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

Abstract

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 gasexchange 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.

Projected Changes in Reference Evapotranspiration in California and Nevada: Implications for Drought and Wildland Fire Danger

“…For California and Nevada, the same holds true with increasing ET 0 for both CO2-adjusted (based on Yang et al, 2019) and unadjusted estimates ( Figure S12). Although we acknowledge this concern, experiments on actual vegetative responses to elevated CO2 find minimal or no drought reducing effects on 10.1029/2020EF001736 various types of vegetation and ecosystems (e.g., Bachofen et al, 2018;Birami et al, 2020;Dikšaitytė et al, 2019;Duan et al, 2014;Jiang et al, 2020;Nackley et al, 2018). Furthermore, the 2012-2016 extreme hot drought in California and Nevada, intensified by CO2 driven warming (e.g., Shukla et al, 2015;Williams et al, 2015), resulted in major declines in vegetative health for parts of California (Dong et al, 2019) further calling into question whether future CO2 increases will mitigate drying, drought, and fire danger.…”

Dying by drying: Timing of physiological stress thresholds related to tree death is not significantly altered by highly elevated CO2

“…Numerous studies on a variety of tree species found rising C a to mitigate restrictions on C assimilation under declining water availability due to increased leaf internal CO 2 concentrations ( C i ) and suppressed photorespiration (Ainsworth & Rogers, 2007; Birami et al, 2020; Dusenge, Duarte, & Way, 2019; Pushnik et al, 1995). CO 2 enrichment generally triggers decreases in stomatal conductance ( g s ) and increases in water‐use efficiency (WUE) often going along with reduced transpiration rates ( E ) (Birami et al, 2020; Dusenge et al, 2019; Haworth, Heath, & McElwain, 2010). As a possible outcome of this, soil water may be preserved, potentially slowing water stress development.…”

“…While in a previous study we could show that eCO 2 can modestly impact heat and hot‐drought stress responses in Aleppo pine seedlings (Birami et al, 2020), we followed here the assumption that tree survival under prolonged drought stress might benefit from greater NSC reserves (e.g., Mitchell et al, 2013), stimulated by eCO 2 . Hence, we assessed the time‐to‐mortality during a fast and a slow lethal drought to identify the importance of NSC for survival.…”

Indications for a Central Role of Hexokinase Activity in Natural Variation of Heat Acclimation in Arabidopsis thaliana

“…Such comparative approaches potentially provide detailed molecular insight into the complex regulation of plant metabolism, which promotes our understanding of how global climatic changes affect plant communities and ecosystems and might indicate breeding strategies to increase stress tolerance of crop plants. As outlined before, it is necessary to study plant acclimation and response to an abiotic stress combination because it remains hardly possible to reliably predict specific plant responses from single stress experiments [ 5 , 6 ].…”