Dying by drying: Timing of physiological stress thresholds related to tree death is not significantly altered by highly elevated CO2_{2}

Drought‐induced tree mortality is expected to occur more frequently under predicted climate change. However, the extent of a possibly mitigating effect of simultaneously rising atmospheric [CO$_{2}$] on stress thresholds leading to tree death is not fully understood, yet. Here, we studied the drought response, the time until critical stress thresholds were reached and mortality occurrence of Pinus halepensis (Miller). In order to observe a large potential benefit from eCO$_{2}$, the seedlings were grown with ample of water and nutrient supply under either highly elevated [CO$_{2}$] (eCO$_{2}$, c. 936 ppm) or ambient (aCO$_{2}$, c. 407 ppm) during 2 years. The subsequent exposure to a fast or a slow lethal drought was monitored using whole‐tree gas exchange chambers, measured leaf water potential and non‐structural carbohydrates. Using logistic regressions to derive probabilities for physiological parameters to reach critical drought stress thresholds, indicated a longer period for halving needle starch storage under eCO$_{2}$ than aCO$_{2}$. Stomatal closure, turgor loss, the duration until the daily tree C balance turned negative, leaf water potential at thresholds and time‐of‐death were unaffected by eCO$_{2}$. Overall, our study provides for the first‐time insights into the chronological interplay of physiological drought thresholds under long‐term acclimation to elevated [CO$_{2}$]

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 gasexchange 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

Anatomical adjustments of the tree hydraulic pathway decrease canopy conductance under long-term elevated CO2_2

The cause of reduced leaf-level transpiration under elevated CO$_2$ remains largely elusive. Here, we assessed stomatal, hydraulic, and morphological adjustments in a long-term experiment on Aleppo pine (Pinus halepensis) seedlings germinated and grown for 22–40 months under elevated (eCO$_2$; c. 860 ppm) or ambient (aCO$_2$; c. 410 ppm) CO$_2$. We assessed if eCO$_2$-triggered reductions in canopy conductance (g$_c$) alter the response to soil or atmospheric drought and are reversible or lasting due to anatomical adjustments by exposing eCO$_2$ seedlings to decreasing [CO$_2$]. To quantify underlying mechanisms, we analyzed leaf abscisic acid (ABA) level, stomatal and leaf morphology, xylem structure, hydraulic efficiency, and hydraulic safety. Effects of eCO$_2$ manifested in a strong reduction in leaf-level g$_c$ (−55%) not caused by ABA and not reversible under low CO$_2$ (c. 200 ppm). Stomatal development and size were unchanged, while stomatal density increased (+18%). An increased vein-to-epidermis distance (+65%) suggested a larger leaf resistance to water flow. This was supported by anatomical adjustments of branch xylem having smaller conduits (−8%) and lower conduit lumen fraction (−11%), which resulted in a lower specific conductivity (−19%) and leaf-specific conductivity (−34%). These adaptations to CO$_2$ did not change stomatal sensitivity to soil or atmospheric drought, consistent with similar xylem safety thresholds. In summary, we found reductions of g$_c$ under elevated CO$_2$ to be reflected in anatomical adjustments and decreases in hydraulic conductivity. As these water savings were largely annulled by increases in leaf biomass, we do not expect alleviation of drought stress in a high CO$_2$ atmosphere

Gas exchange dynamics of Aleppo Pine seedlings in response to elevated [CO2], drought and increasing temperatures

The data describes plant gas exchange dynamics (CO2, H2O) including stomatal conductance and water use efficiency of Pinus halepensis seedlings exposed to drought and increasing temperatures under either ambient (c. 400 ppm) or elevated (c. 900 ppm) atmospheric [CO2]. Measured in a scientific glasshouse facility at KIT IMK-IFU Garmisch-Partenkirchen, Germany, via seperately controlled growth chambers setup

Heat and drought: Stress and recovery responses of Aleppo pine seedlings

Aleppo pine trees growing at the arid timberline (Yatir forest, Israel) are facing extreme seasonal drought combined with hot temperatures during summer and short heat wave periods in spring. These heat waves have large effects on the forest carbon and water exchange, and might likely increase in severity with predicted climate change scenarios. To better understand the trees’ physiology during stress and subsequent ability to recover from heat and drought, we performed a greenhouse experiment on one-year-old Pinus halepensis. Soil moisture was kept close to the winter optimum in the control and heat treatment and close to summer values in the drought and heat-drought treatment. The trees of the heat treatment were exposed to two short heat waves (4-days each) with temperature maxima of 43°C, about 15°C above the control and drought trees. Aboveground carbon exchange, transpiration and emissions of selected volatile organic compounds (VOC) were measured continuously using automated tree chambers (n=4 per treatment). To detect changes in biomass allocation and carbohydrate pools, additional trees were sampled before, during and at the end of the stress periods. We will present first results of stress and recovery responses of carbon and water exchange including VOC emissions. In combination with changes in biomass allocation and carbohydrate pool analysis, this can give us a better view on pine seedling performance during periodic combined environmental stress

Le Conservateur bazadais : journal politique, commercial, agricole et financier

01 février 19021902/02/01 (N1026)-1902/02/01