2007), and species climate range
(Bourne et al., 2017). For instance, in drier climates species tend to
have higher wood density, which provides greater resistance to xylem
conduit implosion under high xylem tensions and is strongly correlated
with P50 (Barotto et al., 2018; Hacke et al., 2001;
Jacobsen et al., 2005). Huber value (HV: ratio of sapwood area to leaf
area) is observed to be negatively related to site water availability
and P50, such that species in drier climates have higher
HV and more negative P50 than species in wetter climates
(Gotsch et al., 2010; Markesteijn et al., 2011). Leaf size has been
observed to be negatively related to drought tolerance
(P50) such that species with small leaves tend to be
more cavitation resistant (Markesteijn et al., 2011; Schreiber et al.,
2016). Studies have quantified and explored species vulnerability to
climate-induced drought (P50) in relation to functional
traits across biomes (Blackman et al., 2017, 2014; Bourne et al., 2017;
Larter et al., 2017; Li et al., 2019, 2018; Lucani et al., 2019;
Martorell et al., 2014; Nardini and Luglio, 2014; Pita et al., 2003).
However, our knowledge on the drought tolerance (P50) of
diverse related species in relation to the interactive effects of
functional and life-history traits on species survival across biomes
remain limited.
The Hakea genus is an ideal candidate for exploring variation in
physiological drought tolerance across contrasting biomes. This is
because Hakea is one of the two genera (the other beingGrevillea ) within the ancient Gondwana plant family Proteaceae,
that have successfully transitioned into the arid biome in the
Australian continent. Hakea also display a wide variation in functional
and life history traits within and among biomes. For instance, some
species re-sprout either from root suckers, epicormic or lignotuber buds
after disturbance such as fire and drought (e.g. H. purpurea, H.
drupacea and H. bakeriana ), while other Hakea species
must rely on seed production (e.g. H. sericea ) (Clarke et al.,
2013; Groom and Lamont, 1996; Weston, 1995). Leaf morphology varies
greatly among species, with broad-leaved (e.g. H. dactyloides ,H. cristata, H. bucculenta ) and terete leaved species (e.g.H. leucoptera, H. tephrospermum, H. sericea ) (Groom and Lamont,
1996). The differences in functional traits and life history forms among
the genus could influence species response to stress conditions (Groom
and Lamont, 1996; Zeppel et al., 2015). Studies have shown that
resprouting species tend to allocate more biomass to roots than shoots,
as well as exhibiting lower rates of photosynthesis, hydraulic
conductivity, and transpiration (Groom and Lamont, 1996; Hacke et al.,
2001; Hernández et al., 2011; Vilagrosa et al., 2014; Zeppel et al.,
2015). Leaf shape influences species exposure to drought, as such within
warmer and drier sites, species tend to be needle-leaved (Groom and
Lamont, 1996; Wright et al., 2017). Resprouting capacity and
needle-leaves support a drought avoidance strategy, and as such may have
variable drought tolerance (Groom and Lamont, 1996; Vilagrosa et al.,
2014; Zeppel et al., 2015).
In this study we aimed to determine the drought tolerance ofHakea species that differ in life history and climatic niches to
investigate what attributes are predictive of aridity. Firstly, we
hypothesized that there will be significant variation in drought
tolerance between species, and that this would be predicted by different
life-histories. Specifically, resprouting species will have higher
P50 than non-resprouting species, and that needle-leaved
species will have higher P50 than broad-leaved species
(Groom and Lamont, 1996; Hernández et al., 2011; Zeppel et al., 2015).
Secondly, we hypothesized that drought tolerance (P50)
will be predicted by species climate such that species in drier climates
will have higher P50 than species in wetter climates
(Bourne et al., 2017; Larter et al., 2017; Trueba et al., 2017).
Thirdly, drought tolerance (P50) will be positively
correlated to Huber Value (HV) and wood density (WD), and negatively
correlated to specific leaf area (SLA). This study will therefore
provide empirical evidence on species drought tolerance
(P50) to inform conservation management of diverse
native flora.
Materials and
Methods
Experimental design and Species
selection
All samples were collected from the same site, the Australian Botanic
Garden (ABG), Mount Annan, NSW, Australia (GPS location: Lat. -34.0703,
Log. 150.7668, average annual rainfall of 759 mm (2007-2016)) and were
well-watered via irrigation systems (simulating a common garden design).
Comparing multiple species from the same Hakea genus in a common
garden minimizes environmental effects and allows quantification of
genetically determined trait variation. Using this approach, we examined
the variation in functional and hydraulic traits of a diverse array of
species sampled from across the Hakea phylogeny (Cardillo et al.
2017). A total of 16 species were selected to represent a wide range of
vegetation type, biome, climate, and life histories (Table 1). Species
occurrence records were downloaded from the Australian Living Atlas
(ALA) (https://www.ala.org.au/, 2019). Vegetation type was defined
according to the World Wildlife Fund (WWF) as abbreviated by Cardillo et
al. (2017); Arid (Deserts and Xeric Shrublands), Mediterranean
(Mediterranean Forests, Woodlands and Scrub), Forest (Temperate
Broadleaf and Mixed Forests), Grasslands (Temperate Grasslands,
Savannas, and Shrublands). The vegetation harboring greater than 50% of
the species occurrence records was assigned as its vegetation type.
Biome was defined based on the aridity index (UNEP, 1997) as broadly
humid and arid (aridity index > 0.5, < 0.5,
respectively). The climate summary details for each species distribution
was obtained from The Atlas of Living Australia using R v3.6.3
(RCoreTeam, 2020).
Sampling of Plant material
Three individuals for each species were sampled from the AGB. Terminal
full sunlight, north-facing branch that were ca. 90 cm long were
sampled and placed into a black plastic bag with wet tissue paper and
transported immediately to the laboratory (<90 minutes).
Samples were stored in a cold room at 4oC until they
were processed (within 10 days). A standardized 50 cm branch was cut
under water from the terminal end of the collected samples, from which
the bottom 10 cm was excised, barked removed to estimate the sap flow
area and then oven dried to obtain the wood density (WD: oven dry
mass/volume). All leaves were removed from the remaining 40 cm branch
and leaf area measured using the Li Cor 3100 leaf area meter. Leaf
material was oven dried at 70 C for 48 h prior to obtaining the dry
mass. Specific leaf area (SLA,
mm2mg-1) was obtained by dividing
the total leaf area by the leaf oven dry mass. The ratio of the sapwood
area to leaf area was described as the Huber value (HV).