The influence of physiological traits on tree drought resilience
Our results confirmed that inter-specific variations of radial growth
responses to extreme drought event are significantly influenced by tree
physiological traits pertinent to drought
tolerance
(Anderegg et al., 2015; Fang & Zhang, 2019; Gazol et al., 2017).
Trees
face more negative xylem pressures than normal during drought events
that can induce serious hydraulic dysfunction through embolism formation
(Anderegg et al., 2012, 2013). The conifer species with higher wood
density usually have narrower tracheids and show stronger resistant to
drought-induced xylem embolism (Pockman and Sperry, 2000; Montwé et al.,
2014). Under the same water conditions, tree species with larger
hydraulic safety margins would have lower risk of reachingP 50 (Anderegg et al., 2012; Choat et al., 2012),
which is regarded as a critical threshold of embolism formation causing
catastrophic hydraulic failure in conifers (Brodribb et al., 2010;
Schuldt et al., 2016). The hydraulic functions of the studied pine
species with higher HSM and WD would be less likely harmed during
non-lethal drought events (Anderegg et al., 2012; Santiago et al., 2018;
Tyree & Sperry 1989), which would hence show lower degrees of decrease
in photosynthetic carbon assimilation in face of drought and require
less input for damage recovery posterior drought events. The positive
correlations observed here between growth resilience to drought (RT and
RS calculated from tree-ring analyses) and wood functional traits
related to drought tolerance (HSM and WD) indicate the importance of
hydraulic safety in guaranteeing tree growth over relatively long
periods in water-limited environments.
The positive correlation between leaf turgor loss point and growth
resilience indicates that the pine species with less drought-tolerant
leaves tend to show higher radial growth resistance to extreme drought
event in the water-limited environment of the study site. Leaf turgor
loss point can be used to predict species’ capacity of osmotic
adjustment and the degree of isohydry-anisohydry (Bartlett, Scoffoni, &
Sack, 2014; Meinzer et al., 2016). Species with more negative leaf
turgor loss points are able to resist leaf dehydration better, and
thereby maintaining stomatal conductance, hydraulic conductance and
photosynthetic gas exchange at lower water potentials (Bartlett et al.,
2014, 2015). In contrast, species with less negative leaf turgor loss
points tend to be more isohydric and enable leaves to close their
stomata earlier as water potential decreases and hence can better avoid
hydraulic failure during drought stress (Brodribb & Holbrook, 2003;
Meinzer et al., 2016). Species with different leaf turgor loss points
are linked to different capacity in regulating carbon and water balances
under drought events of different duration and intensity (McDowell et
al., 2008; Mitchell et al., 2013). During a drought event of
high-intensity but relatively short duration, more adverse effects on
tree growth would be caused by xylem embolism due to sharp decrease of
water potentials in more anisohydric species than adverse effects of
stomatal closure on photosynthetic carbon assimilation in more isohydric
species (Manrique-Alba et al., 2018; Mitchell et al., 2013).
Hydraulic dysfunction caused by extreme drought events impairs
post-drought growth and results in legacy effects of drought, which have
been reported to be most prevalent among pine species and in dry
ecosystems (Anderegg et al., 2013, 2015; Gazol et al., 2017; Wu et al.,
2017). Drought-induced xylem embolism limit water transport capability
and hinder the recovery of photosynthetic rate over a relatively long
period after drought events (Anderegg et al., 2013; Rehschuh et al.,
2020; Skelton et al., 2017). A growing body of literature indicates that
drought-induced hydraulic dysfunction in pines is nonreversible and is
compensated primarily by xylem development over the next few growing
seasons rather than embolism refilling (Anderegg et al., 2013; Brodribb
et al., 2010; Hammond et al., 2019). Our data supported the viewpoint
that hydraulic safety influences the post-drought radial growth probably
through the residual effect of drought-induced hydraulic dysfunction,
since the studied species with larger hydraulic safety margin tend to
have higher RS, and
RT.
The results also indicate that physiological traits reflecting plant
resistance to drought (e.g. HSM and WD) can be used as good proxies for
predicting specific-specific growth response to drought events
(Skelton
et al., 2017; Zhang et al., 2017; Fu & Meinzer, 2018).