Anatomical adjustments of the tree hydraulic pathway decrease canopy conductance under long-term elevated CO$_2$

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%). ... mehrThese 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.