Hot drought reduces the effects of elevated CO₂ on tree water-use efficiency and carbon metabolism

• Published in: The New phytologist Vol. 226; no. 6; pp. 1607 - 1621
• Main Authors: Birami, Benjamin, Nägele, Thomas, Gattmann, Marielle, Preisler, Yakir, Gast, Andreas, Arneth, Almut, Ruehr, Nadine K.
• Format: Journal Article
• Language: English
• Published: Lancaster Wiley 01.06.2020; Wiley Subscription Services, Inc
• Subjects: carbon balance; Carbon dioxide; Climate change; Conductance; Drought; elevated CO2; Environmental impact; Heat; Heat exchange; Heat stress; Heat tolerance; Metabolic response; Metabolism; Metabolites; photosynthesis; Physiological responses; Pine; primary metabolites; protein; Resistance; respiration; Stability; Stomata; Stomatal conductance; Stress response; Trees; Uptake; Water loss
• ISSN: 0028-646X; 1469-8137
• DOI: 10.1111/nph.16471

• Trees are increasingly exposed to hot droughts due to CO₂-induced climate change. However, the direct role of [CO₂] in altering tree physiological responses to drought and heat stress remains ambiguous. • Pinus halepensis (Aleppo pine) trees were grown from seed under ambient (421 ppm) or elevated (867 ppm) [CO₂]. The 1.5-yr-old trees, either well watered or drought treated for 1 month, were transferred to separate gas-exchange chambers and the temperature gradually increased from 25°C to 40°C over a 10 d period. Continuous whole-tree shoot and root gas-exchange measurements were supplemented by primary metabolite analysis. • Elevated [CO₂] reduced tree water loss, reflected in lower stomatal conductance, resulting in a higher water-use efficiency throughout amplifying heat stress. Net carbon uptake declined strongly, driven by increases in respiration peaking earlier in the well-watered (31–32°C) than drought (33–34°C) treatments unaffected by growth [CO₂]. Further, drought altered the primary metabolome, whereas the metabolic response to [CO₂] was subtle and mainly reflected in enhanced root protein stability. • The impact of elevated [CO₂] on tree stress responses was modest and largely vanished with progressing heat and drought. We therefore conclude that increases in atmospheric [CO₂] cannot counterbalance the impacts of hot drought extremes in Aleppo pine.