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

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.