Abstract
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 [CO2 ] 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 eCO2 , the seedlings were grown with ample of water and nutrient supply under either highly elevated [CO2 ] (eCO2 , c. 936 ppm) or ambient (aCO2 , 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 eCO2 than aCO2 . 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 eCO2 . Overall, our study provides for the first-time insights into the chronological interplay of physiological drought thresholds under long-term acclimation to elevated [CO2 ].
Highlights
Rising temperatures, changing precipitation patterns, more frequent as well as more severe extreme weather events are well-known projections of global climate change (Dai, 2013; Huang, Yu, Guan, Wang, & Guo, 2016; IPCC, 2014; Reichstein et al, 2013)
Our hypotheses were: (a) larger net C uptake and enhanced water-use-efficiency as long-term acclimation to highly elevated [CO2] benefit the survival of P. halepensis seedlings during drought, (b) the [CO2] advantage is more pronounced during a slow lethal drought compared with a fast lethal drought as non-structural carbohydrates (NSC) reserves are more crucial when carbon starvation becomes the main mortality risk, and (c) the [CO2] benefit is apparent in the change of critical thresholds of mortality risk factors resulting in the prolonging of the time-to-mortality in P. halepensis seedlings
Lower levels of photosynthetic active radiation (PAR) are not untypical at the forest floor and we found that our seedlings reached 77 %/67 % of their maximum photosynthesis as derived from light response curves at a PAR of 500 μmol m−2 s−1 (Supplement 3)
Summary
Our hypotheses were: (a) larger net C uptake and enhanced water-use-efficiency as long-term acclimation to highly elevated [CO2] benefit the survival of P. halepensis seedlings during drought, (b) the [CO2] advantage is more pronounced during a slow lethal drought compared with a fast lethal drought as NSC reserves are more crucial when carbon starvation becomes the main mortality risk, and (c) the [CO2] benefit is apparent in the change of critical thresholds of mortality risk factors resulting in the prolonging of the time-to-mortality in P. halepensis seedlings
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