Journal of Hydrology / Sensitivity of evapotranspiration and seepage to elevated atmospheric CO2 from lysimeter experiments in a montane grassland

The response of vegetation to elevated atmospheric carbon dioxide (??CO2) has opposing effects on terrestrial water fluxes, especially evapotranspiration. In particular, CO2 fertilization may enhance plant growth, therefore increasing canopy transpiration, while a partial stomatal closure decreases leaf transpiration. In this study, the effects of ??CO2 on water fluxes at a managed montane grassland were monitored using two high precision weighable lysimeters exposed to ambient and elevated (+300 ppm relative to ambient conditions) CO2 concentrations. During the growing season, ??CO2 led to a general decrease in evapotranspiration and an increase in seepage. It was shown that ??CO2 influences evapotranspiration primarily through the change in stomatal resistance, as no effect of ??CO2 on the leaf area index was confirmed. A CO2-dependent Penman– Monteith equation and actual evapotranspiration data from the lysim ters made it possible to estimate the effect of ??CO2 on stomatal resistance. ??CO2 was found to increase stomatal resistance by 50% relative to ambient conditions. A HYDRUS-1D model allowed a temporal extension of the experimental findings to the period of 1990–2021 and revealed high variability in the relative change in annual se page under ??CO2. The modeling exercise demonstrated that the effect of ??CO2 can be easily implemented in the Penman–Monteith equation, and if neglected, leads to overestimation of transpiration and underestimation of seepage, especially in the first post-drought seepage events. Our findings show that ??CO2 positively affects the water balance in montane grasslands, thus ??CO2 might be an important factor in mitigating the effects of warming and droughts in the future. We propose that the vegetation response to ??CO2, in particular the effect on stomatal resistance, should be taken into account when assessing the effects of climate change on grassland water fluxes. This may also have large implications when studying the impact of climate change at a watershed scale Version of record