Impacts of elevated CO levels and temperature on photosynthesis and stomatal closure along an altitudinal gradient are counteracted by the rising atmospheric vapor pressure deficit.

The efficiency of water use in plants, a critical ecophysiological parameter closely related to water and carbon cycles, is essential for understanding the interactions between plants and their environment. This study investigates the effects of ongoing climate change and increasing atmospheric CO concentration on intrinsic (stomata-based; iWUE) and evaporative (transpiration-based; eWUE) water use efficiency in oak trees along a naturally small altitudinal gradient (130-630 m a.s.l.) of Vihorlat Mountains (eastern Slovakia, Central Europe). To assess changes in iWUE and eWUE values over the past 60 years (1961-2020), stable carbon isotope ratios in latewood cellulose (δC) of annually resolved tree rings were analyzed. Such an approach was sensitive enough to distinguish tree responses to growth environments at different altitudes. Our findings revealed a rising trend in iWUE, particularly in oak trees at low and middle altitudes. However, this increase was negligible at high altitudes. Warmer and drier conditions at lower altitudes likely led to significant stomatal closure and enhanced efficiency in photosynthetic CO uptake due to rising CO concentration. Conversely, the increasing intracellular-to-ambient CO ratio (Ci/Ca) at higher altitudes indicated lower efficiency in photosynthetic CO uptake. In contrast to iWUE, eWUE showed no increasing trends over the last 60 years. This suggests that the positive impacts of elevated CO concentrations and temperature on photosynthesis and stomatal closure are counteracted by the rising atmospheric vapor pressure deficit (VPD). These differences underscore the importance of the correct interpretation of stomata-based and transpiration-based WUEs and highlight the necessity of atmospheric VPD correction when applying tree-ring δC-derived WUE at ecosystem and global levels.