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).