Dying by drying: Timing of physiological stress thresholds related to tree death is not significantly altered by highly elevated CO2_{2}

Drought‐induced tree mortality is expected to occur more frequently under predicted climate change. However, the extent of a possibly mitigating effect of simultaneously rising atmospheric [CO$_{2}$] on stress thresholds leading to tree death is not fully understood, yet. Here, we studied the drought response, the time until critical stress thresholds were reached and mortality occurrence of Pinus halepensis (Miller). In order to observe a large potential benefit from eCO$_{2}$, the seedlings were grown with ample of water and nutrient supply under either highly elevated [CO$_{2}$] (eCO$_{2}$, c. 936 ppm) or ambient (aCO$_{2}$, c. 407 ppm) during 2 years. The subsequent exposure to a fast or a slow lethal drought was monitored using whole‐tree gas exchange chambers, measured leaf water potential and non‐structural carbohydrates. Using logistic regressions to derive probabilities for physiological parameters to reach critical drought stress thresholds, indicated a longer period for halving needle starch storage under eCO$_{2}$ than aCO$_{2}$. Stomatal closure, turgor loss, the duration until the daily tree C balance turned negative, leaf water potential at thresholds and time‐of‐death were unaffected by eCO$_{2}$. Overall, our study provides for the first‐time insights into the chronological interplay of physiological drought thresholds under long‐term acclimation to elevated [CO$_{2}$]

Relationships between xylem embolism and tree functioning during drought, recovery, and recurring drought in Aleppo pine

Recent findings suggest that trees can survive high levels of drought-induced xylem embolism. In many cases, the embolism is irreversible and, therefore, can potentially affect post-drought recovery and tree function under recurring droughts. We examined the development of embolism in potted Aleppo pines, a common species in hot, dry Mediterranean habitats. We asked (1) how post-drought recovery is affected by different levels of embolism and (2) what consequences this drought-induced damage has under a recurring drought scenario. Young trees were dehydrated to target water potential (Ψx) values of −3.5, −5.2 and −9.5 MPa (which corresponded to ~6%, ~41% and ~76% embolism), and recovery of the surviving trees was measured over an 8-months period (i.e., embolism, leaf gas-exchange, Ψx). An additional group of trees was exposed to Ψx of −6.0 MPa, either with or without preceding drought (Ψx of −5.2 MPa) to test the effect of hydraulic damage during repeated drought. Trees that reached −9.5 MPa died, but none from the other groups. Embolism levels in dying trees were on average 76% of conductive xylem and no tree was dying below 62% embolism. Stomatal recovery was negatively proportional to the level of hydraulic damage sustained during drought, for at least a month after drought relief. Trees that experienced drought for the second time took longer to reach fatal Ψx levels than first-time dehydrating trees. Decreased stomatal conductance following drought can be seen as “drought legacy,” impeding recovery of tree functioning, but also as a safety mechanism during a consecutive drought