Abstract
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 (421ppm) or elevated (867ppm) [CO2 ]. The 1.5-yr-old trees, either well watered or drought treated for 1month, were transferred to separate gas-exchange chambers and the temperature gradually increased from 25°C to 40°C over a 10d 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.
Highlights
Forests are exposed to a rapidly changing climate world-wide, and extreme weather events such as heatwaves and drought spells are predicted to increase in frequency and severity as atmospheric [CO2] a[CO2]) is rising (Coumou & Rahmstorf, 2012; Baldwin et al, 2019; Pfleiderer et al, 2019)
Growing Aleppo pines for 18 months under e[CO2] of c. 870 ppm had a stimulating effect on tree biomass (+40%), but did not result in larger tree H2O loss due to reductions in stomatal conductance reflected in a nearly proportional increase in WUE maintained throughout a 10 d heatwave (25°C, 30°C, 35°C, 38°C, 40°C)
Considering the tree C balance, we found a stimulation of the net C uptake under e[CO2] largely due to reduced tissue respiration alongside lower protein content
Summary
Forests are exposed to a rapidly changing climate world-wide, and extreme weather events such as heatwaves and drought spells are predicted to increase in frequency and severity as atmospheric [CO2] a[CO2]) is rising (Coumou & Rahmstorf, 2012; Baldwin et al, 2019; Pfleiderer et al, 2019). Hot droughts are stressful because evaporative demand is high, while H2O availability is low and trees need to tightly regulate H2O loss (Ameye et al, 2012; Ruehr et al, 2016; Birami et al, 2018). This typically induces stomatal closure to maintain the integrity of the H2O transporting system (Tyree & Zimmermann, 2002). A C imbalance can arise under progressing stress, which triggers a cascade of metabolic adjustments
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