Hot drought reduces the effects of elevated CO 2 on tree water‐use efficiency and carbon metabolism

Summary Trees are increasingly exposed to hot droughts due to CO 2 ‐induced climate change. However, the direct role of [CO 2 ] 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) [CO 2 ]. 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 [CO 2 ] 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 [CO 2 ]. Further, drought altered the primary metabolome, whereas the metabolic response to [CO 2 ] was subtle and mainly reflected in enhanced root protein stability. The impact of elevated [CO 2 ] on tree stress responses was modest and largely vanished with progressing heat and drought. We therefore conclude that increases in atmospheric [CO 2 ] cannot counterbalance the impacts of hot drought extremes in Aleppo pine.